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WATER’S ROLE IN THE ATMOSPHERE HUMIDITY Amount of water vapor in the air Water vapor adds pressure (called vapor pressure) to the air Saturated air is air that is filled with water vapor to capacity Capacity is temperature dependent – warm air has a much greater capacity Measurements of humidity Specific humidity Quantity of water vapor in a given mass of air Often measured in grams per kilogram Relative humidity Ratio of the air’s actual water vapor content to its potential water vapor capacity, at a given temperature Expressed as a percent Saturated air Content equals capacity Has a 100 percent relative humidity Relative humidity can be changed in two ways Add or subtract moisture to the air •Adding moisture raises the relative humidity •Removing moisture lowers the relative humidity Changing the air temperature •Lowering the temperature raises the relative humidity •Raising the temperature lowers the relative humidity Dew point Temperature at which the air is saturated and the relative humidity is 100 percent Cooling the air below the dew point causes condensation •E.g., cloud formation •Water vapor requires a surface condense on Two types of hygrometers are used to measure humidity Psychrometer •Compare the temperatures of Wet-bulb thermometer, and Dry-bulb thermometer •If the air is saturated (100 percent relative humidity) then both thermometers read the same temperature •The greater the difference between the thermometer readings, the lower the relative humidity Hair hygrometer – reads the humidity directly ADIABATIC HEATING/COOLING Adiabatic temperature changes occur when Air is compressed Motion of air molecules increases Air will warm Descending air is compressed due to increasing air pressure Air expands Air parcel does work on the surrounding air Air will cool Rising air will expand due to decreasing air pressure Adiabatic rates Dry adiabatic rate Unsaturated air Rising air expands and cools at 1°C per 100 meters Descending air is compressed and warms at 1°C per 100 meters Wet adiabatic rate Commences at condensation level Air has reached the dew point Condensation is occurring and latent heat will be liberated Heat released by the condensing water reduces the rate of cooling Rate varies from 0.5°C to 0.9°C per 100 meters STABILITY OF AIR Two types of air stability STABLE AIR Resists vertical displacement (sinking air, or air sitting at the surface) Cooler than surrounding air Denser than surrounding air Wants to sink No adiabatic cooling Stability occurs when the environmental lapse rate is less than the wet adiabatic rate Often results in widespread clouds with little vertical thickness Precipitation, if any, is light to moderate UNSTABLE AIR Acts like a hot air balloon Rising air Warmer than surrounding air Less dense than surrounding air Continues to rise until it reaches an altitude with the same temperature Adiabatic cooling Air is unstable when the environmental lapse rate is greater than the dry adiabatic rate Clouds are often towering Often result in heavy precipitation Conditional instability occurs when the atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel of air Determines to a large degree Clouds that develop Intensity of the precipitation PROCESSES THAT LIFT AIR Orographic lifting Elevated terrains act as barriers Result can be a rain shadow desert Frontal wedging Cool air acts as a barrier to warm air Fronts are part of the storm systems called middle-latitude cyclones Convergence where the air is flowing together and rising LIFTING AIR CONDENSATION AND CLOUD FORMATION Condensation Water vapor in the air changes to a liquid and forms dew, fog or clouds Water vapor requires a surface to condense on Possible condensation surfaces on the ground can be grass, a car window, etc. Possible condensation surfaces in the atmosphere are tiny bits of particulate matter Called condensation nuclei Dust, smoke, etc Ocean salt crystals which serve as hygroscopic (“water seeking) nuclei Clouds Made of millions and millions of Minute water droplets, or tiny crystals of ice Classification based on Form (three basic forms) Cirrus – high, white, thin Cumulus •Globular cloud masses •Often associated with fair weather Stratus •Sheets or layers •Cover much of the sky Height High clouds •Above 6000 meters •Types Cirrus Cirrostrat us Cirrocumu lus Middle clouds •2000 