AMS Weather Studies Introduction to Atmospheric Science, 5th Edition Chapter 1 Monitoring the Weather © AMS

Driving Question What are some basic characteristics of the atmosphere and weather? This chapter covers: Difference between weather and climate Time keeping Overview of weather systems and weather maps Introduction to satellite imagery and radar © AMS

Case-in-Point Blizzards Then and Now The Blizzard of 1888 There have been monumental advances in monitoring of weather The blizzard of 1888 4 nor’easters hit the mid-Atlantic during the winter of 2009-2010 Today, though not exact, much better at monitoring and forecasting weather Understanding of coastal winter storms Satellites/Radar Radiosondes Surface observations February 2010 nor’easter © 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, processes that cause weather, and the life cycle of weather systems. Climate is weather conditions at some locality averaged over a specified time period Climate is an average of weather, computed for the last 30-years and updated every decade A locale’s climate also includes weather extremes © AMS

Accessing Weather Information Internet Television The Weather Channel and local newscasts Radio NOAA Weather Radio Continuous broadcasts repeated every 4-6 min Interrupted with warnings and watches Weather Apps © AMS

Time Keeping Time zones UTC Measured degrees east/west of the prime meridian (0 degrees longitude) UTC Based on atomic clock Expressed as a 24-hour clock 7:45 a.m.=0745 2:20 p.m.=1420 Does not shift to Daylight Savings Time and therefore must adjust local time 0700 CDT=0600 CST=1200 Z Surface observations are taken at least 4 times per 24 hours (U.S. takes hourly observations). Upper air takes measurements at 0000 Z and 1200 Z © AMS

Weather Systems & Weather Maps 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 2 Types High Pressure Systems or “Anticyclones” Low Pressure Systems or “Cyclones” © AMS

Weather Systems & Weather Maps A. clockwise and outward flow from a high-pressure system. B. counterclockwise and inward into a low-pressure system. © AMS

Weather Systems & Weather Maps Air Masses Huge volume of air covering hundreds of 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

Weather Systems & Weather Maps Air Masses Polar air masses are colder in winter, 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 Taking on the characteristics of the area passing over © AMS

Fronts Warm Front Cold Front © AMS

Weather Systems & Weather Maps 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 May occur 12-24 hours or longer Usually light precipitation Cold front clouds and precipitation occur over a narrow band, May occur over minutes to hours May be very heavy precipitation Wind directions are different on the two sides of a front © AMS

Weather Systems & Weather Maps Some fronts have no clouds or precipitation Passage indicated by wind shift, temperature/humidity changes In summer, temperature can be nearly the same on both sides of a cold front Difference will be humidity Fronts 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 fronts © AMS

Weather Systems & Weather Maps 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 US 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 Maximum Temperature Usually occurs in early to mid-afternoon Minimum temperature Usually occurs around sunrise Dewpoint (frost point) Temperature when air is cooled, at constant pressure, so it is saturated with water vapor; dew (frost) forms Relative humidity Percentage/ratio of actual water vapor concentration of air vs water vapor concentration if air was saturated Relative humidity changes through the day as temperature varies Generally highest around sunrise and lowest when warmest Precipitation amounts General rule: 10 in. of snow = 1 in. of precipitation © AMS

Describing the State of the Atmosphere Air Pressure Tendency (rising or falling) Falling may indicate approaching cold front Wind direction and speed Wind direction is the direction wind is blowing from Example: west wind is blowing from the west, toward the east Sky cover Fraction of the sky covered in clouds Weather watch Issued by NWS when hazardous weather is considered possible Weather warning Issued by NWS when hazardous weather is imminent or happening © AMS

Weather Satellite Imagery Geostationary High orbits 36,000 km (22,300 miles) high Orbits planet at same rate as Earth’s rotation, in same direction Currently, 2 provide a view of North America and adjacent oceans, to latitudes of ~60° Positioned over equator at 75° W and 135° W longitude © AMS

Weather Satellite Imagery Polar orbiting Low orbits 800-1000 km (~500-600 mi) high Provides overlapping north-south strips of images Passes over the same point twice every 24 hrs © AMS

Weather Satellite Imagery Visible Satellite Image 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 Satellite Image Available anytime, not just during daylight Provides temperature comparison of features within image Higher cloud tops appear whiter, because they are colder © AMS

Weather Satellite Imagery Water vapor satellite images 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

Weather Radar Composite of infrared satellite image and radar echoes. © AMS

Sky Watching Clouds Aggregates of tiny water droplets, ice crystals or both High thin cirrus clouds appear fibrous because they are composed of tiny ice crystals. Relatively low clouds composed of tiny water droplets with more sharply defined edges. © AMS

Sky Watching Fog Cloud forms: Cloud in contact with the ground Cloud forms: Stratiform clouds Sheet-like clouds formed in horizontal layers Form where air ascends gradually over a broad region Cumuliform clouds Puffy, like cotton balls Form with a more vigorous ascent of air over smaller area Under the right conditions can build vertically into a cumulonimbus (thunderstorm) cloud High, wispy feather-like clouds in western sky First sign of approaching warm front © AMS

Sky Watching Cumulus clouds Fair weather Most common during warmest time of day Vaporize after sunset © AMS

Sky Watching Clouds of vertical development Merging and vertically-growing cumulus clouds Cumulonimbus clouds Nimbo, nimbus prefix or suffix = rain producing Produce lightning, heavy rain, hail, strong, gusty surface winds © AMS

Sky Watching Clouds move in different directions at different altitudes Indicates horizontal wind shifts with altitude © AMS