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United States Army Aviation Center Fort Rucker, Alabama JUNE 2004 STUDENT HANDOUT TITLE: WEATHER FILE NUMBER: 63-0677-5 PROPONENT FOR THIS LESSON PLAN IS: Aviation Training Brigade ATTN: ATZQ-ATB-AD Fort Rucker, Alabama 36362-5000 FOREIGN DISCLOSURE RESTRICTIONS: This product/publication has been reviewed by the product developers in coordination with the USAAVNC foreign disclosure authority. This product is releasable to students from all requesting foreign countries without restrictions. D-3 NOTES D-4 TERMINAL LEARNING OBJECTIVE (TLO): At the completion of this lesson the student will: ACTION: Determine weather requirements for VFR/IFR flights. CONDITIONS: While performing instrument flight examiner (IE) duties. STANDARDS: In accordance with (IAW) FM 1-230 and Department of Defense Flight Information Publications (DOD FLIP). SAFETY REQUIREMENTS: None. RISK ASSESSMENT LEVEL: Low. ENVIRONMENTAL CONSIDERATIONS: None. EVALUATION: Each student will be evaluated on this block of instruction by completing a 50 minute (.9 hour) Criterion Referenced Test. The test consists of 36 questions divided into 5 scoreable units. Each student must receive a GO on each scoreable unit to successfully complete the examination. A. ENABLING LEARNING OBJECTIVE (ELO) #1: ACTION: Identify the weather principles and theory. CONDITION: STANDARD: 1. While performing instrument flight examiner (IE) duties. IAW FM 1-230. Learning Step/Activity 1. The Atmosphere (Chapter 2). a. The atmosphere is the envelope of air which surrounds the earth. Approximately ½ of the air, by weight, is within the lower 18,000’. The air particles, however, become less numerous with increasing altitude until they gradually overcome the earth’s gravitational force and escape into space. b. A given volume of dry air contains about 78% nitrogen; 21% oxygen, and 1% argon, carbon dioxide, and minute amounts of other gases. Natural air contains, in addition to the gases present in dry air, a variable amount of water vapor, most of which is concentrated below 30,000’. The maximum amount of water vapor the air can hold depends primarily on the temperature of the air; the higher the temperature, the more vapor it can hold. c. Weather is defined as the state of atmosphere with respect to temperature, moisture content, turbulence, and cloudiness. These interact in various combinations to form the following six major meteorological elements: air temperature, humidity, clouds, precipitation, atmospheric pressure, and wind. 2. Learning Step/Activity 2. a. Temperature (Chapter 3). Temperature is a measurement of the amount of heat (Para 3-1). (1) Different substances have different molecular structures that absorb, retain, and radiate heat at different rates. Land heats and cools faster than water. (2) The earth's surface is heated during the day by incoming solar radiation called insolation. Heat radiated from the earth by outgoing radiation is called terrestrial radiation. D-5 (3) Fahrenheit and Celsius (centigrade) scales used by aviators. b. Temperature distribution (Para 3-4) over the earth's surface depends first on the seasons and second on the composition and distribution of land and sea surfaces over the earth. c. Heat transfer (Para 3-5). another in four different ways. Heat can be transferred from one body to (1) Radiation. The transfer of heat by electromagnetic waves. Heat waves are filtered and absorbed by gases in the atmosphere. When heat waves are absorbed, the energy is transferred to the absorber, raising its temperature. Heat waves can also be reflected. Clouds and snowfields are effective reflectors and reflect 50 to 80 percent of the sun's heat and light radiation. (2) Conduction. The transfer of physical contact between molecules. Air is only exchanged at the earth's surface where atmosphere are actually in contact with the heat energy through direct a poor conductor of heat. Heat is the lowest few centimeters of the ground or water. (3) Convection. The process of a heated substance carrying heat as it moves from one place to another. Water heated in a container is warmed at the bottom, expands, becomes less dense, and rises to the top, bringing heat with it. The cold water at the top, being denser, sinks to the bottom. (4) Advection. The term used for the horizontal transport of heat by wind. Advection moves about 1,000 times more heat in the earth's atmosphere than convection. d. Adiabatic process (Para 3-6). The change in the temperature of a gas which results from a change in pressure without a gain or loss of heat energy to or from an outside source. (1) Rising air. When air rises, it is cooled because it is expanding by moving to an altitude where pressure and density are less, a process called adiabatic cooling. (2) Descending air. When the air is forced downward, it is compressed, causing it to heat, a process called adiabatic heating. (3) Rising unsaturated air. Cools at the dry adiabatic lapse rate of 3°C for each 1,000 feet of altitude. (4) Rising saturated air. Cools at the moist adiabatic lapse rate of 1.1°C to 2.8°C for each 1,000 feet of altitude. A small decrease in temperature of ascending saturated air causes a relatively large amount of moisture to condense as it cools which releases latent heat into the air slowing the temperature decrease. (5) Standard lapse rate (Para 3-7a). A basis for calibrating aircraft instruments and preparing performance charts -2°C for each 1,000 feet of altitude. (6) Effects of nonstandard temperature on pressure and density (figure 3-8). The altimeter reads too high in cold air; you are lower than altimeter indicates. The altimeter reads too low in warm air; you are higher than the altimeter indicates. D-6 (7) Inversion (Para 3-7b). An increase in temperature with an increase in altitude. The most frequent type inversion is through terrestrial radiation on a clear night. Inversions are also associated with fronts. 3. Learning Step/Activity 3. Moisture (Chapter 4). a. Water continually evaporates, cools, condenses, and falls to the earth in various forms of precipitation. This process is called the hydrologic cycle which keeps moisture in the atmosphere and causes temperature and pressure changes. b. States [Para 4-2a(1)]. Water is found in three states, solid, liquid, and gaseous. The change from one state to another involves the exchange of heat energy. As the moisture changes state, heat is either absorbed or released to the environment. (1) As water changes from solid to liquid to gas or direct from solid to gas, heat is absorbed. (2) As water changes from gas-liquid-solid or direct from gas to solid, latent heat is released. c. Moisture content (Para 4-3a). At a given temperature there is a limit to the amount of water vapor the air can hold. When this limit is reached, the air is saturated. The higher the air temperature the more water vapor the air can hold. The lower the air temperature, the less water vapor the air can hold before saturation is reached and condensation occurs. d. Relative humidity (Para 4-3b). The ratio of the amount of water vapor in the air to the maximum amount the air can hold when