Atmospheric Temperature, Pressure and The Wind

Insolation is the solar radiation that reaches the earth’s surface. It is measured by the amount of solar energy received per square centimetre per minute. Insolation affects the temperature at any place on earth. The more the Insolation at a place, the higher is the temperature there. In any given day, the strongest insolation is received at noon.

Factors affecting insolation are Angle of the sun, Distance between the sun and the earth, Duration of daylight and atmospheric conditions preventing sun’s rays from reaching the earth.

Received radiation is unevenly distributed because the Sun heats equatorial regions more than polar regions. Energy is absorbed by the atmosphere, hydrosphere, and lithosphere, and the heat energy is redistributed through evaporation of surface water, convection, rainfall, winds, and ocean circulation. When the incoming solar energy is equal to flow of heat to space, global temperatures will be stable.

To quantify Earth’s heat budget or heat balance, let the insolation received at the top of the atmosphere be 100 units. Called the albedo of Earth, around 35 units are reflected back to space: 27 from the top of clouds, 2 from snow and ice-covered areas, and 6 by other parts of the atmosphere. The 65 remaining units are absorbed: 14 within the atmosphere and 51 by the Earth’s surface. These 51 units are radiated to space in the form of terrestrial radiation: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of condensation, 9 via convection and turbulence, and 6 directly absorbed). The 48 units absorbed by the atmosphere (34 units from terrestrial radiation and 14 from insolation) are finally radiated back to space. These 65 units (17 from the ground and 48 from the atmosphere) balance the 65 units absorbed from the sun; thereby demonstrating no net gain of energy by the Earth.

The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight but are also transparent to outgoing thermal (infrared) radiation. However, water vapour, carbon dioxide, methane and other trace gases are opaque to many wavelengths of thermal radiation. The Earth’s surface radiates the net equivalent of 17 percent of the incoming solar energy in the form of thermal infrared. However, the amount that directly escapes to space is only about 12 percent of incoming solar energy. The remaining fraction, 5 to 6 percent, is absorbed by the atmosphere by greenhouse gas molecules. This absorption of heat energy by greenhouse gases leads to increase of Earth’s average temperature. This is why it is important to ensure that the proportion of gases in the atmosphere is constant to prevent an increase in Earth’s temperature beyond inhabitable levels.

We already know that vertical rays of the sun are warmer than slanting rays of the sun. So, the region around the equator will be comparatively warmer than other regions while polar regions will be comparatively colder. Because of this differential heating, the five main latitude regions of the Earth’s surface comprise geographical zones, divided by the major circles of latitude.

The Torrid or Tropical Zone is also known as the Tropics. The zone is bounded on the north by the Tropic of Cancer and on the south by the Tropic of Capricorn; these latitudes mark the northern and southern extremes of regions in which the sun seasonally passes directly overhead. At those two latitudes this happens once a year, but in the region between them, the sun passes overhead twice a year.

In the Northern Hemisphere, in the sun’s apparent northward migration after the March equinox, it passes overhead once, then after the June solstice, at which time it reaches the Tropic of Cancer, it passes over again on its apparent southward journey. After the September equinox, the sun passes into the Southern Hemisphere. It then passes similarly over the southern tropical regions until it reaches the Tropic of Capricorn at the December solstice, and back again as it returns northwards to the Equator.

In the two Temperate Zones, the Sun is never directly overhead, and the climate is mild, generally ranging from warm to cool. The four annual seasons, spring, summer, autumn and winter, occur in these areas. The North Temperate Zone includes Europe, Northern Asia, and North and Central America. The South Temperate Zone includes Southern Australasia, southern South America, and Southern Africa.

The two Frigid Zones, or polar regions, experience the midnight sun and the polar night for part of the year – at the edge of the zone there is one day in the winter when the Sun is invisible, and one day at the summer solstice when the sun remains above the horizon for 24 hours, while in the center of the zone (the pole), the day is literally one year long, with six months of daylight and six months of night. The Frigid Zones are the coldest parts of the earth and is generally covered with ice and snow.

Atmospheric pressure is a force in an area pushed against a surface by the weight of air in Earth’s atmosphere. The earth is covered in a layer of air. However, this layer is not distributed evenly around the globe. At different times, the layer of air is thicker in some places than in others. Where the layer of air is thicker, there is more air. Since there is more air, there is a higher pressure in that spot. Where the layer of air is thinner, there is a lower atmospheric pressure.

Atmospheric temperature is a measure of temperature at different levels of the Earth’s atmosphere. It is governed by many factors, including incoming solar radiation, humidity and altitude. We have already seen the temperature at different latitudes and heights. When the temperature is high the air expands reducing the pressure at that area. Hence, the geographic zones are not only temperature zones, but also pressure zones with tropics which is heated to maximum having a low pressure compared to poles which have a high pressure because of very low temperatures.

The Wind is the movement of air. Short bursts of fast winds are called gusts. Strong winds that go on for about one minute are called squalls. Winds that go on for a long time are called many different things, such as breeze, gale, hurricane, and typhoon. Sunlight and differential heating causes the Earth’s atmospheric circulation. The resulting winds blow over land and sea, producing weather. The wind is named after the direction from which it flows. For example, a wind blowing from the east is called easterly.

