Seasonal Distribution of Pressure, Winds, Type of Winds Part 2

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Seasonal Distribution of Pressure

The variation of pressure from place to place and from season to season over the earth plays an important role in affecting the weather and climate. While studying pressure distribution through isobar maps, the pressures of all places are reduced to sea level to avoid the effect of altitude on air pressure.

January Conditions

In January, with the southward apparent movement of the sun, the equatorial low-pressure belt shifts a little south of the mean equatorial position. Areas of lowest pressure occur in South America, southern Africa, and Australia. This is because the land tends to get hotter rapidly than water. Sub-tropical high-pressure cells are centered over the ocean in the southern hemisphere. The belt of high pressure is interrupted by the continental land masses where the temperature is much higher.

Sea level pressure in millibars

Sea Level Pressure in Millibars

Sea level pressure in millibars

In the northern hemisphere, ridges of high pressure occur in the sub-tropical latitudes over the continent. A well-developed high-pressure cell occurs in the interior parts of Eurasia. This is due to the fact that land cools more rapidly than oceans. Its temperatures are lower in winter than the surrounding seas. In the southern hemisphere, the sub-polar low-pressure belt circles the earth as and is not divided into cells, because there is virtually no landmass. In northern hemisphere two cells of low pressure namely Iceland low and Aleutian low develop over the North Atlantic and the North Pacific Oceans respectively.

July Conditions

In July, the equatorial low-pressure belt shifts a little north of the mean equatorial position because of the northward apparent movement of the Sun. All the pressure belts shift northwards in July.

Image of Sea-level Pressure in Millibars

Image of Sea-Level Pressure in Millibars

Image of Sea-level Pressure in Millibars

The Aleutian and Icelandic lows disappear from the oceans while the landmasses, which developed high pressure during winter months, have extensive low-pressure cells now. In Asia, a low pressure develops. The subtropical highs of the northern hemisphere are more developed over the Pacific and Atlantic oceans. In the southern hemisphere, the sub-polar low and the sub-tropical high-pressure belt is continuous. In the northern hemisphere, there is only a faint oceanic low.

Winds

Air attempts to balance the uneven distribution of pressure. Hence, it moves from high pressure areas to low pressure areas. Horizontal movement of air in response to difference in pressure is known as wind while vertical or nearly vertical moving air is called air current.

Pressure Gradient and Winds: The greater the difference in air pressure between the two points, the steeper is the pressure gradient and greater is the speed of the wind. The gentler the pressure gradient slower is the speed of the wind.

The Coriolis Effect and Wind: Winds do not cross the isobars at right angles as the pressure gradient directs them. They get deflected from their original paths. One of the most potent influences on wind direction is the deflection caused by the earth’s rotation on its axis. Demonstrated by Coriolis in 1844, it is known as the Coriolis effect or Coriolis force. Coriolis force tends to deflect the winds from their original direction. In northern hemisphere winds are deflected towards their right, and in the southern hemisphere towards their left This is known as Ferrell’s law. The Coriolis force is absent along the equator but increases progressively towards the poles.

Type of Winds

The winds are generalized under three categories:

Planetary Winds: Planetary winds blow from high pressure belts to low pressure belts in the same direction throughout the year. They are:

Image of Idealized Global Pattern of Surface Winds

Image of Idealized Global Pattern of Surface Winds

Image of Idealized Global Pattern of Surface Winds

The Easterlies: The winds that blow from sub-tropical high-pressure areas towards equatorial low-pressure areas called trade or easterly winds. Because of the Coriolis Effect the northern trade winds move away from the subtropical high in northeast direction. In southern hemisphere the trade winds diverge out of the sub-tropical high towards the equatorial low from the southeast direction. As the trade winds tend to blow mainly from the east, they are also known as the tropical easterlies.

The Westerlies: The winds that move poleward from the sub-tropical high pressure in the northern hemisphere are deflected to the right and thus blow from the south west. These in the southern hemisphere are deflected to the left and blow from the north-west. Thus, these winds are called westerlies.

Polar Easterlies: Polar easterlies blow from Polar Regions towards sub-polar low-pressure regions. Their direction in the northern hemisphere is from northeast to southwest and from southeast to northwest in the southern hemisphere.