II. The Atmosphere

7. Winds and wind systems on the Earth

Wind is formed by the flow of air in horizontal directions. In meteorological definitions the coming direction of the wind is called wind direction. For example, Taipei will usually experience easterly winds in winter, meaning that the wind comes from the east. In weather observation, wind direction is presented by 16 directions or by angle degree (Fig 8). Common units for wind speed are as follows: meters per second (m/s), kilometers per hour (km/hr), miles per hour (mph), nautical miles per hour or knots. The converting of these units lists as follow:

1 m/s = 3.60 km/hr = 2.24 mph = 1.94 knot

Wind bearing diagram

Fig. 8: Wind bearing diagram.

In terms of weather, the Earth has several major wind zones: the low-latitude trade wind zone, mid-latitude west wind zone and the polar east wind zone. Were the Earth a smooth and immobile sphere, extra solar heat energy absorbed at the Equator would drive the hot air up to a certain altitude and then flow to the poles. The air would lose heat on its way until it finally descended in the polar region. The falling air would push the air on the polar surface back to the equatorial region creating an endless cycle (Fig. 9).

In reality, the rotation of the Earth changes everything. The rise of air in the equatorial region reduces surface air pressure, creating an equatorial low pressure zone (Equatorial Low). In this zone the weather is humid and rainy, with feeble wind flowing in with no prevailing direction. It is also called the equatorial doldrums. The south-north flowing air over the Equator gradually flows east due to the rotation of the Earth. When this upper level air reaches 30° N or 30° S, it will be moving totally eastward. As a result, a lot of air will accumulate over the sky around 30° N and 30° S, causing the surface air pressure to rise and creating a subtropical high. A Pacific high, for example, belongs to this type of subtropical high pressure; the air in this region generally descends, therefore the moisture is low. The biggest deserts in the world are located in this region. Since the air in this region is pushed down to the surface of the land, some of it flows back towards the Equator, and under the influence of Earth rotation, gradually bear to the right and becomes a northeast wind in the Northern Hemisphere (while in the Southern Hemisphere the left bearing wind forms a southeast wind). Since such wind direction is steady, in the era of sailboats, trade boats depended on this wind to travel from Europe to the New World. Therefore, it is also known as a trade wind. Other part of descending air flowing north is again influenced by Earth rotation which causes an eastward deviation. The west wind is the dominant wind in mid-latitude regions. (Fig. 10)

Warm air from the Equator does not only descend to the ground near 30° N and 30° S, but some of it continues to flow to the polar regions, and is cooled by radiation, subsiding around the polar regions and forming the polar highs. At the same time, the descending air pushes the air across polar land surfaces flowing southward and then gradually westward, creating the polar easterlies. The eastward flowing winds from the two poles come in contact with the warmer westward flowing winds at mid-latitude areas and create the polar fronts, bringing rain, snow, storms to these areas.

The atmospheric circulation in a motionless earth Atmospheric circulation on the surface of a rotating earth

Fig. 9: The atmospheric circulation in a motionless earth.

Fig. 10: Atmospheric circulation on the surface of a rotating earth.