ppl_04_e2

Energy from the Sun, often referred to as solar radiation, is of short wavelength. The Earth’s atmosphere is almost totally transparent to this incoming short-wave solar radiation. Consequently, incoming solar energy passes through the atmosphere without heating the atmosphere to any significant extent. Only when solar radiation encounters the Earth’s surface and is re-radiated into the atmosphere, is there any significant raising of the temperature of the atmosphere.

Clouds reflect some incoming solar energy out to space again. The remaining radiation continues through the atmosphere to the Earth’s surface. (See Figure 4.6.) On a cloud-free day, approximately 85% of solar radiation reaches the Earth’s surface. At the Earth’s surface, surfaces of different characteristics absorb different amounts of solar radiation. Generally, the darker the surface the more radiation will be absorbed, and the warmer that surface will become. Reflective surfaces, such as areas of ice or snow, will reflect short wave solar radiation back into the atmosphere. It is the surfaces which absorb solar radiation which have the greatest heating effect on the atmosphere.

TERRESTRIAL RADIATION.

Where the incoming short-wave solar radiation, also known as insolation, is absorbed by the Earth’s surface, the Earth’s surface heats up. Where this happens, the Earth now acts as a heat source, which continually radiates the absorbed heat energy back into the atmosphere, but now as long wave radiation.

Land, water and concrete all absorb short-wave solar radiation. Land and concrete heat up quickly and re-radiation of solar energy to the Earth’s atmosphere, from those surfaces, is the main source of atmospheric heating during the day. Water absorbs solar radiation much more slowly, and so re-radiation to the atmosphere from large water surfaces does not commence for many hours, taking place mostly at night.

Short wave

radiation from the

Sun has no

direct heating effect on the atmosphere.

C H A P T ER

The atmosphere

is not heated directly by

short-wave solar radiation, but from beneath, by long wave terrestrial radiation.

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Figure 4.6 Insolation.

Heat energy in the form of long wave radiation is much easier for the atmosphere to absorb than the short-wave solar radiation. The atmosphere, then, is heated up, not by the short-wave solar radiation which comes directly from the Sun, but, from beneath, by the long wave terrestrial radiation emitted by the Earth’s surface.

(See Figure 4.6.)

HEATING THE ATMOSPHERE.

So how does terrestrial radiation heat up the atmosphere? Well, the most important factor in atmospheric heating is the role played by two so-called greenhouse gases: water vapour and Carbon Dioxide.

As we have seen, short-wave solar radiation passes through the Earth’s atmosphere without heating up the air to any appreciable extent. But, after being absorbed by the Earth’s surface, the long-wave terrestrial radiation emitted back to the atmosphere is trapped in the atmosphere by greenhouse gases, causing the atmosphere to heat up.

The upper atmosphere is cold because it experiences very little heating from the Earth’s surface. There is very little water vapour and carbon dioxide present in the upper atmosphere, and, the greater part of the terrestrial radiation has already been absorbed by the greenhouse gases in the lower atmosphere. This atmospheric heating mechanism is known as “the greenhouse effect” because the process is similar to that which heats up the air within a typical garden greenhouse.

However, the atmosphere is heated by a number of other methods, too, as we shall now see.

C o n d u c t i o n .

Conduction is the process by which heat is transferred from one body to another through direct physical contact between the two bodies. In the case of atmospheric heating, the two bodies in contact are the Earth’s surface and the lower atmosphere. The red line on the graph in Figure 4.8 shows that, during the day, the lowest layers of the atmosphere, in direct contact with the Earth, are the warmest areas, because the atmosphere is heated from the Earth’s surface upwards. As altitude increases, the red line shows that day-time temperature decreases linearly.

Figure 4.8 During the day, the atmosphere is heated by conduction, from the surface upwards. At night, the surface cools and the lowest part of the atmosphere is also cooled, leading to a temperature inversion.

However, during the night, especially when there is no low-level cloud, as the land cools, the temperature of the air in contact with the cooling land falls more rapidly than areas higher up in the atmosphere. This night-time temperature profile arises because air is a poor conductor of heat. Consequently, while at altitudes beyond the immediate influence of the Earth’s surface, daytime temperatures are retained, the air in direct contact with the Earth’s surface is colder than the air layers immediately above it. As a result of this phenomenon, a temperature inversion arises, as shown by the lower part of the blue line in Figure 4.8.

