air masses


  • Warm air expands (density decreases) and rises up leaving an area of low pressure.
  • When air goes up through upper layers of the atmosphere cools down (se enfria). 
  • Cold air contracts going down into the atmosphere, generating a high-pressure area.
  • From high pressure area, air flows near Earth’s surface to low pressures, warming up. 
  • The circuit which air flows continuously: convective cell.
  • If Earth were motionless, air circulation would follow convective cells that would extend from the equator to each one of the two poles.
  • Coriolis effect is the effect that the rotation of Earth has on the movement of air


  • There are 3 convective cells in each hemisphere.

Air mass are defined by their temperature and humidity.

  • The temperature measures how warm or cold the air mass is.
  • The absolute humidity is the amount of water vapor in a volume of air.
  • The lower the temp. of an air mass, the smaller the amount of water vapor it can have.


  • When two air masses with different characteristics move into one another, phenomena such as cloud formation and precipitation happen. 
  • front is the boundary between two masses of different temperature and humidity.

atmosphere has gases (nitrogen, carbon dioxide, oxygen, ozone) and suspended particles such as water.

found in the trotoshphere that occur the weather phenomena.

WARM AIR: rises and generates and low-pressure areas associated with cyclones.
COLD AIR: drop and generates high-pressure areas associated with anticyclones.

warm front: forms when a warm air mass overtakes a cooler air mass.

occuluded front: forms when a warm front is overtaken by a rapidly moving cold front.

stationary front: forms when a warm air mass and a cool mass push against one another but NO replace
cold front: forms when a cold air mass advances into a warmer air mass.

isobars: are lines shown on a weather map joining places of equal atmospheric pressure.

H: high pressure anticyclone. air descending. fine weather
L: low pressure cyclone air rises. bad weather 

  • Differences in density and pressure in the troposphere cause air circulation.
  • Vertical motion: (happens bc different temperatures).
  • warmer air masses rise bc they are less dense that the surrounding air.
  • Colder air masses fall bc they are densest that the surrounding air.
  • Horizontal motion: 
  • When air rises, it leaves behind a low-pressure area. When air falls, it generates a high-pressure area.
  • The horizontal motion of air is called wind


  • cyclone is a spiraling wind that forms around centers of low pressure. bad weather
  • An anticyclone is a spiraling wind that forms around centers of high pressure. fair weather


  • Air movements depend on pressure and temperature.
  • An air mass is a large body of air that is in direct contact with earth’s surface.

Classification of the main air masses on Earth

  • Humidity: the maritime air masses that form over the oceans are more humid that the continental air masses.
  • Temperature: the distance of an air mass from the poles causes its temperature.  
  • Precipitation is water, in liquid or solid state that falls from clouds to Earth’s surface.
  • Types of precipitation depends on the atmospheric conditions of the location.

    • Are classified by them form and altitude, they are 4 categories.
    • Nimbus: grey dense clouds linked to precipitation and storms. Altitude 4 
    • Stratus: uniform horizontal clouds that stretch. NOT linked to precipitation, they can produce light rain. Altitude 2.
    • Cumulus: rounded vertical clouds with a cottonlike shape. Linked to good weather. Altitude 5. 
    • Cirrus: white clouds. Formed at high altitudes. Altitude 12.

4 types of fronts:

  • COLD FRONTS: mass of cold air pushes a warm air mass, cold front arises. Vapor of warm air mass condenses. Produce heavy precipitation, rain or snow.
  • WARM FRONTS: mass of warm air pushes a cold air mass, cold front arises. warm air expands and cools as it rises. Vapors condense into horizontal clouds that produce light precipitation.
  • OCCLUDED FRONTS: occur when an active cold front overtakes a slower warm front. Produce a wide variety of weather conditions.
  • STATIONARY FRONTS: occur when a warm air mass and a cold air mass move into one another in such a way that neither mass can overtake the other. Cause persistent precipitation.


  • Clouds move along with the air masses in the atmosphere.
  • They form through the condensation of water vapor in air.

For clouds to form they may need:

  • The air must contain suspended particles on which the droplets can condense.
  • The air must rise up to the troposphere, cool down and reach a relative humidity of 100%.
  • The lower temperature of the air, the lower amount of vapor it can contain. The excess vapor condenses and forms clouds.