Brief notes on Weather and Diurnal Variation of Temperature

The cloud cover also affects the daily march of temperature. An overcast day receives a much smaller amount of incoming solar radiation resulting in much reduced day-time heating.

At night the cloud cover retards the loss of heat from the ground by nocturnal radiation. The clouds, on the other hand, re-radiate heat towards the earth.

Thus, the night temperatures are not allowed to drop to that level as they otherwise would have done. A cloudy day is, therefore, likely to have smaller daily range of temperature than a clear day.

Controlling Factor:

The following factors affect the daily range of temperature: condition of the sky, stability of air, nature of surface, wind speed, water vapour content, soil composition, and latitude.

Condition of the sky:

Cloud cover in the sky has a dominant control over the daily ranged temperature. When the day is overcast, the passage of incoming solar radiation to the ground is blocked, so that the maximum temperature for the day is reduced.

The amount of cloudiness affects the minimum temperature as well. Under the cloudless sky there is unchecked loss of heat by earth radiation, whereas the deck of clouds retards the terrestrial radiation so that the minimum, temperature tends to be higher. This results in a smaller diurnal range of temperature.

Stability:

Atmospheric stability is directly related to the fluctuations of daily range of temperature. If there is an inversion layer in the atmosphere at a small distance from the ground, then the heat radiated from the earth has to warm relatively smaller volume of air.

Therefore, nocturnal cooling is retarded and the minimum temperature is a little higher. Thus, the inversion of temperature lowers the daily range of temperature.

Nature of the surface:

Because of the differential heating of land and water, the places with marine influence, like annual ranges have smaller diurnal range of temperature.

According to Haurwitz and Austin, the highest temperatures over the oceans are recorded only half an hour past noon, while on the land areas it takes about 2 to 3 hours more to attain the maximum temperature.

On the water surface the maximum temperatures are relatively lower and the minimum temperatures higher than those on the land surfaces. Therefore the places situated far away from the oceans or shut off from the moderating influence of the oceans have higher diurnal range of temperature.

Wind speed:

Gusty winds promote mixing in the atmospheric layers. Under these conditions, the terrestrial heat is distributed in a thicker layer of air so that the maximum temperature on a windy day is certainly lower than that on a calm day.

On the other hand, mixing does not allow the layer of air adjoining the earth’s surface to become so cold by nocturnal radiation as it would have been otherwise. Thus, on a day with gusty winds the diurnal range of temperature is relatively smaller.

Water vapour content:

Humidity of air also affects the diurnal range of temperature in an effective manner. Lesser the amount of water vapour in the air, greater the amount of heat lost to space by long wave earth radiation.

On the other hand, larger amount of water vapour in the air absorbs a large percentage of radiant heat from the earth’s surface and minimises the decrease in temperature. Thus, other things remaining the same, the dry air tends to have a lower minimum temperature as compared to more humid air.

Therefore more humid the air, smaller is the diurnal range of temperature, and drier the air, larger is the diurnal range. That is why higher daily range of temperature is so common in the deserts, and relatively lower range characterises the humid regions.

Soil composition:

Heat conductivity of soil increases in direct proportion to its moisture content. In humid soil regions a fraction of insolation received at the ground is expended in evaporating the moisture. This is an additional factor that causes the diurnal range of temperature to be higher in dry regions.

Effect of latitude:

Latitude is a very important control over the diurnal range of the temperature, because the diurnal as well as annual variations of temperature are directly related to the local radiation budget.

The diurnal range is highest at the tropics over land areas, but in the equatorial regions the diurnal variation of heating and cooling exceeds the annual variation. This is partly due to small seasonal variation in the angle of the sun at the equator.

However, the diurnal range over the oceans in the equatorial region is everywhere less than the annual range, and generally nowhere exceeds .56° C.

But the land areas, on the contrary, in this very zone have higher daily range of temperature. In the vicinity of the equator, the diurnal range over land surface is much larger than the annual range which is very low indeed.

In the vertical plane, the diurnal range is highest near the ground and decreases upward.