AMS Weather Studies Introduction to Atmospheric Science, 4th Edition Chapter 1 Monitoring the Weather © AMS

Case-in-Point There have been monumental advances in monitoring of weather Case-in-point – the blizzard of 1888 A similar storm in December 2000 Still lacked data on sea surface temperatures – overestimated snowfall Today, though not exact, we are much better at monitoring and forecasting weather Satellites Radiosondes Surface observations The Blizzard of 1888 © AMS

Driving Question What are some basic characteristics of the atmosphere and weather? This chapter will introduce and describe some of the tools and basic understandings that will guide your investigation of the atmosphere © AMS

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 © AMS

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 © AMS

Weather Systems and Weather 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) © AMS

Pressure Systems 2 Types High Pressure Systems, or “Anticyclones” Low Pressure “Cyclones” © AMS

Pressure Systems, Continued High and low refer to air pressure High pressure area is relatively high compared to surrounding air Low pressure area is relatively low compared to surrounding air Highs 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 © AMS

Pressure Systems, Continued High and Low Pressure Centers © AMS Arrows indicate surface horizontal winds

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 © AMS

Fronts Warm Front Cold Front © AMS © American Meteorological Society

Characteristics of Air Masses & Fronts Polar air masses are colder in winter, and milder in summer Tropical air masses have less seasonal variation, due to nearly uniform sunlight duration and intensity Air masses modify as they move away from source region Starts taking on the characteristics of the area it is passing over Fronts are 3-dimensional Where the front intersects the earth surface is shown with the front symbol on the map Warm front clouds and precipitation occur over a wide band, and may occur over 1 to 2 days Usually light precipitation Cold front clouds and precipitation occur over a narrow band, and may occur over minutes to hours May be very heavy precipitation © AMS

Characteristics of Air Masses & Fronts, Continued Wind directions are different on the two sides of a front Some fronts have no clouds or precipitation. Passage indicated by wind shift, and temperature/humidity changes 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. 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 © AMS

Other Weather Characteristics In coastal areas Lake/sea breezes may lower summer temperatures May push 10-50 km (6 -30 miles) inland from the coast/shore Heavy lake-effect snow on Great Lakes or Great Salt Lake Late fall and winter Downwind (eastern and southern shores) Central U.S. Most common location of tornadoes in the spring Thunderstorms Common in FL, western High Plains and eastern slopes of the Rockies Rare along Pacific Coast and in HI Tropical storms and hurricanes impact Atlantic and Gulf coasts Mostly August through October Rare on West Coast © AMS

Describing the State of the Atmosphere What do Forecasters Tell Us? Maximum Temperature Usually occurs in early to mid-afternoon Minimum temperature Usually occurs around sunrise Dewpoint (frost point) 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 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 © AMS

Describing the State of the Atmosphere What do Forecasters Tell Us 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 Example; a west wind is blowing from the west, toward the east Sky cover 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 © AMS

Weather Satellite Imagery Two major types of satellite orbits Geostationary High orbits 36,000 km (22,300 miles) high 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 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 © AMS

Orbit of a Geostationary Satellite © AMS

Orbit of a Polar-Orbiting Satellite © AMS

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 © AMS

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 © AMS

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 © AMS

Weather Radar Complements satellite surveillance Doppler radar detects movement Excellent tool to forecast tornadoes © AMS

Combination of Images Composite of IR and Radar © AMS

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 warm front © AMS

Cloud Forms These high thin cirrus clouds appear fibrous because they are composed of mostly tiny ice crystals © AMS

Cloud Forms These relatively low clouds are composed of tiny water droplets and have more sharply defined edges than ice-crystal clouds © AMS 27

Fog, stratus clouds in contact with the ground, reduces visibility Cloud Forms Fog, stratus clouds in contact with the ground, reduces visibility © AMS

Cloud Forms Fair weather cumulus clouds are most common during the warmest time of day and then vaporize after sunset © AMS 29

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 © AMS

Cloud Forms Clouds may move in different directions at different altitudes Indicates horizontal wind shifts with altitude © AMS