Weather Studies Introduction to Atmospheric Science American Meteorological

Weather Studies Introduction to Atmospheric Science American Meteorological Society Chapter 1 Monitoring The Weather Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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 The Blizzard of 1888 § § § Satellites Radiosondes Surface observations 2

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 3

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 4

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 5

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 6 System (ASOS)

Pressure Systems 2 Types High Pressure Systems, or “Anticyclones” Low Pressure Systems, or “Cyclones” 7

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 8

Pressure Systems, Continued High and Low Pressure Centers Arrows indicate surface horizontal winds 9

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

Fronts Warm Front Cold Front © American Meteorological Society 11

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 12

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 13 and warm front

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 14

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 15

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 16

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 17

Orbit of a Geostationary Satellite 18

Orbit of a Polar. Orbiting Satellite 19

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 20

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 21

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 22

Weather Radar § Complements satellite surveillance § Doppler radar detects movement – Excellent tool to forecast tornadoes 23

Combination of Images § Composite of IR and Radar 24

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 25 warm front

Cloud Forms § These high thin cirrus clouds appear fibrous because they are composed of mostly tiny ice crystals 26

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

Cloud Forms § Fair weather cumulus clouds are most common during the warmest time of day and then vaporize after sunset 28

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 29

Cloud Forms § Clouds may move in different directions at different altitudes – Indicates horizontal wind shifts with altitude 30

Essay: Weather Maps, Historical Perspective U. S. Army Signal Corps weather map dated 1 September 1872