saturated at that temperature. e. Dew point (Para 4-4). An indicator of the amount of moisture in the air. It is that temperature to which air would have to be cooled for saturation to occur. As the air temperature and dew point spread is within 2ºC (4ºF), the possibility of the formation of fog or low clouds is increased. f. Precipitation (Para 4-7). Liquid or solid moisture that falls from the atmosphere in the form of rain, drizzle, ice pellets, snow, or combinations of these. Occurs when cloud particles grow in size and weight and fall because of gravitational pull. 4. Learning Step/Activity 4. Atmospheric pressure (Chapter 5). a. Atmospheric pressure is the force-per-unit area exerted by the weight of a column of air lying directly above a point. It may be expressed in pounds per square foot (PSF), inches of mercury (Hg), or millibars (MB). Pressure patterns are shown on weather charts by a series of lines, called isobars, which connect points of equal pressure and outline pressure areas. The standard procedure on maps in North America is to draw isobars for every 4 millibars, with intermediate 2 millibar lines as needed. (1) Low pressure system (Para 5-5a). A pressure system in which the barometric pressure decreases toward the center and the wind flow around the system is counterclockwise in the Northern hemisphere. Air flows inward and up, converges, and cools adiabatically. The cooling increases relative humidity and produces cloudiness, precipitation, turbulence, strong winds, and generally poor flying weather. Convergence may also occur in areas where there are no lows or fronts. D-7 (2) High pressure system (Para 5-5b). A pressure system in which the barometric pressure increases toward the center and the wind flow around the system is clockwise in the Northern Hemisphere. Air flows downward and out, subsides, and heats adiabatically, relative humidity decreases producing generally good flying weather. This outflow of air is called divergence. (3) Col. The neutral area between two highs and two lows. (4) Trough. An elongated area of low pressure, with the lowest pressure along the trough line. Weather is frequently violent. (5) Ridge. An elongated area of high pressure with the highest pressure along the ridgeline. Weather is generally favorable. b. Pressure gradient (Para 5-6). The rate of change in pressure in a direction perpendicular to the isobars is called pressure gradient. The closer the spacing of the isobars, the stronger the pressure gradient and the stronger the winds. c. Altimeter (Para 5-7). An altimeter reads accurately only in a standard atmosphere and when the correct altimeter setting is used. Standard conditions seldom exist. (1) The atmospheric pressure frequently differs at the point of landing from that at takeoff. Adjustment of altimeter may be necessary from the takeoff point to the landing point. (2) Error due to variation from standard temperature. An aircraft flying in warmer than standard air would normally be higher than the altimeter indicates. Conversely, if the air temperature at flight altitude is colder than standard air the aircraft will normally be lower than the altimeter indicates. d. Density altitude (Para 5-8). Density altitude is defined as the pressure altitude corrected for temperature deviations for the standard atmosphere. Density altitude bears the same relation to pressure altitude as true altitude does to indicated altitude. Pressure altitude is the distance measured from 29.92” pressure level (The standard datum plain). (1) Formula for obtaining density altitude from a known pressure altitude: DA= PA + (120xTv). Where DA = density altitude. PA = pressure altitude. 120 = temperature constant. Tv = variation of the actual temperature from standard at the pressure altitude. (2) Density altitude computation (Para 5-8b). The first step in computing density altitude is to determine the pressure altitude by setting the altimeter to 29.92 in the Kollsman window of the altimeter. The second step is to determine the effect of actual air temperature of the air density. The standard temperature of the atmosphere is 15 degrees C at sea level with a decrease of 2 degrees C per thousand feet. Each 1 degree C variation from standard temperature changes the density altitude approximately 120 feet. Temperature variation is incorporated into a formula for obtaining Density Altitude from a known pressure altitude. 5. Learning Step/Activity 5. Atmospheric circulation (Chapter 6). a. Circulation and wind (Para 6-2a). Circulation is the movement of air over the surface of the earth. Differences in air density and temperature cause the air to circulate. We identify this circulation as wind. D-8 b. Semi-permanent pressure areas may be created by local surface deviations from the primary circulation. These semi-permanent pressure systems are important in the formation and movement of air masses which produce frontal systems. (1) Two high-pressure cells form one over the Pacific Ocean and one over the Atlantic Ocean near the 30° north latitude. (2) Two high-pressure cells exist one over Canada (the North American high) and one over Siberia (the Siberian high) between the 45° to 60° north latitude. (3) Two low-pressure cells form the Icelandic and Aleutian low pressure areas. c. Secondary circulation (Para 6-6). Consists of atmospheric disturbances and irregularities in the lower levels of the troposphere brought about by movement of high and low pressure systems and movement of air within these pressure systems. These moving systems are smaller than the semipermanent cells of general circulation. d. e. Factors that influence actual circulation. (1) Irregular distribution of oceans and continents. (2) Heating differences between water and land. (3) Irregular terrain. (4) Daily variation in temperature. (5) Seasonal changes. Forces affecting air motion (Para 6-7). (1) Pressure gradient force. The initiating force which produces wind. The force always acts directly across isobars toward the lower pressure. (2) Coriolis force. Perpendicular to the air flow, deflects air to the right in the Northern Hemisphere, is strongest at the Poles and decreases to zero at the Equator, and increases as wind speed increases. (3) Centrifugal force. The centrifugal force that results from the rotation of the earth neutralizes some of the effect of gravity. The effect of centrifugal force would cause all air masses to travel directly to the equator. (4) Frictional force. Greatest near the ground and tends to retard air movement and reduce the Coriolis force. The resultant surface wind flows at an angle of about 30° to 45° to the isobars over land surfaces. Above 2000 feet the effect is negligible. As the frictional force decreases with altitude the wind speed will increase. Beyond the frictional layer, increases in wind speed are due to variations in the pressure force with altitude. 