to 6000 meters •rainy) •Types (alto as part of the name_ Altocumulus Altostratus Low clouds •Below 2000 meters •Types Stratus Stratocumulus Nimbostratus (nimbus means Clouds of vertical development •From low to high altitudes •Called cumulonimbus •Often produce Rain showers Thunderstorms HIGH CLOUDS MIDDLE CLOUDS LOW CLOUDS FOG Considered an atmospheric hazard Cloud with its base at or near the ground Most fogs form because of Radiation cooling, or Movement of air over a cold surface FOG Types of fog Fogs caused by cooling Advection fog – warm, moist air moves over a cool surface Radiation fog Earth’s surface cools rapidly Forms during cool, clear, calm nights ADVECTION FOG RADIATION FOG Upslope fog Humid air moves up a slope Adiabatic cooling occurs Evaporation fogs Steam fog Cool air moves over warm water and moisture is added to the air Water has a steaming appearance Frontal fog, or precipitation fog Forms during frontal wedging when warm air is lifted over colder air Rain evaporates to form fog UP-SLOPE FOG EVAPORATION FOGS Steam Fog Frontal Fog PRECIPITATION Cloud droplets Less than 10micrometers in diameter Fall incredibly slow Formation of precipitation Bergeron process Temperature in the clouds is below freezing Ice crystals collect water vapor Large snowflakes form and Fall to the ground as snow, or Melt on their descent and form rain Collision-coalescence process Warm clouds Large hygroscopic condensation nuclei Large droplets form Droplets collide with other droplets during their descent Forms of precipitation Rain and drizzle Rain – droplets have at least a 0.5 mm diameter Drizzle – droplets have less than 0.5 mm diameter Snow – ice crystals, or aggregates of ice crystals Sleet and glaze Sleet Wintertime phenomena Small particles of ice Occurs when •Warmer air overlies colder air •Rain freezes as it falls Glaze, or freeing rain – impact with a solid surface causes freezing Hail Hard rounded pellets •Concentric shells •Most diameters range from 1-5 cm Formation •Occurs in large cumulonimbus clouds with violent up-anddowndrafts •Layers of freezing rain are caught up in up-and-downdrafts in the cloud •Pellets fall to the ground when they become too heavy Rime Forms on cold surfaces •Freezing of super-cooled fog, or •Cloud droplets Rime Measuring precipitation Rain Easiest form to measure Measuring instruments Standard rain gauge •Uses funnel to collect and conduct rain •Cylindrical measuring tube measures rainfall in centimeters or inches Recording gauge Snow has two measurements Depth Water equivalent General ratio is 10 snow units to 1 water unit Varies widely AIR PRESSURE AND WIND Atmospheric Pressure Force exerted by the weight of the air above Weight of the air at sea level 14.7 pounds per square inch 1 kilogram per square centimeter Decreases with increasing altitude Units of measurement Millibar (mb) – standard sea level pressure is 1013.2mb Inches of mercury – standard sea level pressure is 29.92 inches of mercury Instruments for measuring Barometer Mercury barometer Invented by Torricelli in 1643 Uses a glass tube filled with mercury Aneroid barometer “Without liquid” Uses an expanding chamber Barograph (continuously records air pressure) WIND Horizontal movement of air Out of areas of high pressure Into areas of low pressure Controls of wind Pressure gradient force Isobars – lines of equal air pressure Pressure gradient – pressure changes over distance Coriolis Effect Apparent deflection in the wind direction due to Earth’s rotation Deflection is To the right in the northern hemisphere To the left in the southern hemisphere Friction with earth’s surface Only important near the surface Acts to slow the air’s movements Upper air winds Generally blow parallel to isobars – called geostrophic winds Jet stream “River” of air High altitude High velocity (120-140 km/h) CYCLONES AND ANTICYCLONES Cyclones A center of low pressure Pressure decreases toward the center Winds associated with In the Northern Hemisphere Inward (convergence) Counterclockwise In the Southern Hemisphere Inward (convergence) Clockwise Associated with rising air Often bring clouds and precipitation Anticyclone A center of high pressure Pressure increases toward the center Winds associated with In the Northern Hemisphere Outward (divergence) Clockwise In the Southern Hemisphere Outward (divergence) Counterclockwise Associated with subsiding