Chapter 20: Weather Air Masses & Fronts

Air Masses a body of air in the lower troposphere that has similar characteristics temperature & humidity depends on where they form named for where they form

Characteristics Temperature Humidity depends on if air mass originates in polar, arctic, or tropical region Humidity depends on if air mass originates over a continent (continental) or ocean (maritime)

5 Types of Air Masses

#1 Continental Arctic cA extremely cold & dry originates in arctic regions- coldest temperatures NO precipitation

#2 Continental Polar cP warmer than cA, but only slight difference in temperature & humidity creates some precipitation creates lake-effect snow around Great Lakes

#3 Maritime Polar mP originates over oceans in high latitudes cold, damp air mass not as cold as cP because water is warmer than land when mP cools to its dew point fog, clouds, and precipitation is formed

#4 Maritime Tropical mT originates over warm, tropical oceans warm, moist air thunderstorms likely to develop mT

#5 Continental Tropical cT originates over deserts hot, dry can exceed 100ºF may originate as a maritime tropical but dries out over land

Fronts

Fronts boundaries between different air masses air masses do not easily mix frontal systems are the cause of weather in the mid-latitudes since this is where air masses mix bring storms and precipitation types & severity of weather depend on type of air mass and how fast air masses are moving

Four kinds of Fronts Cold front Warm front Stationary front Occluded front

#1: Cold Fronts the boundary between an advancing cold air mass and the warmer air mass it is replacing cold air is more dense so it slides under warm and pushes it up move quickly so precipitation doesn’t last long steep slopes so band of precipitation on ground isn’t wide type of weather depends on type of air it is replacing

Cold Front

#2 Warm Fronts the boundary between an advancing warm air mass and the cold air mass it is displacing warm air rises, cold air slowly retreats Slope is more gradual than a cold front upper level clouds form first, then lower level clouds weather changes are less dramatic thunderstorms uncommon, prolonged rain and flooding are very common large surface area receives precipitation

Warm Front

#3 Stationary Front front that is not moving forward caused because the warm air is not moving fast enough to push cold air away, the front stalls prolonged precipitation & flooding can occur

#4 Occluded Front cold front “catches up” to a warm front warm air is wedged above cold air weather varies, depending on the time of year– usually big storms usually the air mass becomes dried out

Occluded Front

Ch. 20: Weather Severe Weather

Thunderstorms Storms with lightning, thunder, & hail warm, moist, unstable air- cumulonimbus clouds usually occur in the afternoon or evening because the Earth has been heating up all day convection within the cumulonimbus gives the storm it’s energy form along low pressure systems and cold front boundaries

Squall Line line of severe thunderstorms that precede a cold front bring torrential rains, hail, tornadoes, and lightning

Lightning discharge of electricity from a thundercloud caused by friction can occur between clouds or cloud to ground Positive charge on top and bottom of cloud & negative charge in the middle

Tornadoes violently rotating column of air that usually touches the ground in order for a tornado to form, there must be a rotating updraft called a mesocyclone when a tornado has not yet reached the ground its called a funnel cloud

The Fujita Scale measures the intensity of tornadoes using wind speed and damage also called the F-scale

Tornado Alley area of the USA where tornadoes are likely to form; extends from northern Texas to North Dakota warming of the ground and the constant wind in this region are ideal for mesocyclones place where warm, moist air from the Gulf of Mexico meets and mixes with cooler, dry air from the west and southwest

Storm/Tornado Watches & Warnings conditions are favorable for a storm or tornado WARNINGS a severe storm or tornado has been seen and is approaching

Hurricanes large rotating storms of tropical origin that have sustained winds of at least 119 km/h last for 1+ days have low pressure centers that draw energy from warm oceans wind and rain is mild compared to outer edge of eye strongest winds and rain occur around eye wall inside the eye, winds are mild with little to no rain air rises in the eye wall

Formation of Hurricanes need warm, moist air for extended period of time June 1st – November 30th form between 5º – 20º N and S of equator Start off as tropical depressions and evolve to tropical storms Hurricanes weaken when they make landfall Steered by global wind patterns in Atlantic  form off west coast of Africa and move west towards North America

The Saffir-Simpson Scale categorizes hurricanes based on their wind speed and storm surge gives a general idea about impending damage hurricanes change categories throughout their life

Hurricane Damage A storm surge is when strong eye wall winds blow ocean water into a dome that makes landfall storm surges can raise the sea level several meters higher than normal

Hurricane Watches & Warnings A Hurricane will arrive in 24-36 hours WARNING A hurricane will arrive in less than 24 hours

Winter Storms Snowstorms are storms with steady snowfall Blizzards are winter storms with heavy snow, winds, and low temperatures 3 criteria must be met for blizzards: Winds over 56 kph Temperatures -7oC or lower Reduction of visibility as a result of falling or blowing snow

Forecasting Weather Meteorologists collect data to create weather maps & station models Use: Satellite images Radiosondes: instruments attached to balloons Weather stations Weather radar Airplanes Ships Information is organized into a station model to be put on a map

Station Models include temperature, dew point, cloud coverage, barometric pressure, wind speed and direction international symbols so meteorologists around the world can read them

Station Models

Station Model Symbols

Forecasting Weather Weather forecasts are based on computer models Computers organize the data from many weather stations Meteorologists draw in the fronts based on temperature, dew point, and wind direction Computers use mathematical equations to make forecasts over the next day to few days Meteorologists edit forecasts based on previous weather patterns for an area Urbanization is the tendency for a city to be warmer than the surrounding area based on the fact that buildings hold heat better than vegetation

Weather Map