Regional variations in Solar Radiation and Temperature

All parts of the Earth do not receive the same amount of solar radiation. There are regional, and even local variations. We can often measure it by using the concept of Temperature.

Temperature

Heat is measured in terms of temperature, which is the measurement of hotness/coldness of something/some place.

Significance of Temperature

• Temperature influences the moisture-carrying capacity of the air, measured by relative humidity. The amount of moisture in the air dictates the nature and types of cloud that will form and the amount of precipitation too.

• It also affects the rate of evaporation and condensation.

Regional variations in Incoming Insolation & Temperature

The insolation received and temperature of air at any place depends on:

• Latitude of that place
• Altitude of that place
• Distance from the Sea
• Airmass Circulation & Ocean Currents

Let’s see how these factors cause regional and local variations in insolation received and temperature.

Effect of Latitude on Insolation

Lower latitude places get comparatively more exposure to sun and more vertical sunrays. So, places near the equator have higher average temperature.

The revolution of earth on its tilted axis varies the incoming insolation at any place throughout the year. It influences:

• the length of the day;
• the angle of inclination of the sun’s rays.

These two factors cause seasons, i.e. summer, winter etc.

One other factor is the transparency of the atmosphere – clearer the atmosphere, more the sunlight that will reach the ground.

Interesting facts

• Equator receives less insolation than the tropics.

• Subtropical deserts receive the maximum insolation. It’s mainly because the cloudiness is the least here.

• Generally, at the same latitude, continents receive more insolation than the oceans.

Effect of Altitude on Temperature

Atmosphere is indirectly heated by terrestrial radiation from below and not directly by sunrays. It means that the places near the sea-level with more dense air will have higher temperature than the places at higher elevations, where air density is lower.

So, we notice that the temperature generally decreases with increasing height (we are talking about troposphere here).

The rate of decrease of temperature with height is 6.5°C per 1,000 m (called Normal Lapse Rate).

Effect of Distance from the Sea (Continentality)

Compared to land, the sea heats up slowly and also loses heat slowly. In comparison, land heats up and cools down faster. (it requires around three times more energy to raise the temperature of water by the same degree, as compared to land.)

This is because of these factors:

• Higher latent heat of water
• As water is always in motion, its absorbed heat is distributed over a greater depth and area. While on land the insolation is concentrated at the surface.
• Opaque nature of land leads to greater absorption, while transparent water absorbs heat slowly.
• Evaporation over water surface – evaporation has a cooling effect.

So, sea and places near it witness less temperature variation. Sea extends its effect onto nearby places via sea and land breezes which moderate the temperature.

Effect of Airmass Circulation, Ocean Currents and Local Winds

Places under the influence of warm airmasses experience higher temperature (and vice-versa).

Places located near the coast where warm ocean currents flow will also experience higher temperature (and vice-versa).
For example: Gulf Stream or the North Atlantic Drift keep the coastal areas of western Europe warm and the ports ice-free. Other ports located at the same latitude, but under the influence of cold currents, e.g. those in north-east Canada (washed by cold Labrador current) remain frozen for several winter months.

Local winds, e.g. Fohn, Chinook, Loo, also influence temperature a lot.

Summary

Heat received by different parts of the earth is not the same → temperatures vary → causes pressure differences in the atmosphere → Winds transfer the heat from one region to the other

We will study more about pressure differences and winds in other articles.

Global distribution of Temperature

Isotherms - lines joining places having equal temperature.

In July the isotherms generally run parallel to the latitude. But they do show deviations from this general trend in other months, with the month of January showing the most extreme deviations.

Why is the deviation of isotherms more pronounced over northern hemisphere?

The deviation from general trend is more pronounced in January than in July, especially in the northern hemisphere. It is because percentage of land surface area is more in the northern hemisphere than in the southern hemisphere. Land heats and cools faster leading to more variation in temperature.

Isotherms are comparatively more parallel to the latitudes in the southern hemisphere. It’s because here majority of the area is ocean. Effect of the ocean reduces the variation in temperature.

Why do the isotherms deviate to the north over the ocean and to the south over the continent in January?

It’s because in the northern hemisphere there is winter at this time and land is cooler than the ocean (so places on land even closer to equator have same temperature as places on ocean at much higher latitudes).

Similarly, in the southern hemisphere there is summer at this time and land is warmer than the ocean (so places on land even closer to southern pole have same temperature as places on ocean at much lower latitudes).

Inversion of Temperature

Normally, temperature decreases with increase in elevation in troposphere (called normal lapse rate).

Inversion of temperature – when the normal lapse rate is inverted. That is, temperature starts increasing with increase in elevation in troposphere.

It is generally only for a short duration.

Situations giving rise to Inversion of Temperature

• When earth is cooler than the air above – E.g. in polar areas.
  It’s possible even in other places – E.g. cloudless sky in winter night may allow the heat of the day to be radiated off during the night. So, in early morning, the earth may be cooler than the air above.

• In hills and mountains due to air drainage – cold air is heavier than warm air, and so flows down to the valleys, with warm air above it.

Effects of Inversion of Temperature

• Fog and Smog - Smoke and dust particles get collected beneath the inversion layer. It gives rise to fogs and smog in mornings (especially during winter season.)

• In hills as the cold air flows down at night, plants and crops on the hills escape from frost damages.