air Usually bring “fair” weather GENERAL ATMOSPHERIC CIRCULATION Underlying cause is unequal surface heating On the rotating earth, there are three pairs of atmospheric cells that redistribute the heat Idealized global circulation Equatorial low pressure zone Rising air Abundant precipitation Subtropical high pressure zone Subsiding, stable, dry air Near 30° latitude Location of great deserts Air traveling equatorward from the subtropical high produces the trade winds Air traveling poleward from the subtropical high produces the westerly winds Subpolar low pressure zone Warm and cool winds interact Polar front – an area of storms Polar high pressure zone Cold, subsiding air Air spreads equator-ward and produces polar easterly winds Polar easterlies collide with the westerlies along the polar front Influence of continents Seasonal temperature differences Influence is most obvious in the Northern Hemisphere Monsoon Seasonal change in wind direction Occur over continents During warm months •Air flows onto land •Warm, moist air from the ocean Winter months •Air flows off the land •Dry, continental air CIRCULATION IN THE MID-LATITUDES Complex Occurs in the zone of the westerlies Air flow is interrupted by cyclones Cells move west to east in the Northern Hemisphere Create anti-cyclonic and cyclonic flow Paths of the cyclones and anticyclones are associated with the upper-level airflow I CAN… I can explain the dynamics of the El NinoSouthern Oscillation and its effect on continental climates. I can explain differences between maritime and continental climates with regard to oceanic currents. I can describe the various conditions of formation associated with severe weather. I can describe the seasonal variations in severe weather. EL NINO A counter current (ocean current that flows the opposite way) that flows southward along the coasts of Ecuador and Peru THINK ABOUT THESE QUESTIONS… Can you think of another ocean current that travels along the coast of a continent? How does it affect the areas weather? Can you explain what the weather would be like without an ocean current’s effect? Warm Usually appears during the Christmas season Blocks upwelling of colder, nutrient filled water, and anchovies starve from lack of food Strongest El Nino on record occurred in 1997 and 1998 and caused Heavy rains in Ecuador and Peru Ferocious storms in California NOW, THINK ABOUT THESE QUESTIONS… How could the loss of the anchovies affect us? What do you think a warm ocean current would do to the atmosphere of a location that normally had cold ocean water near it? How do you know this? Related to large-scale atmospheric circulation Pressure changes between the eastern and western Pacific called the Southern Oscillation Changes in trade winds creates a major change in the equatorial current system, with warm water flowing eastward WHICH MAP SHOWS CONDITIONS THAT YOU WOULD LIKE TO EXPERIENCE? Do you feel we are experiencing one of these phenomena this year? Explain. Effects are highly variable depending in part on the temperatures(how warm or cold they are) and size of the warm water pools LOCAL WINDS Produced from temperature differences Small scale winds Types Sea and land breezes Valley and mountain breezes Chinook and Santa Ana winds WIND MEASUREMENT Two basic measurements Direction Speed Direction Winds are labeled from where they originate (e.g., North wind – blows from the north toward the south) Instrument for measuring wind direction is the wind vane Direction indicated by either Compass points Scale of 0° to 360° Prevailing wind comes more often from one direction Speed – often measured with a cup anemometer Changes in wind direction Associated with locations of Cyclones Anticyclones Often bring changes in Temperature Moisture conditions GLOBAL DISTRIBUTION OF PRECIPITATION Relatively complex pattern Related to global wind and pressure patterns High pressure regions Subsiding air Divergent winds Dry conditions e.g., Sahara and Kalahari deserts Low pressure regions Ascending air Converging winds Ample precipitation e.g., Amazon and Congo basins RELATED TO DISTRIBUTION OF LAND AND WATER Large landmasses in the middle latitudes often have less precipitation than toward their centers Mountain barriers also alter precipitation patterns Windward slopes receive abundant rainfall from Orographic lifting Leeward slopes are usually deficient in moisture