6. Learning Step/Activity 6. Stability (Chapter 7). a. Stability is the atmosphere's depends on the vertical distribution of Hotter air is lighter (less dense) than than dry air. If air is warmer or more resistance to vertical motion. It the air's temperature and moisture. colder air and moist air is lighter moist than its surroundings, it is D-9 unstable and will rise. If the air is colder or drier than its surroundings, it is stable and will sink. The degree of stability of the atmosphere helps determine what type clouds, if any. b. Absolute Stability (Para 7-4b). A parcel of absolutely stable air which is lifted becomes cooler than the surrounding air and sinks back to its original position as soon as the lifting force is removed. c. Absolute Instability (Para 7-4d). A parcel of air lifted even slightly will at once be warmer than its surroundings: and, like a hot air balloon, it will be forced to rise rapidly. B. ENABLING LEARNING OBJECTIVE (ELO) #2: ACTION: masses. Identify the classification and characteristics associated with air CONDITION: STANDARD: 1. While performing instrument flight examiner (IE) duties. IAW FM 1-230. Learning Step/Activity 1. General (Chapter 9). a. Air masses (Para 9-1). A large body of air (1,700 KM or more across) whose physical properties (temperature and humidity) are horizontally uniform. b. Characteristics of an air mass (Para 9-2). (1) High pressure areas stagnate over large uniform surfaces called source regions. Typical surfaces regions include: large uniform land areas, ice caps, snow fields, and large bodies of water. The air assumes the temperature and moisture characteristics of the surface. (2) Some semi-permanent, seasonally migrating air masses are associated with low pressure areas. Examples are the air masses associated with the heat at the equator and those associated with large desert areas. c. Classification (Para 9-3). Air masses are classified with a three letter identifier indicating the temperature, moisture content, and stability of the air. (1) Moisture content [Para 9-3a(2)]. A lower-case letter indicates the type source region of the air mass. A land-source air mass produces dry air and is designated by the small letter “c” to indicate a continental air mass. A water-source air mass produces moist air and is designated by the small letter “m” to indicate a maritime air mass. (2) Temperature is identified by the source region [Para 9-3a(1)]. Capital-letter abbreviations which identify the latitude of the source region are used to indicate the air mass temperature. The following designations identify air masses that frequently affect the northern temperate zone: (a) A = Artic air mass (70º to 90º Latitude). (b) P = Polar air mass (40º to 60º Latitude). (c) T = Tropical air mass (10º to 30º Latitude). (d) E = Equatorial air mass (0º to 10º Latitude). D-10 (3) Stability (Para 9-3b). A lower-case letter indicates the temperature of the air relative to the temperature of the surface beneath which indicates stability or instability. from below. (a) Warm "w". Air is warmer than surface beneath it, cooled This is generally a stable air mass. (b) Cold "k". Air is colder than surface beneath it, heated from below. This is generally an unstable air mass. The letter "k" comes from the German word "kalt," meaning cold. d. General characteristics of air masses (Para 9-5). of typical air masses with which they are identified. Weather condition (1) Cold air mass (k). The amount and severity of the clouds, precipitation, and associated weather depends on the amount of moisture in the air mass. Air is colder than the surface beneath it and is heated from below. The heated air expands, rises, and is unstable. The vertical currents associated with the rising air result in the following types of weather. (a) Cumulus and cumulonimbus type clouds. (b) Generally good (high) ceiling (except in precipitation (c) Excellent visibility (except in precipitation areas). areas). (d) Pronounced air instability (turbulence) in lower levels due to convective currents. (e) Occasional local thunderstorms, heavy showers, hail, or snow flurries. (2) Warm air mass (w). The amount of clouds and precipitation depends on the amount of moisture in the air mass. Air is warmer than the surface beneath it, cooled from below. The cooled air would have no tendency to rise and is stable. The absence of vertical currents results in the following types of weather: (a) Stratus and stratocumulus type clouds and/or fog. (b) Low ceilings (often below 1000’). (c) Poor visibility (since haze, smoke, and dust are held in (d) Smooth stable air with little or no turbulence. lower levels). 2. Learning Step/Activity 2. 9-7). Air masses affecting the United States (Para a. General. As the different air masses migrate across the United States, typical weather conditions can be expected in each type air mass. Local influences, such as mountains, lakes, etc., can alter these conditions. (1) Maritime polar (mP)(Para 9-8) air masses arrive in the US from two different source regions, the North Pacific Ocean and the North Atlantic Ocean. (a) Winter – Pacific Coast [Para 9-8a(1)]. Originate in the interior of Siberia. They have a long over water trajectory and become D-11 unstable in the lower levels. As they enter the US they are cooled from below and become more stable with stratus and stratocumulus clouds. As they move eastward up the slopes of the mountains, they form cumuliform clouds with extensive shower activity. East of the mountains, the air descends and is warmed with decreased humidity and clear skies. (b) Winter - Northeast Coast [Para 9-8a(2)]. As the move into the New England States from the northeast, they are usually colder and more stable than those entering the West Coast from the northwest direction. Low stratiform clouds with light continuous precipitation and generally strong winds. (c) Summer (Para 9-8b). Since water temperatures are cooler than the adjacent land temperatures in the summer, maritime polar air masses entering the Pacific Coast become unstable because of surface heating. In the afternoon, cumuliform clouds form and scattered showers occur. At night, fog and low stratiform clouds are common on the coastal regions. (2) Continental polar (cP)(Para 9-9) air is cold and dry, and their source region is very stable. Originate over Canada and Alaska. (a) Winter. As they enter the United States from central or western Canada, they are heated by the underlying surface. During daylight hours, the air is generally unstable near the surface and the sky is usually clear. At night the air becomes more stable. As the air passes over the warm water of the Great Lakes, it acquires heat and moisture and becomes unstable in the lower levels. Cumuliform clouds form and produce snow over the lakes and on the leeward side of the lakes. Cumuliform clouds intensify at the Appalachians with unfavorable flying conditions. (b) Summer (Para 9-9b). These air masses are warmer and contain more moisture; the air is less stable in the surface layers. Scattered cumuliform clouds form during the day and dissipate at night when the air becomes more stable. (3) Maritime Tropical (mT)(Para 9-10). Originate over the Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. More common in the Southeastern US. They bring warm moist stable air. (a) In the winter the land is colder than the water. Warm moist air masses are cooled from below and become stable as