This latter effect is most marked on clear nights in winter and late autumn, or early spring. It is on clear nights, at these times of the year, that surface frosts are experienced.

The

atmosphere is heated from

the Earth’s

surface. Most atmospheric heating takes place in the lower atmosphere where

C O2 and w a t e r arev a p o very effective absorbers of

heat. Temperature therefore, decreases with a l t i.t u d e

The very

lowest layer of the

atmosphere is warmed by conduction.

At night,

the Earth’s surface cools

more rapidly

than the air. Consequently, the air in direct contact with the surface is often colder than the air above it, leading to a

t e m p e r a t u r. e i n

Atmospheric convection is

the mechanism which permits

glider pilots to soar in the rising columns of air which they call thermals.

Different surfaces on the Earth absorb heat to different extents. For example, under the influence of the Sun’s radiation, a large area of concrete will become warmer than a green field lying next to it. The warmer, less dense air above the concrete will rise, by the process of convection, while air above the cooler green field will sink.

On hot summer days, this localised convection can be seen as a shimmering heat haze and is often made dramatically visible by the presence of large, verticallygrowing cloud masses called cumulus clouds.

Glider pilots exploit these rising columns of air, which they call thermals, to gain height in their sailplanes and, therefore, remain airborne for many hours, and cover hundreds of miles cross-country.

Figure 4.10 Advection - the horizontal transfer of heat.

C H A P T ER

Temperature will generally decrease with

height, but it is possible for temperature to remain constant with altitude. In such a case, the temperature is said to be

isothermal. Temperature may even increase with height. This phenomenon is called a temperature inversion.

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Figure 4.12 An Isotherm - temperature remains constant with height.

Under certain circumstances, such as on clear, cloudless nights in winter, temperature may increase with altitude; this phenomenon, known as a temperature inversion, was mentioned earlier in the chapter under the heading Conduction.

Figure 4.13 Inversion - temperature increasing with height.

In general, temperature

varies with time over a 24 hour period. Because of

“thermal inertia” the hottest time is 2 to 3 hours after the sun has reached its highest point, at local noon, and the coldest about 30 minutes after dawn.

Di u r n a l V a r i a t i o n .

The temperature of the air will vary greatly in a given location through a 24-hour period. This is called diurnal variation (from the Latin diurnus meaning daily.) The Sun is at its highest at noon, with the maximum incoming solar radiation occurring at that time. However, due to ‘thermal inertia’, the surface of the Earth continues to receive more incoming solar radiation than it emits as terrestrial radiation. Therefore, the actual maximum air temperature usually occurs some two to three hours after maximum solar elevation, usually at about 1500 local time. Similarly, as a result of

‘thermal inertia’, the time at which the minimum temperature is reached is around half an hour after dawn. Figure 4.14 shows diurnal variation over a twenty four hour period.

C l o u d s a n d Di u r n a l V a r i a t i o n .

Clouds have a great influence on the maximum and minimum air temperatures. Clouds reflect some incoming solar radiation during the day, thereby reducing the incoming solar radiation, which, in turn, will reduce the maximum daily temperature. But, at night, cloud acts in a very different manner. Heat energy stored inside the cloud layer, by water vapour, is radiated back to the atmosphere, especially to that part of the atmosphere lying beneath the cloud. In this way, cloud acts like a blanket around the Earth. The release of latent heat from cloud, as it forms by the process of convection, also contributes to the warming effect.

Figure 4.15 Cloud cover reduces diurnal variation.

When it is cloudy, therefore, minimum night time temperatures are higher than if skies were clear.

Cloud cover, then, will reduce the extent of diurnal variation of the temperature of the atmosphere. Note, however, that the basic form of the diurnal variation curve is almost the same, whether or not cloud is present. Figure 4.15, shows how the diurnal variation is modified by cloud cover. With cloud cover, maximum daytime temperature is lower, and the minimum temperature, occurring just after dawn, is higher.

A cloud layer

will modify the diurnal

variation. Cloud

will reduce the maximum temperature, and increase the minimum temperature.