Question

What role does length of daylight play in the climate system? How does length of daylight vary by time of year and by latitude?

Answer 1

Among the several factors that control the climate of a region, the length of daylight plays a very significant role. The length of daylight can easily be defined as the period of time during which sunlight is received at any location on earth. This time period vary from location to location based on their longitude, latitude and even elevation of a region as well.

In most practical applications, climates are classified based on temperature, humidity, precipitation and also the boundary of natural vegetation. The length of daylight effects the mean seasonal temperature of a place. As a result the equatorial region and tropics experience hot summers with an average of 12 hours of sunlight. This is not the case everywhere. Even though the poles get six months of sunlight, the annual average temperature is always less than -10 degree Celsius and always remains covered in ice.

In middle and higher latitudes, the variation in length of daylight creates cool summers and cold, harsh winters. The ground surface thaws with the arrival of summer. Seasonal temperature differences are quite marked due to the daylight time.  

We know that the earth's axis is tilted at an angle of 23.5 degrees to the orbital plane. As a result of this we obtain summer and winter solstices and vernal and autumnal equinoxes. Overall, the equatorial regions and the tropics in both the northern and southern hemispheres receive direct rays of the sun whereas the mid-latitudes and polar regions receive slanting rays.

The sun rays fall on the equator (zero degree) perpendicularly all round the year causing it to receive 12 hours of daylight all round the year. This time span vary everywhere else with seasons.

On Summer solstice (June 21), it is the major part of the Northern hemisphere is tilted towards the sun. The sun rays fall directly on the Tropic of Cancer (23.5 degree N) and we obtain the longest period of daylight. It is summer in the northern hemisphere and vice-versa. North of the Arctic circle (66.5 degree N) the midnight sun is a natural phenomenon where the sun remains visible at local midnight. The North pole (90 degree N) experience six months of daylight.

The exact opposite scenario is observed during the Winter Solstice (December 22) when it is the maximum part of the southern hemisphere that is titled towards the sun and so it is summer in the southern hemisphere and vice-versa. On this day sun-rays fall directly on the Tropic of Capricorn (23.5 degree S) and southern hemisphere experiences the longest day. During this period the South Pole (90 degree S) experience six months daylight.

On the equinoxes (March 21 and September 23) however, equal lengths of day and night are observed on both halves of the earth.