they move inland over the south Atlantic and Gulf States. Fog and Statiform clouds form at night and tend to dissipate during the afternoon. (b) In the summer the land is normally warmer than the water; the air moves inland and is heated from below by the land and becomes unstable. Along the coast, stratiform clouds are common during the morning, change to cumuliform by late morning, and thunderstorms by late afternoon. (4) Continental Tropical (cT)(Para 9-11). Source region MexicoTexas-Arizona-New Mexico area and only in the summer. Characterized by high temperatures, low humidity, and, although rare scattered cumuliform clouds. Flying is often rough especially during the daylight hours. Occasional dust storms may extend to altitudes in excess of 10,000 feet and reduce visibility for many hours. C. ENABLING LEARNING OBJECTIVE (ELO) #3: ACTION: Identify the types of fronts to include the associated weather phenomena. D-12 CONDITION: STANDARD: 1. While performing instrument flight examiner duties. IAW FM 1-230. Learning Step/Activity 1. General (Para 10-1). a. Fronts are transition zone (boundaries) between air masses of different density. The density of the air is primarily controlled by the temperature of the air. Types: Cold, Warm, Stationary, and Occluded. b. Air Mass discontinuities (Para 10-2). A front is a boundary in the atmosphere along which certain physical properties between the air masses are discontinuous. Discontinuities are used to identify a front and determine its location. 2. (1) Temperature change. (2) Wind shift (Approximately 90º). (3) Pressure tendency (falling then rising). (4) Dew point change. Learning Step/Activity 2. Cold front (Para 10-3). a. Cold front is the leading edge of an advancing mass of relatively cold air. The colder air mass is denser and under rides the warmer air forcing it upward. The frontal slope will be quite steep as compared to other type fronts with a range of 1:50 to 1:150 depending on the forward speed of the front. A typical slope is 1:80. b. Cold front characteristics (Para 10-3b). (1) Wind ahead of the front is generally from the southwest and behind the front is from the northwest. (2) Warm air ahead of the front and cold air behind the front. (3) Due to surface heating weather band of cumuliform clouds develop 50 to 100 mile in width at the frontal boundary. With rapid moving cold fronts, a prefrontal squall line may form up to 300 miles ahead of the surface front. (4) Direction of movement is generally from northwest to southeast with average speed 22 knots and slope ratio of 1 to 80. (5) The colder air behind the front will have lower dew points where the warmer air ahead of front will have a higher dew point. Even in exceptional temperature situations, a distinct dew point change should occur across a front. c Cold frontal weather depends upon the amount of moisture in the warm air and the steepness of the slope. If the warm air is comparatively dry, no appreciable clouds and precipitation would occur. If the warm air has high moisture content and a steep frontal surface, expect the following typical weather pattern. (1) Narrow weather band. (2) Rapid lifting of the warm air causes towering cumulus and cumulonimbus clouds. D-13 (3) Thunderstorms with associated hazards including: heavy rain showers, gusty surface winds, extreme turbulence aloft, lightning, hail, and possible tornadoes. (4) Squall lines which are instability lines may develop ahead of fast moving cold fronts. The line may be solid or broken and consist of towering cumulus and cumulonimbus clouds with rain showers, thunderstorms and possible tornadoes. 3. Learning Step/Activity 3. Warm front (Para 10-4). a. The air mass boundary formed between the trailing edge of the retreating mass of cool air and the warm air mass moving in to replace it, is a warm front. The advancing warm air is less dense than the retreating colder air and overrides the colder air. b. Warm front characteristics (Para 10-4b). (1) Wind ahead of the front is from the southeast and warm sector behind the front is from the southwest. (2) As the front passes a location the temperature increases. (3) With the warm air riding over the slope of the cooler air stratiform clouds develop. They will range from nimbostratus at the front to cirrus as far as 1,000 miles ahead of the front. An area of rain can extend as far as 300 miles ahead of the front. The typical warm front is characterized by wide area stratiform clouds with low ceiling and poor visibility. (4) Warm front usually moves from southwest to northeast at an average speed of 10 knots. (5) The slow speed of movement is due to opposing wind components. The average slope ratio of the warm frontal surface is 1 to 200. c. Warm frontal weather depends primarily upon the amount of moisture and stability of the warm air mass and the steepness of the frontal slope. If the warm air mass is comparatively dry, no appreciable clouds or precipitation occurs. If the warm air has high moisture content, expect the following typical weather pattern. (1) Wide weather band ahead of the surface position of the warm front can be hundreds of miles. Gradual overriding of the warm air producing predominately stratiform clouds. (2) Precipitation normally in the form of drizzle, light rain, ice pellets, freezing rain, and snow. (3) Hazards, including low ceilings and visibility, rime icing in clouds, clear icing in freezing rain, and possibility of embedded thunderstorms. 4. Learning Step/Activity 4. Occluded front (Para 10-5). a. Occluded front is the portion of the surface front where the fast cold front has overtaken the warm front. The warm front is lifted off the surface. There are two types of occlusions: cold front occlusions and warm front occlusions. D-14 b. Occluded front characteristics. (1) If the air behind the cold front is colder than the air ahead of the warm front, the warm front will be lifted. This is called a cold front type occlusion. (2) If the air ahead of the warm front is colder than the air behind the cold front, the cold front will ride over the warm front surface. This is called a warm front type occlusion. A line of imbedded thunderstorms is often associated with a warm front occlusion within an overcast sky. (3) Weather in an occluded front is a combination of cold and warm frontal weather. (4) The most severe weather associated with an occluded front is 50 to 100 miles north of the surface apex of the three fronts. 