Define tertiary circulation and describe how tertiary circulations affect local weather and wind direction and speed.

TERTIARY CIRCULATION 

Tertiary (third order) circulations are localized circulations directly attributable to one of the following causes or a combination of them: local cooling, local heating, adjacent heating or cool-ing, and induction (dynamics). 

Many regions have local weather phenomena caused by temperature differences between land and water surfaces or by local topographical features. These weather phenomena show up as circulations. These tertiary circulations can result in dramatic local weather conditions and wind flows. The most common tertiary circulations are discussed in this lesson. However, there are numerous other circulations and related phenomena in existence around the world.

Learning Objective: Define tertiary circula-tion and describe how tertiary circulations affect local weather and wind direction and speed.

The term monsoon is of Arabic origin and means season. The monsoon wind is a seasonal wind that blows from continental interiors (or large land areas) to the ocean in the winter; they blow in the opposite direction during the summer. The monsoon wind is most pronounced over India, although there are other regions with noticeable monsoon winds.

Monsoon winds are a result of unequal heating and cooling of land and water surfaces. During winter a massive area of cold high pressure develops over the extensive Asiatic continent. This high pressure is due primarily to cold arctic air and long-term radiational cooling. To the south, the warm equatorial waters exist and, in contrast, the area has relatively lower surface pressures. The combination of high pressure over Asia and low pressure over the Equatorial Belt sets up a pressure gradient directed from north to south. Because of the flow around the massive Siberian high, northeast winds begin to dominate the regions from India to the Philip-pines. (See fig. 3-3-1.)

During the winter months, clear skies pre-dominate over most of the region. This is caused by the mass motion of air from a high-pressure area over land to an area of lower pressure over the ocean. As the air leaves the high-pressure area over land, it is cold and dry. As it travels over land toward the ocean, there is no source of moisture to induce precipitation. The air is also traveling from a higher altitude to a lower altitude; consequently, this downslope motion causes the air to be warmed at the adiabatic lapse rate. This warming process has a still further clearing effect on the skies.

During the summer the airflow over the region is completely reversed. The large interior of Asia is heated to the point where the continent is much warmer than the ocean areas to the south. This induces relatively low pressure over Asia and higher pressure over the equatorial region. This situation produces a southwesterly flow as shown in figure 3-3-2.

The weather associated with the summer mon-soon winds is thunderstorms, almost constant heavy rain, rain showers, and gusty surface winds. This condition is caused by mass motion of air from the relatively high-pressure area over the ocean to a low-pressure area over land. When the air leaves the ocean, it is warm and moist. As the air travels over land toward the low-pressure area, it is also traveling from a lower altitude to a higher altitude. The air is lifted by a mechanical force and cooled to its condensation point by this upslope motion (pseudoadiabatic process).

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