If there is a high-pressure system near a low-pressure system, the air will move from the high pressure to the low pressure to try and even out the pressures. A big difference in pressure can make high winds. In some storms, such as hurricanes, typhoons, cyclones, or tornadoes, the pressure differences can cause winds faster than 320 kilometres per hour.

The Wind can also be caused by the rising of hot air or the falling of cool air. When hot air rises, it creates a low pressure underneath it, and air moves in to equalise the pressure. When cold air drops (because it is denser or heavier than warm air), it creates a high pressure and flows out to even out the pressure with the low-pressure around it.
Atmospheric circulation is the large-scale movement of air through the troposphere and the means (with ocean circulation) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth’s rotation rate and the difference in solar radiation between the equator and poles.

The Earth’s weather is a consequence of its illumination by the Sun and the laws of thermodynamics. The atmospheric circulation can be viewed, from that standpoint, as a heat engine driven by the Sun’s energy, and whose energy sink, ultimately, is the blackness of space. The work produced by that engine causes the motion of the masses of air and in that process, it redistributes the energy absorbed by the Earth’s surface near the tropics to space and incidentally to the latitudes nearer the poles.

In tropics, air is warmed by the Earth’s surface, decreases in density and rises. The rising air creates a low-pressure zone near the equator. Air mass rising on all sides of the equator forces those rising air masses to move poleward. As the air moves poleward, it cools, becomes denser, and descends at about 30th parallel, creating a high-pressure area. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop. This atmospheric circulation pattern that George Hadley described was an attempt to explain the trade winds and is called Hadley Cell.

The Coriolis effect is a force that is found in a rotating object. Gaspard-Gustave de Coriolis first described the Coriolis effect in 1835. The Coriolis effect can best be seen in hurricanes. In the northern hemisphere or part of the earth, they spin clockwise, in the southern hemisphere they spin the other way. This happens because the earth spins on its tilt.
One example of the Coriolis effect that is the winds in northern hemisphere tilts rightward from their direction of motion and the winds in southern hemisphere tilt leftwards.

The poleward movement of the air in the upper part of the troposphere deviates toward the east due to the Coriolis force. At the ground level, however, the movement of the air toward the equator in the lower troposphere deviates toward the west producing a wind from the east. The winds that flow to the west (from the east, easterly wind) at the ground level in the Hadley cell, are called the Trade Winds.

Though the Hadley cell is described as located at the equator, in the northern hemisphere, it shifts to higher latitudes June and July and toward lower latitudes December and January as it is caused by the Sun’s heating of the surface. The zone where the greatest heating takes place is called the “thermal equator” or Inter-Tropical Convergence Zone(ITCZ). As the southern hemisphere summer is December to March, the movement of the thermal equator to higher southern latitudes takes place then.

The Polar cell, likewise, is a simple system. Though cool and dry relative to equatorial air, the air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop. At the 60th parallel, the air rises to the tropopause (about 8 km at this latitude) and moves poleward. As it does so, the upper-level air mass deviates toward the east. When the air reaches the polar areas, it has cooled and is considerably denser than the underlying air. It descends, creating a cold, dry high-pressure area. At the polar surface level, the mass of air is driven toward the 60th parallel, replacing the air that rose there, and the Polar circulation cell is complete. As the air at the surface moves equatorward, it deviates toward the west. Again, the deviations of the air masses is due to conservation of energy, which is also referred to as the Coriolis effect. The air flows at the surface are called the Polar easterlies (easterly from the east).

Part of the air rising at 60° latitude diverges at high altitude toward the poles and creates the polar cell. The rest moves toward the equator where it collides at 30° latitude with the high level air of the Hadley cell. There it subsides and strengthens the high pressure ridges beneath. A large part of the energy that drives the Ferrel cell is provided by the Polar and Hadley cells circulating on either side and that drag the Ferrel cell with it. The Ferrel cell, theorised by William Ferrel (1817–1891), is, therefore, a secondary circulation feature, whose existence depends upon the Hadley and Polar cells on either side of it; it behaves much as an atmospheric ball bearing between the two.

Apart from these planetary winds, there are local winds due to relative heating within the same locality. This is called longitudinal circulation as they are not dependent on parallels, but can be within same latitudinal areas. Longitudinal circulation is a result of the heat capacity of water, it’s absorptivity, and it’s mixing. Water absorbs more heat than does the land, but its temperature does not rise as greatly as does the land. As a result, temperature variations on land are greater than on water. The Hadley, Ferrel, and Polar cells operate at the largest scale of thousands of kilometres (synoptic scale). But, even at mesoscales (a horizontal range of 5 to several hundred kilometres), this effect is noticeable. During the day, air warmed by the relatively hotter land rises, and as it does so it draws a cool breeze from the sea that replaces the risen air. At night, the relatively warmer water and cooler land reverse the process, and a breeze from the land, of air cooled by the land, is carried offshore by night. This described effect is daily (diurnal). They are also called sea breeze and land breeze.

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