5. Learning Step/Activity 5. Stationary front (Para 10-6). a. The boundary between different air masses with little or no movement (less than 5 knots) is called a stationary front. The wind on each side of the front generally parallel each other, but are opposite in direction. b. The associated weather is primarily of the warm front type except when the air is unstable and the stationary front is slowly moving southward. In this case, the associated weather resembles cold frontal weather. c. The possibility exists of a closed, low pressure system forming which could develop later into an occluded front. D. ENABLING LEARNING OBJECTIVE (ELO) #4: ACTION: Identify the conditions that produce weather hazards to flight. CONDITION: STANDARD: 1. While performing instrument flight examiner duties. IAW FM 1-230. Learning Step/Activity 1. Turbulence (Chap 11). a. Causes of turbulence (Para 11-2). Caused by random fluctuations of airflow which are instantaneous and irregular. The four degrees of intensity are light, moderate, severe, and extreme. b. Turbulence will be divided according to the following causes: (1) Thermal causes (Para 11-3). Localized vertical convective currents due to surface heating or unstable lapse rates and cold air moving over warmer ground or water. (a) Normally extends to about 3,000 feet AGL - can be higher. (b) Generally light to moderate - can be severe. (2) Mechanical causes (Para 11-4). over irregular terrain or obstructions. Resulting from wind flowing (a) When the air near the surface of the earth flows over obstructions, irregular terrain (bluffs, hills, mountains) and buildings, the normal horizontal wind flow is disturbed. D-15 (b) Mountain wave turbulence. Formed in stable air with wind flow of 25 knots or more perpendicular to a mountain range. The stronger the wind, the larger the wave. One or more waves may form on the leeward side of the mountains. 1. Presence of a mountain wave may be indicated by cap clouds at the peak of the mountain and roll and lenticular clouds on the leeward side. 2. Areas of steady updrafts and downdrafts may extend to 70,000 feet and as far as 300 miles downwind from the mountain range. Moderate to severe turbulence can be encountered in these waves. (3) Frontal causes (Para 11-6). Frontal turbulence is caused by lifting of warm air by a frontal surface, leading to instability and/or mixing or sheer between the warm and cold air masses. (4) Wind shear causes (Para 11-7). Wind shear is a change of wind speed and/or direction over a short distance. A relatively steep gradient in wind velocity along a given line of direction (either vertical or horizontal) and produces churning motions (eddies) which result in turbulence. It is often referred to as clear air turbulence (CAT). (a) Generally associated with jet streams (strong winds of 50 (b) Hard to forecast or detect until encountered. (c) Can produce extreme turbulence. knots or more). 2. Learning Step/Activity 2. Thunderstorms (Chapter 12). a. Thunderstorm formation (Para 12-2). The minimum factors essential to the formation of a thunderstorm are conditionally unstable air with relatively high moisture content and some type of lifting action. b. Stages of a thunderstorm (Para 12-5b). (1) Cumulus stage. The chief distinguishing feature of the cumulus or building stage is an updraft (3,000 to 6,000' FPM) that prevails throughout the entire cell. Turbulence will light to severe. Temperature will be warmer in the cloud. Icing will be clear and rime at the appropriate levels. (2) Mature stage. The beginning of surface rain and adjacent updrafts and downdrafts initiates the mature stage and is considered the most dangerous. Precipitation amounts may be heavy with the possibility of hail. With the up and down draft creating wind shear, turbulence levels will be severe to extreme. High wind and first gust may be found in any sector, usually 10 to 20 miles from the storm. Wind speeds average about 15 knots above prevailing velocity with about a 40° change in direction. Speed may be 75 knots or more. Icing can be expected Icing (clear and rime) can be expected at different levels above the freezing level. Lightning will be at its maximum at freezing level. Cloud tops extend up to 40,000 feet, sometimes in excess of 70,000 feet. (3) Dissipating stage. As the updraft begins to weaken the downdrafts continue to build in area until the storm is mostly downdrafts. Turbulence levels are light to moderate. Precipitation is light to none. Icing will be clear and rime at the appropriate levels. c. Steady state thunderstorms. D-16 (1) In a steady state thunderstorm, precipitation falls outside the updraft allowing the updraft to continue unabated. This allows the mature stage updrafts to become much stronger and last much longer than the normal air mass storm. Such a cell may last for several hours. (2) Such storms usually are associated with weather systems such as fronts, converging winds, troughs aloft, and often form into squall lines. d. Thunderstorm hazards. thunderstorms.) (Solicit student experiences with (1) Heavy rain and/or snow possible tornadoes. (2) Hail and structural damage. (3) Icing. (4) Lightning, instrument error, loss of communications, and (5) Severe surface gusts and low level wind shear. (6) Pressure variations. (7) Low ceiling and visibility in rain showers. (8) Turbulence - greatest hazard - light to extreme. static. e. Recommended flight procedures (Para 12-8). Avoid all thunderstorms. AR 95-1 - Aircraft will not be intentionally flown into thunderstorms. f. g. 3. Microburst formation and hazards. (1) Evolution of a microburst. (2) Symmetric microburst. (3) Asymmetric microburst. (4) Dry microburst. Weather evaluation to avoid thunderstorms. (1) METAR and Terminal aerodrome forecasts (TAF). (2) Aviation severe weather watch. (3) LLWAS. (4) Convective SIGMETs. (5) Visual cues. (6) PIREPs. (7) Weather radar. Learning Step/Activity 3. Icing (Chapter 14). D-17 a. Factors necessary to produce structural icing on aircraft in flight are free-air temperature at or below freezing (±4ºC to -20ºC) and presence of visible liquid moisture in the form of clouds or precipitation (Para 14-2). Intensity levels are reported as trace, light, moderate, and severe. b. Types and characteristics (Para 14-8). (1) Clear icing - most hazardous. (a) Formed from large super-cooled water droplets. (b) Forms as clear, dense, or solid ice, out and back from the leading edge of the airfoil as a blunt shape and is hard to break loose, cohesive. (c) Found in cumuliform clouds between temperatures 0°C to -10°C. (d) Found in liquid precipitation at temperatures lower than 0°C as freezing rain (usually associated with warm fronts). (2) Rime icing - most common. (a) Formed from tiny super-cooled water droplets. (b) Forms as granular, opaque, and rough deposit of ice and extends outward from leading edge of airfoils. (c) More brittle and easier to dislodge than clear ice. (d) Found in stratiform clouds at temperatures 0°C to -20°C and cumuliform clouds at temperatures -10°C to -20°C. (3) Frost. Sublimation of water vapor to ice crystals. (a) Generally formed on clear nights when the metal of the aircraft is radiation cooled to temperatures below 0°C. (b) Can be formed when an aircraft descends from temperatures below 0°C to warmer humid air. (c) Causes loss of lift and collecting agent for other types (d) Should be removed before takeoff. of icing. c. Aircraft icing is more probable and severe over mountainous or steep terrain than over low or flat elevations. The presence of a mountain range causes strong upward air currents on its windward side which are capable of supporting larger than average water droplets, thereby compounding the icing hazard. Severest icing occurs above the crest and to the windward side of the ridges. This zone usually extends 4,000 to 5,000 feet above the mountain and can