Air Masses and Their Impact on Weather

Air Masses

• Large bodies of air that can be 1600 kilometers (1000 miles) or more across
• Air masses acquire properties from their source region
• Air masses are classified by their source region and the characteristics of the surface in the source region
• Polar (P) air masses form in high latitudes and are cold
• Tropical (T) air masses form in low latitudes and are warm
• Continental (c) air masses form over land and are likely to be dry
• Maritime (m) air masses form over water and are humid
• The four basic types of air masses are Continental Polar (cP), Continental Tropical (cT), Maritime Polar (mP), and Maritime Tropical (mT)
• In North America, cP and mT air masses have a particularly important influence east of the Rocky Mountains

Air Masses and Weather in North America

• Continental Polar (cP) air masses are responsible for lake-effect snows
  • cP air mass crosses the Great Lakes picking up moisture from the lakes
  • Snow occurs on the leeward shores of the Great Lakes
• Continental Polar air masses from Northern Canada and interior of Alaska can bring cold, dry air in the winter and cool relief in the summer
• Maritime Tropical (mT) air masses from the Gulf of Mexico and the Atlantic Ocean bring warm, moist, unstable air, which leads to precipitation in the eastern United States
• Continental Tropical (cT) air masses form over the Southwest and Mexico and are hot and dry
• Maritime Polar (mP) air masses bring precipitation to the western mountains and can influence the northeastern United States, causing "Nor'easters" in New England with cold temperatures and snow

Fronts

• Boundaries that separate air masses of different densities
• Fronts force warmer, less dense air aloft and form a wedge of cooler, denser air
• Warm fronts are shown on maps with a red line with semicircles and are characterized by a gradual slope, lower clouds as the front nears, a slow rate of advance, and light-to-moderate precipitation
• Cold fronts are shown on maps with a line with triangles and are characterized by a steep slope, faster advance than warm fronts, and more violent weather than warm fronts
• Stationary fronts have nearly parallel airflow on both sides of the front and remain relatively stationary
• Occluded fronts occur when an active cold front overtakes a warm front, forcing the warm air upward and creating complex weather conditions with precipitation

Thunderstorms

• Occur when cumulonimbus clouds produce heavy rainfall, lightning, and sometimes hail
• Approximately 2000 thunderstorms are in progress at any one time, and 100,000 occur annually in the United States
• Occur frequently in Florida and the eastern Gulf Coast region
• Thunderstorms require warm, moist, and unstable air (lifting)
  • High surface temperatures are common in the afternoon and early evening
• Thunderstorms require a continuous supply of warm air and moisture, which leads to updraft and downdraft formation
  • Heavy precipitation indicates the most active stage of the storm
  • The cooling effect of precipitation marks the end of thunderstorm activity

Hurricanes

• Winds exceeding 119 kilometers (74 miles) per hour with rotary cyclonic circulation
• Form between 5 degrees and 20 degrees latitude
• Are also known as typhoons (western Pacific) and cyclones (Indian Ocean).
• The North Pacific has the highest number of hurricanes per year
• The hurricane's eye wall is characterized by rising air, intense convective activity, a wall of cumulonimbus clouds, highest wind speeds, and heaviest rainfall.
• The eye of a hurricane is the calm center with a diameter of about 20 kilometers (12.5 miles)
• Hurricanes can have winds reaching 300 km/hr and generate 50 foot waves at sea

Hurricane Formation and Decay

• Hurricanes can form in all tropical waters except the South Atlantic and Eastern South Pacific
• Hurricanes obtain energy from condensing water vapor
• Develop most frequently in late summer when warm water temperatures provide energy and moisture
• The initial stage isn't well understood
• Tropical depressions have wind speeds under 61 kilometers (38 miles) per hour
• Tropical storms have wind speeds between 61 and 119 kilometers (38 and 74 miles) per hour, at which point they are given names
• Hurricanes diminish in intensity when they move over cooler ocean water, move onto land, or when the large-scale flow aloft is unfavorable

Hurricane Damage

• Factors that influence hurricane damage include:
  • Hurricane strength (most important factor)
  • Size and population density of the affected area
  • Shape of the ocean bottom near the shore
• The Saffir-Simpson scale ranks the relative intensities of hurricanes
• Hurricane damage includes:
  • Storm surge: a large dome of water that sweeps across the coast where the eye makes landfall, spanning 65 to 80 kilometers (40 to 50 miles) wide
  • High winds, heavy rain, and flooding

Explore the characteristics and classifications of air masses, including Polar and Tropical types. Learn how these air masses affect weather patterns, specifically in North America, such as lake-effect snow. This quiz covers essential concepts related to meteorology and climate.