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January History Great Flood of 1937 • 70% of Louisville was submerged • 3.3 billion in damages • Crest was 85.4 feet and flood stage is only 55 feet. • 15 inches of rain in 12 days Chapter 1: Monitoring the Weather (Basics) Difference between Weather and Climate Weather is the state of the atmosphere at some place and time Described with quantitative variables Temperature, humidity, cloudiness, precipitation, wind speed, wind direction Meteorology is the study of the atmosphere and the processes that cause weather Climate is weather conditions at some locality averaged over a specified time period Climate is an average of the weather, figured over the last 30-years and updated every decade A locale’s climate also includes weather extremes Seattle? Arizona? 3 Sources of Weather Information Television The Weather Channel and local newscasts Radio NOAA Weather Radio Continuous broadcasts repeated every 4 - 6 minutes Interrupted with warnings and watches The Internet 4 Weather Systems and Maps Weather info received via TV, radio, or the Internet includes Weather maps National Regional Satellite/radar images Data on current/past conditions Weather forecasts Short-term 24 – 48 hours Long-term Up to 7 days or longer Automated Surface Observing System (ASOS) 5 2 Types of Pressure Systems High Pressure Systems, or “Anticyclones” Low Pressure Systems, or “Cyclones” 6 Pressure Systems, Cont. High and low refer to air pressure Highs High pressure area is relatively high compared to surrounding air Low pressure area is relatively low compared to surrounding air Fair weather Clockwise rotation of sinking air (in Northern Hemisphere) Generally track toward the east and southeast Lows Stormy weather Counterclockwise rotation of rising air (in Northern Hemisphere) Generally track toward the east and northeast Lows tracking across the northern U.S. or southern Canada produce less moisture than lows tracking across the southern U.S. Weather to the west and north – usually cold Weather to the south and east – usually warm 7 Pressure Systems, Continued High and Low Pressure Centers Arrows indicate surface horizontal winds 8 Air Masses Huge volume of air covering thousands of square kilometers Horizontally relatively uniform in characteristics Temperature Humidity Gathers characteristics from its source region Cold, dry air masses form at higher latitudes over continents Cold, humid air masses form at higher latitudes over maritime surfaces Warm, dry air masses form over continents in subtropical regions Warm, humid air masses form near the equator or in the subtropics over maritime surfaces 10 AIR MASSES – large volume of air that has a relatively uniform temperature and humidity 1. Continental Polar – cP – cold and dry (Canada) 2. Maritime Polar – mP – cold and wet (Gulf of Alaska) 3. Continental Tropical – cT – hot and dry (Mexico) 4. Maritime Tropical – mT – hot and wet (Gulf of Mexico) 5. Arctic – A – very cold and dry (Northern Canada) Fronts Warm Front Cold Front © American Meteorological Society 13 Fronts – the boundary between air masses 1. Cold Front a. Generally, a narrow band of precipitation along or just ahead of the surface front, where precipitation is brief (couple of minutes to a few hours) b. Precipitation can be severe c. Boundary between advancing cold air and retreating warm air d. Plotted on a map as a blue line with triangles pointed in the direction of motion 2. Warm Front a. Generally, a wide band of precipitation along or just ahead of the surface warm front, where precipitation can be persistent (12-24 hours) b. Precipitation is generally light to moderate c. Boundary between advancing warm air and retreating cold air d. Plotted on a map as a red line with semi-circles pointed in the direction of motion A cyclone with the warm and cold fronts extending outward from the low pressure center. Showers generally form along the warm front, while more severe weather can occur along the cold front. This shows how the warm and cold fronts act as boundaries between different air masses. Notice how the wind directions are different on either side of the fronts, and that the flow is counterclockwise and convergent. Ways to locate a front on a surface weather map: 1. Precipitation/Clouds 2. Wind Shift 3. Temperature Difference 4. Dew Point Difference (change in relative humidity) Other Fronts 1. Stationary – a non-moving front where winds on either side blow in opposite directions 2. Occluded – when the air behind the cold front overtakes the air ahead of the warm front Characteristics of Air Masses & Fronts, Continued Wind directions are different on the two sides of a front Some fronts have no clouds or precipitation. In summer, temperature can be nearly the same