go higher in cumuliform clouds. d. Effects of icing on aircraft (Para 14-10). Ice can cause mechanical or visual obstruction. It may modify the profile of part of the aircraft, reducing its aerodynamic efficiency. Ice can alter the frequency of some parts of the aircraft (vibration may be induced). Ice may break off and cause serious mechanical damage, asymmetric condition for rotor mechanism, or injury to nearby persons. D-18 e. Recommended flight procedures. (1) Remove all ice and snow from the aircraft before takeoff. (2) Use all necessary anti-icing/deice equipment. (3) Avoid flying in clouds when the outside air temperature is between 0ºC and -20ºC. (4) If ice is encountered, climb or descend to an altitude where the temperature is warmer than 0ºC or colder than -20ºC. 4. Learning Step/Activity 4. Fog (Chapter 15). a. Fog is minute water droplets or ice crystals suspended in the atmosphere with no visible downward motion. Fog is similar to stratus clouds. The base of fog is at the earth's surface and the base of a cloud is at least 50 feet above the surface. Fog is hazardous because it restricts surface visibility. b. Fog formation (Para 15-2). (1) High relative humidity (By adding moisture to the air or cooling to the dew point). (2) Light wind less than 5 knots for mixing action (spreads surface cooling action). (3) c. Fog types (Para 15-4). (1) surface. Condensation nuclei. Radiation fog. Radiation cooling with light wind. (2) Advection fog. Movement of warm moist air over a colder If fog forms over the ocean, it is referred to as sea fog. (3) Upslope fog. and cools by expansion. Moist stable air flows up a sloping land surface (4) Valley fog. In the evening cold dense air accumulates in the valleys causing the air temperature to decrease to dew point temperature. (5) Ice Fog. At temperatures of -25 and below, water vapor sublimates into ice crystal without passing through a liquid state. (6) Evaporation fog. Moisture added to the air. (a) Frontal fog. Forms when precipitation falls from the maritime tropical systems and evaporates in the polar air below. Common with slow moving winter frontal systems and active warm fronts during all seasons. (b) Steam fog. Cold stable air flows over a non-frozen water surface that is several degrees warmer than the air. Evaporation of moisture into the cold air saturates the air. E. ENABLING LEARNING OBJECTIVE (ELO) # 5: ACTION: Decode METAR and TAF reports. CONDITION: While performing instrument flight examiner duties. D-19 STANDARD: 1. IAW FM 1-230 and DOD FLIP. Learning Step/Activity 1. Aviation routine weather reports (METAR). a. Aviation routine weather reports (METAR) is the observation code to report meteorological data. METAR was internationally for worldwide use but each country can modify the code. When METAR data is missing, it is simply omitted. (AIM and DOD FLIP). b. The following describe the elements in a METAR report: (1) Report type. METAR is a scheduled observation taken between 55-59 minutes past the hour (hourly observation). SPECI (special report is an unscheduled observation taken when a predefined condition criteria change occurs. (2) ICAO station identifier. station identifiers. The METAR code uses ICAO 4-letter (3) Date and time of report. The date and time the observation is taken are transmitted as six-digit date/time group appended with Z to denote UTC. The first two digits are the date followed with two digits for hour and two digits for minutes. (4) Modifiers (As required). “AUTO” identifies a METAR/SPECI report as an automated weather report with no human intervention. “COR” indicates a corrected report that is sent out to replace an earlier report with an error. AO1 is an ASOS without a rain vice snow discriminator and AO2 has a discriminator. (5) Wind. Reported as 5 digit group (six if over 99 Kts). Reference to true north, reported to nearest 10 degrees or “VRB” is direction is variable. If the wind is gusty it is reported as a “G” after the speed followed by the highest gust reported. The abbreviation “KT” is appended to denote the use of knots for wind speed. (6) Visibility. Prevailing visibility is reported in statute miles with “SM” appended to it. Overseas locations use meter vice statue miles with 9999 (7 SM or greater) as the largest value. (a) Prevailing visibility is the greatest distance that can be seen over at least one half of the horizon, not necessarily continuous. (b) Variable/Sector visibility shown in the Remarks (when prevailing visibility is less than three miles). (c) Visibility less than three miles is reported in fractions of a mile (smallest fraction - 1/16). Visibility between 3 and 15 miles is reported to the nearest mile. Visibility greater than 15 miles is reported to the nearest 5 miles. (7) Runway visual range. “R” identifies the group followed by the runway heading “/” and the visual range if feet (meters in other countries) followed with “FT” (i.e. R06/4000FT). M is RVR less than lowest reportable sensor value, P is greater than highest value, and V means RVR is variable. (8) Significant weather is reported in the format: Intensity, Proximity, Descriptor, Precipitation, Obstruction to visibility, and Other. Reported whenever phenomena are occurring at the time of report in the order D-20 of predominance of effect on visibility or ceiling. to decode. (9) See weather table in FIH Sky condition is reported in hundreds of feet above the surface (AGL). (a) The amount of sky cover is reported in eights of sky cover (cumulative), using the contractions: 1. SKC: clear (no clouds). 2. FEW: few (0/8 to 2/8). 3. SCT: scattered (3/8 to 4/8). 4. BKN: broken (5/8 to 7/8). 5. OVC: overcast (8/8). (b) Ceiling. The heights above the earth’s surface of the lowest layer of clouds or obscuring phenomena that is reported as “broken,” “overcast,” or “obscuration,” and is not classified as thin or partial. Vertical visibility (indefinite ceiling) is preceded by “VV” and followed by three digits indicating the vertical visibility in hundreds of feet. When the ceiling is variable, the remark “CIG” will be shown followed by the lowest and highest ceiling heights separated by a “V.” (c) Suffixed by CB or TCU - cumulonimbus or towering cumulus when present. (10) Temperature and dew point are reported two, two digit groups in degrees Celsius, separated by a “/”. Temperatures below zero are prefixed with an “M.” If the temperature is missing, the group is omitted. (11) Altimeter setting is reported in a four digit format in inches of mercury prefixed with an “A” to denote the units of pressure. (12) Remarks group will be included in all observations, when appropriate. The “RMK” denotes the start of the remarks section. There are two categories of remarks: (1) Automated, manual, and plain language; (2) Additive and automated maintenance data. See remarks table in FIH. 2. Learning Step/Activity 2. Automated weather observation system (AWOS). a. AWOS is a real time system consisting of various sensors, a processor, a computer generated voice subsystem, and transmitter to broadcast local minute-by-minute weather directly to the aircraft. b. AWOS observations derived from an automated system will