Weather

• Atmospheric Lifting Mechanisms


  • Orographic Lifting
  • Conventional Lifting
  • Frontal Lifting
  • Convergent lifting
• Orographic Lifting


  1. Warm, moist air from the ocean surface rises and is cooled by the adiabatic process

  2. Air cools, water droplets condense, clouds form and precipitation begins

  3. Air descends, is compressed, and gets warmer

  4. Air is warmer and drier, moisture has been removed by precipitation, creating a rain shadow on the leeward slope
• Convectional Lifting


  • Dark surfaces
  • Urban heat island (cities)
  • Wildfires
• Convectional Lifting


  1. Moist air warmed at the surface expands, rises, cools to the dew point, and condensation occurs forming clouds

  2. Continued updrafts lead to thunderstorms, common on summer days in tropical interior continents, local and intense
• Air Masses


  A homogeneous body of air that has taken on the temperature and moisture characteristics of its source region
• Air Mass Formation


  Warm air forms over tropical regions, cold air forms over polar regions, wet air forms over water, dry air forms over land
• Air Mass Classification


  • Humidity: maritime (m) = moist, continental (c) = dry
  • Temperature: Arctic (A) = cold/very cold, Polar (P) = cool/cold, Tropical (T) = warm/hot, Equatorial (E), Antarctic (AA)
• Fronts are where two air masses meet, their names are determined by the moving air mass, precipitation occurs along all fronts, commonly associated with moisture gradient, temperature gradient, and wind shift
• Types of Fronts


  • Cold Front
  • Warm Front
  • Stationary Front
  • Occluded Front
• Cold Front


  • When cold air moves into an area of warmer air, causes thunderstorms and heavy rain
• Warm Front


  • When warm air moves into an area of colder air, causes long, steady rains
• Stationary Front


  • When two air masses do not move, causes the same weather for days
• Occluded Front


  • When colder air moves into a warmer area of air at an angle, the masses start to spin, can cause severe weather like tornados
• Midlatitude Cyclone


  Area of low pressure with converging and rising air, traveling cyclones and anticyclones bring changing weather systems, cyclones cause condensation and precipitation, anticyclones cause clear weather
• Midlatitude Cyclone Development


  1. Low pressure forms at surface over polar front due to divergence aloft, a 'wave' develops on the polar front, friction effects cause surface flow around low to converge, mass balance causes large-scale lifting and cloud formation

  2. Surface low is maintained due to divergence aloft exceeding convergence at surface, cold sector air pushes cold front forward, warm sector air flows up warm front, cold front overtakes warm front to form an occlusion

  3. Low starts to weaken as inflowing air 'fills up' the low pressure, low continues to weaken and clouds break up
• Midlatitude Cyclones are also known as extra-tropical or frontal cyclones, they are large, traveling, cyclonic storms up to 1000 kilometers in diameter with centers of low atmospheric pressure, they cause the extreme weather conditions in the middle latitudes
• Effects of Midlatitude Cyclones


  • Various types of precipitation including rain, freezing rain, hail and snow
  • Cause far less damage than tropical cyclones or hurricanes, although rare can have winds as strong as a weak hurricane
• Severe Weather


  • Thunderstorms
  • Tornadoes
  • Tropical Cyclones
• Atmospheric Lifting Mechanisms


  • Air Masses & Fronts
  • Midlatitude Cyclonic Systems
  • Violent Weather
• Severe Weather


  • Thunderstorms
  • Tornadoes
  • Tropical Cyclones
• Severe Weather develops in three places:

  along cold front

  orographic lifting

  warm. moist air masses.

• Thunderstorms


  Produced by cumulonimbus clouds and is accompanied by lightning and thunder. Occurs when the atmosphere becomes unstable—when a vertically displaced air parcel becomes buoyant and rises on its own.
• The intensity and duration of thunderstorms depend on the local conditions that create them.
• Thunderstorms develop in three stages

  1. Uplift & adiabatic cooling

  2. Precipitation starts, lightning, thunder

  3. Downdrafts dominate
• Thunderstorm development stages


  • Cumulus Stage
  • Mature Stage
  • Dissipation Stage
• Cumulus Stage


  Air starts to rise vertically creating updrafts that which transport water vapor to cooler upper regions in the cloud. As water condenses the clouds become larger and heavier until updrafts can no longer sustain them and they fall back to Earth as precipitation.
• Mature Stage


  Updrafts and downdrafts exist side by side. Precipitation cools the air, the new cool air sinks rapidly causing down drafts. The updrafts and down drafts form convection cells which cause winds to develop.
• Dissipation


  Once the supply of warm air is used up convection stops. The cool downdrafts spread out and eventually stop. The storm eventually dissipates or dies out.
• Lightning


  The transfer of electricity generated by the rapid rushes of air in a cloud. Clouds become charged when friction between the updrafts and downdrafts causes atoms to lose or gain electrons.
• Types of Thunderstorms

  • Multi-cell thunderstorms
  • Supercell thunderstorms
  • Single-cell thunderstorms
• Tornadoes


  • Form in severe thunderstorms, Few meters to 100s of meters diameter, Last ~seconds to 10s of minutes, Winds up to 300 mph (485 kph), Average ground speed = 45–55 mph, Average path length = 7 km
• In an average year, 1,200 tornadoes cause 70 fatalities and 1,500 injuries nationwide.
• "Tornado Alley," or the states at the highest risk of getting a tornado, include Arkansas, Iowa, Kansas, Louisiana, Minnesota, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota, and Texas.
• Tornadoes are common during Spring when greatly contrasting air masses collide to produce severe storm systems.
• There is also a distinct Seasonal March of Peak Tornado Activity
• Enhanced Fujita Scale


  • Weak, 69%
  • Strong, 29%
  • Violent, 2%
• Tornadoes


  • Small, intense spiraling vortex of rising air associated with the strong updraft of an intense thunderstorm, Estimates of wind speed run as high as 100 m/s (about 225 mph) – generally closer to 50 m/s (about 110 mph), Center of a tornado is characterized by low pressure, which is typically 10 to 15 percent lower than the surrounding air pressure, Most tornadoes rotate in a counterclockwise direction, but a few rotate the opposite way
• Tornado Development


  A. vertical circulation and significant change in wind speed and direction with height – wind shear, Strong convection can then lift portions of the vortex which results in a vertical tower of slowly rotating air, called a mesocyclone
• How does a tornado form?

  Step 1: Spinning in the lower atmosphere, Step 2: Lifted and tilted from horizontal to vertical, Step 3: Extending and Forming
• Tornado Types


  • Weak Tornadoes (88%, less than 110mph)
  • Strong Tornadoes (11%, 110-125mph)
  • Violent Tornadoes (Less than 1%, greater than 205mph)