What do you mean by Lapse Rates?

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The observed rate of vertical decrease in temperature (also called vertical temperature gradient) is called the ‘lapse rate.’ Lapse rate is not constant, but varies with height, location or season.

The lapse rate at a given place and time can be obtained only by actual observations. It should be noted that the lapse rate indicates the temperature conditions that are found in a stationary column of air characterized by the absence of any vertical motion.

The upward decrease in temperature of the air continues only up to the base of the tropopause beyond which it stops. In the lower layers of tropopause the lapse rate may be very high on clear, sunny days.

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On occasions, the rate of decrease of temperature may even exceed the adiabatic rate. In the tropical regions where insolation is very intense, the lapse rate is generally super-adiabatic up to about 160 meters on most afternoons especially in dry summer season.

Contrary to it, clear and calm nights during winter characterized by rapid nocturnal cooling produce a vertical temperature gradient that may be less than the adiabatic.

It is a diagrammatic representation of different patterns of lapse rates (also called actual lapse rate or environmental lapse rate). Lapse rates may be low or steep depending on the atmospheric conditions. Of course, the actual lapse rate at a particular place may be entirely different at different times.

The fact that temperature in the lower layers of troposphere shows an upward decrease goes to prove that the direct source of atmospheric heat lies at the earth’s surface.

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Thus, as the distance from the direct source of energy (i.e. the earth’s surface) increases, the air naturally becomes progressively cooler, though only up to a certain height.

It is noteworthy that heating of the lower layers of air is not caused because of nearness to the earth’s surface alone, but there are other factors as well. The air close to the earth’s surface is denser than the upper air and contains a larger quantity of water vapour, dust particles and water droplets.

On the contrary, air in the upper strata of the atmosphere is rarefied, dry, and there are little dust particles. Therefore, because of the lesser amount of water vapour and carbon dioxide the upper air does not absorb as much heat received from terrestrial radiation as is done by the lower air.

Besides, the upper air being more transparent to incoming short­wave solar radiation, despite the intensity of solar rays, its temperature is always relatively lower.

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Even on mountain slopes exposed to the sun there is a large difference in the temperature of ground and that of free air. In the same way, on high plateaus there is a large difference in temperature taken in the shade and the sun.

The conditions of atmosphere and the differences in elevations or local relief features also affect the vertical temperature gradient. If a valley in mountainous region is filled with cold and dense air or there is an advection of cold air in the upper part of atmosphere, the vertical lapse rate is likely to become lower.

There is a smaller vertical temperature gradient, if the layer of air near the surface gets colder because of its contact with the chilled surface of the earth.

On the contrary, if the surface is intensely heated during daytime, the air lying close to it is also healing by the processes of heat transfer.

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Under these conditions, the lapse rate becomes steeper, as often occurs on most afternoons during the dry season. Thus, it is clear that sometimes the actual lapse rate is larger than the normal lapse rate, and at times it is smaller than that.

In general, the actual lapse rate or the environmental lapse rate is always different from the normal lapse rate.’ It may also be noted that the temperature variation with altitude is many times greater than latitudinal variation.

Continents and oceans not only influence the horizontal distribution of temperature, but they also affect its vertical distribution. In summer, the vertical temperature gradient is steeper over the continents, while in winter it is steeper over the oceans.

There are certain levels in the atmosphere, where under certain conditions, the normal condition of a decrease in temperature with elevation is reversed, and temperatures increase with altitude on a temporary and local basis.

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Since in these conditions the cold air is overlaid by the warmer air, the normal lapse rate is reversed. That is why this phenomenon is known as a temperature inversion.

In winter, in Polar Regions the layers of air close to the surface become so cold that up to a certain height the temperature increases with elevation. During the colder part of the year, surface inversion in that region is a common phenomenon.

Outside the polar region, inversion of temperature over continents is of common occurrence during winter. But on the oceans, inversion of temperature frequently occurs in summer. However, these inversions are confined only to the lower part of the troposphere.

Austin Miller holds the view that the lapse rate is lower in winter than in summer; it is lower at night than during daytime. Similarly, the plateaus and mountains exhibit varying lapse rates in the air above them.

Air over the plateaus has definitely a lower lapse rate than that over the mountains. As an exception to this rule, in winter the lapse rate in eastern Brazil is 8.5″C per kilometer, while in summer it is only 3.7° C per kilometer.

Another salient feature of the lapse rate is that in tropical regions the decrease in temperature with elevation continues up to a height of 16 to 18 kilometers in the troposphere.

In this zone the temperature at the outer boundary of the troposphere is reduced to -80° C, but in the polar region the lapse rate continues upto 6 kilometers only. In winter, this height is further reduced.

Beyond latitude 60″N and S, the height of tropopause is 10 kilometers in summer and 9 kilometers in winter. The height of tropopause in higher latitudes is relatively less.

That is why the temperature in equatorial tropopause is less than that in middle latitude regions. Thus, at the same height from the Earth’s surface temperature in the stratosphere increases from the equator pole-ward.

During the summer months, this in­crease in stratospheric temperature continues up to the poles, but in winter, the sun being invisible in Polar Regions, the temperature begins to decrease beyond latitude 60° north and south to the poles.

In winter, the stratosphere is warmest between 50″ and 60° of latitudes. Besides latitude, continents and oceans also have influence on this high stratospheric temperature.

The fact that the lapse rate suddenly drops to zero at the outer boundary of the troposphere indicates that convectional currents rise up to this level only. Salient Features

The following are some of the important features of vertical distribution of temperature.

(1) The normal lapse rate is uniform at all levels in the troposphere in all latitudes.

(2) The normal lapse rate abruptly drops to zero at the upper boundary of the tropopause.

(3) In the lower part of the stratosphere temperature at all levels is the same and there is little change in it.

(4) In the stratosphere the temperatures gradually increase from the equator towards the poles at each and every level. This is mainly due to the fact that the stratosphere is lower at the poles.

An accurate knowledge of the vertical distribution of temperature in different layers of the atmosphere is of great help in understanding the exchange of heat by radiation between the earth’s surface and the atmosphere.

Therefore for a correct appraisal of the different weather processes going on in the atmosphere, a comprehensive knowledge of the vertical distribution of temperature as well as humidity, at lease in the troposphere, becomes all the more necessary.

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