include the prefix “AWOS.” If augmented by certified observer, AWOS augmentation will be identified as “OBSERVER WEATHER.” The reported visibility is derived from a sensor near the touchdown of the primary instrument runway and may differ from prevailing visibility. The reported sky condition/ceiling is derived from the ceilometer and may differ from the Observer sky condition. c. There are four operational levels of AWOS. (1) AWOS-A - reports only altimeter setting. (2) AWOS-1 - reports altimeter setting, wind data, temperature, dew point, and density altitude. D-21 (3) AWOS-2 - reports information in AWOS-1 plus visibility. (4) AWOS-3 - reports information in AWOS-2 plus cloud and ceiling data. d. The information is transmitted over a discrete radio frequency or the voice portion of a local NAVAID. Some locations can be received by telephone. The system transmits a 20 to 30 second weather message each minute. The messages are updated each minute. Receivable to a maximum of 25 NM at or above 3,000' AGL up to 10,000' AGL. 3. Learning Step/Activity 3. Automated surface observation System (ASOS). a. ASOS is the primary surface weather observing system of the United States. A joint effort of the National Weather Service, the FAA and the Department of Defense. The ASOS provides continuous minute-by-minute observations. Generates information for an aviation routine weather report. b. Transmits over a discrete VHF radio frequency or the voice portion of a local NAVAID. Maximum of 25 NM and 10,000' AGL. The ASOS report without human intervention will contain only that weather data capable of being reported automatically. “AUTO.” c. METAR format will be used on AWOS/ASOS sequence. METAR remark section will contain the type and number of sensors. A01 - Without precipitation discriminator. A02 - With precipitation discriminator. 4. Learning Step/Activity 4. Terminal Aerodrome Forecasts (TAF). a. A concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). Scheduled four times daily for 24-hour periods beginning at 0000Z, 0600Z, 1200Z, and 1800Z. b. There are two types of TAF issuances: Routine forecast (TAF) and Amended forecast (TAF AMD). Corrected (COR) or delayed (RTD) TAF’s are identified only in the communications header which precedes the actual forecasts. c. The TAF code uses ICAO 4-letter identifiers as described in the METAR section. Date and time of origin when the forecast is actually prepared. The format is a two digit date and four digit time followed by the letter “Z.” d. Forecast Meteorological Conditions. Basic Format: Wind, Visibility, Weather, Sky Condition, and Optional data (i.e. Wind shear). If a significant, lasting change in any of the elements is expected during the valid period, a new time period with the changes is included. c. Forecast change indicators are used when a rapid, gradual, or temporary change is expected in some or all of the forecast meteorological conditions. (1) From (FM) group is used when a rapid change (less than 1 hour) in prevailing conditions is expected. Appended to the “FM” indicator is the four-digit hour and minute the change is expected to begin and continues until the next change group or the end of the current forecast. Generally will start a new line and contains all the required elements - wind, visibility, weather, and sky condition. D-22 (2) Becoming (BECMG) group is used when a gradual change in conditions is expected over a longer time period (2 hours). Time period when the change is expected is a four-digit group with the beginning hour and ending hour of the change period following the “BECMG” indicator. Only the changing forecast meteorological conditions are included. (3) Temporary (TEMPO) group is used for any conditions in wind, visibility, weather, or sky condition which are expected to last for generally less than an hour at a time, and are expected to occur during less than half the time period. “TEMPO” followed by a four-digit group giving the beginning hour and ending hour of the time period during which the conditions are expected. Only the changing forecast meteorological conditions are included. (4) Probability forecast is the probability or chance of thunderstorms or other precipitation events occurring, along with associated conditions (wnd, vis, sky cond.). “PROB” followed by a two digit percentage (30 = 30-39%), then the beginning hour and the ending hour the thunderstorms or precipitation expected. D-23 APPENDIX E WEATHER STUDY GUIDE 1. Air masses are formed by large bodies of air stagnating over a uniform surface and acquiring the temperature and moisture characteristics of the surface. Which type pressure system (high) (low) would be more conducive to the formation of an air mass? 2. The relative moisture content of air masses is denoted by classifying them according to their type source region. (m) and (c) . 3. Air mass temperature classification, using source regions, from the coldest to the hottest are — A , P , T , and E . 4. The “w’ classifier for air masses means the air is (colder, warmer) _________ than the surface beneath it. 5. The “k” classifier for air masses means the air is being (heated, cooled) ____________ from below and is (stable, unstable) _____________. 6. T F A maritime tropical (mT) or continental tropical (cT) air mass will always be classified as a “w” air mass. 7. T F A cP mass, after having traveled a long distance over northern waters, would be redesignated as mP. 8. List the general weather conditions that would be expected with a “k” and “w” classification. “k” ________ “w” ________ Precipitation: ________ ________ Ceilings: ________ ________ Visibility: ________ ________ SFC winds: ________ ________ Turbulence: ________ ________ Cloud type: 9. When an air mass is lifted (frontal or terrain), the temperature will (increase, decrease) _______________ and the relative humidity will (increase, decrease) _____________. 10. T F cP air masses move farther south during winter than summer. 11. T F When mT air masses dominate the Fort Rucker area in the summer, the stability classifier during the day will be “k” and thunderstorms are possible. 12. T F The major hazard associated with cT air masses is clear air turbulence. 13. T F The trajectories of a cold front in the southeastern United States are generally to the southwest. E-1 14. The term “front” normally refers to the boundary between air masses of different _________________. 15. The frontal surface always slopes over the (cold, warm) _______air mass. 16. The (leading, trailing) _____________ edge of advancing polar air masses are called (cold, warm) _____________ fronts in the northern hemisphere, and they generally move toward the (northeast, southeast) ____________. 17. Squall lines sometimes form (ahead, behind) __________ of fast moving (cold fronts, warm fronts) ___________________. 18. The slope of a typical cold front is (steep, shallow) ___________ and the associated band of weather is (wide, narrow) _______________. 19. The type clouds that form with typical cold fronts are predominantly (stratiform, cumuliform) ______________________. 