ELEMENTS OF CLIMATE (II). ATMOSPHERIC PRESSURE AND WIND.

Atmospheric pressure and its factors.

Definition.

Atmospheric pressure is the weight of a column of air on a place. It is meassured with a barometer and is expressed as millibars (mb).



The average atmospheric pressure meassured at sea level it is 1.013 mb, but the atmospheric pressure changes and it is higher or lower in different areas because of two factors:

• altitude, the higher the altitude, the smaller the volume and weight of the column of air over a location becomes.

• air temperature:

1.  cold air is denser and heavier, so its tends to fall. As a consequence, its warmed and produces dry and estable weather, and zones of high pressure known as anticyclones
2. warm air is lighter and less dense, so it tends to rise, then cools and produces precipitation. As a consequence produces zones of low pressure known as depressions.

How heavy is air?

Weather maps.

A weather map is a chart that represent meteorological conditions over a specific area at a specific time. In a weather map appear the following elements:

• isobars, lines that connect all the points of equal atmospheric pressure.
• anticyclones are represented with a capital A (usually are also represented with an H, high pressure). The wind moves between the isobars from left to right in the northern hemisphere and from right to left in the southern hemisphere.
• depressions are represented with a capital B  (usually are also represented with an L, low pressure). The wind moves between the isobars from rigth to left in the northern hemisphere and from left to right in the southern hemisphere.

• fronts, areas of contact between two air massess with different characteristics. Can be:

1. hot fronts, represented with a red line with semicircles.
2. cold fronts, represented with a blue line with triangles.
3. Occluded front, represented with a red line with semicircles and triangles (is the meeting of a hot and a cold front). In this case, the front is about to dissapear.

Weather map.

Coriolis force.

Wind and its factors.

Wind is the horizontal movement of air.

Measurement: 

• its speed is measured in kilometres per hour (km/h) or in metres per second (m/s), using a anemometer.
• its direction is determined by the cardinal point of the source of the wind: east wind, north wind. Is measured using a weather vane.

Weather vane.

Wind is caused by the difference in atmospheric pressure. The wind blows from areas of high pressure towards areas of low pressure.

On Earth we are going to find:

• constants winds that maintain an equilibrium of temperatures, transporting hot air towards the cold zones and vice versa. The most important are the trade winds (in spanish: alisios) which goes from the Tropics to the Equator, the west winds and the polar winds.
• seasonal winds, such as the monsoons (in spanish: monzones) in South East Asia
• daily winds, such as marine and mountains breezes (in spanish: brisas).