Chapter 20: Weather

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

Temperature  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)

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

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

Maritime Polar  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 mP #3

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

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

 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

1. Cold front 2. Warm front 3. Stationary front 4. Occluded front

 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  moves 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

 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

 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

 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

Ch. 20: Weather

 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  forms along low pressure systems and cold front boundaries

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

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

 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

 measures the intensity of tornadoes using wind speed and damage  also called the F-scale

 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

WATCHES  conditions are favorable for a storm or tornado WARNINGS  a severe storm or tornado has been seen and is approaching

 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 in the eye 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

 need warm, moist air for extended period of time  June 1 st – November 30 th  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

 categorizes hurricanes based on their wind speed and storm surge  gives a general idea about impending damage  hurricanes change categories throughout their life

 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

WATCH  A Hurricane will arrive in hours WARNING  A hurricane will arrive in less than 24 hours

 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 -7 o C or lower  Reduction of visibility as a result of falling or blowing snow

 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

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

 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