Understanding Atmospheric Circulation and Climates
Key Meteorological Concepts
- Latitude: The distance between the Equator and a point on the planet, measured along the meridian passing through that point.
- Gale: The clash of air masses from the poles with air masses from the tropics.
- Anticyclone: An area under atmospheric pressure above normal pressure (1016 mb). Anticyclones usually produce stable weather without precipitation.
- Precipitation: Water that falls on Earth’s surface from clouds, in liquid or solid form. It is measured with a rain gauge and is represented by isohyets.
- Barometer: Measures the weight of air at a given location. It is expressed in millibars and is represented by isobars.
- Wind: The horizontal movement of air. Its speed is measured with an anemometer, and its direction with a weather vane; it is represented by arrows.
- Temperature: The degree of heat in the air. It is measured with a thermometer and is represented by isotherms.
Major Climate Types
Oceanic Climate
Located on the western facade of continents between 45º and 60º north and south latitude. Precipitation is abundant, near or exceeding 800mm per year, and is unevenly spread throughout the year. Temperatures, influenced by the ocean, are not contrasted. Winters do not drop below -3ºC, and summers are not very hot.
Mediterranean Climate
Found around the Mediterranean Sea and on the western facades of continents, between 30º and 45º north and south latitude. Precipitation is low or moderate, 300 to 800 mm per year, and is characterized by summer drought. Temperatures are warm in winter and hot in summer.
Continental Climate
Located within the Northern Hemisphere continents, between 45º and 65º north latitude. Precipitation is low or moderate, 300 to 800mm annually. Temperatures show marked contrasts between very cold winters (below -3ºC) and hot summers.
Subtropical or Chinese Climate
Located on the east side of continents between 30° and 45° north and south latitude. It is a transitional climate between tropical and temperate. Rainfall is abundant, more than 800mm per year, and concentrated in summer. Temperatures are mild in winter and hot in summer.
Atmospheric Circulation
Atmospheric circulation is the displacement of air from place to place on Earth. The root cause of this shift is the presence of large contrasts in temperature between hot and cold zones. Temperature differences between latitudes lead to unequal pressure, which causes the movement of wind through the ups and downs of pressure. Thanks to this mechanism, there is heat exchange between the different parts of the Earth, and the planet’s temperature equilibrium is maintained.
At the Equator, heat causes low pressure because hot air is lighter and ascends. It is directed, through the upper layers of the atmosphere, to the tropics of each hemisphere until, at about 30 degrees, it cools and descends, producing high pressure in the subtropical zone. On the surface, winds from the east, called trade winds, return to the Equator from these subtropical high-pressure zones. Winds from the west, called westerlies, are directed towards the temperate zone.
At the poles, cold high pressure occurs as cold air descends. The cold air then leads over the surface to the temperate zone of each hemisphere, causing winds from the east, until, at about 60 degrees latitude, it heats and rises, creating low pressure in the subpolar area. From these areas, the air returns to the upper layers of the atmosphere into the poles. In temperate zones, westerly winds from the tropical anticyclones collide with winds from the east from the polar anticyclones. Both are separated by a surface called the polar front. Along the front, the mass of polar air presses and moves the subtropical zone, and tropical masses press and are displaced to the subpolar zone, thus allowing the exchange of heat from warm to cold areas. Moreover, along the front, precipitation leads to lower pressures, since, as it moves, the mass of cold air is introduced beneath the warm air, forcing it to climb.
This whole movement mechanism moves seasonally north or south and is complicated by the distribution of the Earth’s surface in oceans and continents, which explains the great diversity of climates on Earth.