20. What are the four elements of discontinuity across a front? a. _________________________________________ b. _________________________________________ c. _________________________________________ d. _________________________________________ 21. In order to stay on course after penetration of a cold front, a heading correction to the (left, right) ____________ should be made. 22. The trailing edge of a retreating cold air mass is called a __________ front and generally moves toward the (northeast, southeast) _____________. 23. The weather band associated with a warm front is generally (wide, narrow) ___________; the associated clouds are primarily (cumuliform, stratiform) ______________; and predominate (ahead, behind) _____________of the surface position of the front. 24. List four hazards to aviation associated with a warm front. _________________, ____________________, _________________, ____________________. 25. What type front extends northeastward from the apex and contains the worst elements of both a cold front and a warm front to include embedded thunderstorms? _____________________ 26. The front that moves at five knots or less is a (cold, warm, stationary) ____________ front. 27. After a cold front passes, the temperature will (increase, decrease) _______________ and the pressure will (rise, fall) __________. 28. A front depicted in red with smooth bumps is a ___________ front. 29. Any precipitation that might occur in conjunction with a warm front would probably be (light or heavy) _________, but it would probably be more continuous. E-2 30. The most severe weather associated with an occluded front would be to the ______________ (give geographic direction) of the apex of the front. 31. The greatest possibility of frontal fog exists with a ________ front. 32. The clear skies after frontal passage are probably a result of a (cold, warm) __________ front. 33. Increasing cloudiness from what type of clouds can be expected in advance of a cold front ___________. 34. The hazards of a fast moving cold front include the formation of a _____________ line. 35. Listed below are characteristic features of either clear or rime ice. To the left of each feature, write “C” if it pertains to clear ice, “R” if it pertains to rime ice, and “B” if it pertains to both types. a. b. c. d. e. f. g. ______ ______ ______ ______ ______ ______ ______ 0ºC to -10ºC in cumuliform clouds. Formed by freezing rain or drizzle. 0ºC to -20ºC in stratiform clouds. Should be avoided whenever possible. Milky, granular-type ice. -10ºC to -20ºC in cumuliform clouds. causes loss of lift. 36. T F Ice will not form on the propeller, rotor blades, or rotor head during flight. 37. When encountering freezing rain, you should expect temperatures above you to be (warmer, colder) _________. 38. The extreme turbulence associated with mountain waves are found on the (windward, leeward) side of the mountain. 39. In which stage of thunderstorm development should the most severe turbulence be expected? ______________________________ 40. T F Turbulence in a thunderstorm can be avoided by penetration at an altitude between 4,000 feet and 6,000 feet AGL. 41. METAR is a scheduled observation taken between ________________ minutes past the hour (a.k.a. hourly observation). 42. How much sky coverage (or obscuration) is indicated by each of the following contractions? a. b. c. d. e. SKC FEW SCT BKN OVC __________________ __________________ __________________ __________________ __________________ 43. A ceiling is defined as the (lowest, highest) ____________ condition reported that is either ____________or __________ or ___________________. 44. T F Only 1/8 of clouds at a given level may constitute a ceiling. 45. At a certain weather station, the observer notes that to the northeast he/she can see 3/4 of a mile, to the southeast 1/2 mile, to the southwest 2 E-3 miles, and to the northwest 1/4 of a mile. The observer would report the prevailing visibility at this station as _________ mile. NOTE: Use the following METAR to answer questions 45 through 50. METAR KCBM 111555Z 15015G30KT 3/8SM +TSRA FEW002 SCT006 OVC010CB 20/18 A2989 RMK TSRAB45 SPECI KNMM 111615Z 19025G35KT 1/2SM TSRA BKN007CB 20/18 A2990 RMK SQ50KT RVNO TSB50 46. METAR report for KCBM was transmitted at ___________________. 47. What is the ceiling at Columbus AFB (KCBM)? 48. What weather is occurring at Meridian NAS (KNMM)? 49. What type of clouds are at the ceiling level at Meridian (KNMM)? 50. What is the temperature/dew point spread at Columbus AFB (KCBM)? 51. What is the complete wind information at NAS Meridian (KNMM)? NOTE: Use the following ICAO METARs to answer questions 51 through 57. METAR PAED 012155 27005KT 2100 SN OVC080 M10/M15 A3005 RMK CIGM080 FROPA 2100 METAR PAFB 012155 00000 CAVOK M29/M35 A3125. SPECI PWAK 012115 18010G30KT 9999 +TSRS BKN025 24/21 A2967 RMK CIGM025 CIG RGD. METAR PGUA 012155 VRB05KT 0350 -RAFG VV005 30/30 A2944 RMK CIGW005 FG LYR NE METAR PHIK 012155 07012KT 3/4SM R06R/4000FT -DZ SCT030 BKN180 27/26 A2992 RMK VIS S2. 52. What is the visibility at PAED? 53. What is the runway visual range at PGUA? 54. What is the wind at PHIK? 55. What is the weather at PWAK? 56. What is the ceiling at PGUA? 57. Which station is reporting the lowest altimeter setting? 58. When did PAED experience a frontal passage? NOTE: Refer to Terminal Aerodrome Forecast (TAF) below to answer questions 58 through 61. TAF KMGM 091130Z 091212 25015KT 2SM -RABR SCT002 BKN006 OVC035 FM 2330 27005KT 1/2SM -RAFG SCT002 BKN006 OVC009 PROB30 0002 1/4SM TS TEMPO 0204 00000KT 1/8SM FG BKN003 BECMG 0406 36010G15KT P6SM SCT012 BKN030. 59. What is the valid period of this forecast? E-4 60. What is the visibility forecast for KMGM at 0500Z? 61. What is the worst weather forecast for KMGM? 62. What visibility is forecast for KMGM at 2330Z? E-5 APPENDIX E ANSWER SHEET WEATHER 1. 2. 3. 4 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. high maritime, continental Arctic, Polar, Tropical, Equatorial warmer heated, unstable false true "k" "w" cumuliform stratiform_________ shower type light, continuous__ high low________________ good poor_______________ gusty light______________ moderate to severe none to light______ decrease, increase true true true false densities (temperatures) cold leading, cold, southeast ahead, cold fronts steep, narrow cumuliform a. temperature b. wind c. pressure tendency d. moisture right warm, northeast wide, stratiform, ahead low ceilings, poor visibility, freezing rain, embedded thunderstorms occluded stationary decrease, rise warm light north warm cold cumulus squall a. C b. C c. R d. B e. R f. R g. B false warmer leeward mature false 55-59 minutes a. sky clear (0/8) E-6 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. b. 1/8 to 2/8 c. 3/8 to 4/8 d. 5/8 to 7/8 e. 8/8 lowest, broken, overcast, vertical visibility true 3/4 11th day at 1555Z 1000 feet AGL thunderstorms and rain showers cumulonimbus 2 degrees C 190 degrees at 25 knots, gust 35 knots 2100 meters not reported 070 at 12 knots Thunderstorms and heavy rain vertical visibility 500 feet PGUA 29.44 FROPA 2100Z 9th day from 1200Z until next day 1200Z P6SM more than 6 statute miles TEMPO 0204 1/8 SM, FG, and OVC 003 1/2 SM in light rain and fog E-7