on both sides of a cold front Difference will be humidity Fronts are anchored to lows on a weather map. Passage indicated by wind shift, and temperature/humidity changes Counterclockwise flow brings contrasting air masses together to form fronts Thunderstorms/severe weather often occur in the warm, humid air mass located between the cold and warm front Describing the State of the Atmosphere What do Forecasters Tell Us? Maximum Temperature Minimum temperature The temperature at which air must be cooled at constant pressure to become saturated with water vapor and for dew (or frost) to form Relative humidity Usually occurs around sunrise Dewpoint (frost point) Usually occurs in early to mid-afternoon A percentage; the ratio of the actual concentration of the water vapor component of air compared to the concentration the air would have if saturated with water vapor Relative humidity will change throughout the day as the temperature varies Generally highest around sunrise and lowest when warmest Precipitation amounts General rule – 10” of snow = 1” of precipitation 21 Based on Temperature of 90 F Dew Point Temp F Human Perception R. Humidity 75 + Extremely uncomfortable, oppressive 62% 70-74 Very Humid, quite uncomfortable 52-60% 65-69 Somewhat uncomfortable for most people 44-52% 60-64 OK for most 37-46% 55-59 Comfortable 31-41% 50-54 Very comfortable 31-37% 49 or lower Feels like the western US 30% Describing the State of the Atmosphere What do Forecasters Tell Us? (Continued) Air Pressure And its tendency (rising or falling) Falling may indicate approaching cold front Wind direction and speed Wind direction is the direction wind is blowing from Sky cover Example; a west wind is blowing from the west, toward the east Fraction of the sky covered in clouds NWS Weather watch – issued when hazardous weather is considered possible NWS Weather warning – issued when hazardous weather is imminent or actually taking place 23 Weather Satellite Imagery Two major types of satellite orbits Geostationary High orbits Orbits planet at same rate as Earth’s rotation and in same eastward direction Currently 2 of these provide a complete view of much of N. America and adjacent oceans to latitudes of about 60 degrees 36,000 km (22,300 miles) high Positioned over equator at 750 W longitude, 1350 W longitude Polar orbiting Low orbits 800-1000 km (~500-600 miles) high Provides overlapping north-south strips of images Passes over the same point twice every 24 hours 24 Orbit of a Geostationary Satellite 25 Orbit of a PolarOrbiting Satellite 26 Weather Satellite Imagery Visible Black and white photograph of the planet Only available during daylight hours Highly reflective surfaces appear bright white and less reflective surfaces are darker 27 Weather Satellite Imagery Infrared Available anytime, not just during daylight Provides temperature comparison of features within image Whiter = colder Higher cloud tops appear whiter, because they are colder 28 Weather Satellite Imagery Water vapor imagery Enables tracking of plumes of moisture Shades of white = increasing moisture Upper-level clouds appear milky to bright white GOES Imagery 29 Weather Radar Complements satellite surveillance Doppler radar detects movement Excellent tool to forecast tornadoes 30 Sky Watching You can determine much about the weather by watching the sky Clouds are aggregates of tiny water droplets, ice crystals, or some combination of both A cloud in contact with the ground is fog Cloud forms: Stratiform clouds are sheet-like clouds formed in horizontal layers Form where air ascends gradually over a broad region Cumuliform clouds are puffy, like cotton balls More vigorous ascent of air over a smaller area Under the right conditions can build vertically into a cumulonimbus (thunderstorm) cloud The appearance of high, wispy, feather-like clouds (composed of ice) in the western sky is often the first sign of an approaching 31 warm front Cloud Forms These high thin cirrus clouds appear fibrous because they are composed of mostly tiny ice crystals 32 Cloud Forms These relatively low clouds are composed of tiny water droplets and have more sharply defined edges than ice-crystal clouds 33 Cloud Forms Fog, stratus clouds in contact with the ground, reduces visibility Cloud Forms Fair weather cumulus clouds are most common during the warmest time of day and then vaporize after sunset 35 Cloud Forms Clouds of vertical development Merging and vertically-growing cumulus clouds Can become Cumulonimbus clouds Nimbo, nimbus prefix or suffix = rain producing These clouds always produce lightning and sometimes heavy rain, hail, or strong and gusty surface winds 36 Cloud Forms Clouds may move in different directions at different altitudes Indicates horizontal wind shifts with altitude 37