The diagram shows that the amount of Insolation received by the Earth’s surface does indeed vary.  At the Equator there is a year round gain of Insolation and this region gains the most Insolation of all of the locations on the globe.  The amount of Insolation at the Equator also varies little throughout the year. As you progress north and south of the Equator the amount of Insolation received by the Earth’s surface varies seasonally and decreases in quantity towards the Poles.  The Northern Hemisphere receives its maximum amount of Insolation between March and September, and for the Southern Hemisphere it is between September and March. There are times of the year when Polar Regions receive no insolation at all.

The principle reason for these patterns is the way in which the Earth travels around the sun each year.  This is shown in the images below.

The Earth is tilted at an angle of 23.5° relative to the sun and this has a marked impact upon the amount of Insolation received by different places and at different times of the year.  On December the 21^st (My birthday!) the North Pole is tilted away from the sun, and the maximum overhead sun is located at the Tropic of Capricorn.  Hence the North Pole receives no insolation at this time and the Tropic of Capricorn its maximum amount.  3 months later, the Earth has moved into a position where the maximum overhead sun is over the Equator, which receives the most insolation.  The Northern and Southern hemispheres are in spring and autumn respectively.  On June the 21^st, the Earth has continued its journey so that the maximum overhead sun is over the Tropic of Cancer (23.5° N), and the Northern Hemisphere experiences its maximum amount of insolation received and summer. This accounts for the seasonality of Insolation, but what about the total amount received by different latitudes?

       The total amount of insolation received by the latitudes of the Earth does not vary a great deal, but the curvature of the Earth has a vital role to play in determining how much Insolation is received by different places.   The image below shows this.

As you can see above, because of the curve of the Earth a sunbeam of Insolation hitting the Earth at higher latitudes has to spread out over a larger surface area than one reaching the Equator.  Thus lowering the amount of Insolation per km² in more Northerly and Southerly latitudes.  In addition, because the insolation hits the atmosphere at a greater angle in more northerly and southerly latitudes it has more atmosphere to pass through (despite the lower atmosphere being thicker at the Equator than it is at the Poles because of available heat energy).  The result of this is that at higher latitudes there is greater chance of scattering and absorption which lowers the amount of radiation received.

       The curvature of the Earth and its tilt also combine to affect the length of Day and Night in higher latitudes. As shown on Figure 3, in December the maximum overhead sun is over the Tropic of Capricorn and because of this and the Earth’s curve day length shortens in Northern latitudes and goes to 0 hours at the North Pole.  This means that during this period these areas receive very little Insolation during these short days.

There are other more local and short term factors that can affect the amount of Insolation received by a location.

Altitude has a significant impact upon the temperature of the air.  This is because the mountains higher up have significantly less surface area with which to heat the air that surrounds it (recall that the air is largely heated from the ground upwards).  In addition, because atmospheric air pressure falls with height, there are greater gaps between the molecules in the air so their ability to hold and transfer heat decreases too.

Aspect is another factor that influences the heat budget at a local level; this is because hillsides alter the angle at which the sun hits the ground.  In the Northern hemisphere it is seen as advantageous for homeowners to have a garden that faces towards the south.  This is because the south facing slopes are orientated towards the sun for the majority of the day and hence receive the most sunlight.  North facing slopes are often the ones where glacier creation starts, as snow is protected from the sun’s rays.  North facing slopes are called Adret whilst south facing slopes are called Ubac.

Cloud Cover – this is a hugely important determinant on the local heat budget of a place and is experience d by people in the UK on a regular basis.  On a cloudless night in winter the temperature plummets, whilst on a cloud covered night temperatures are often much milder.  This is because the clouds reduce the losses of outgoing radiation and reradiate that energy back to the Earth’s surface at night.  Conversely, clouds can also reduce incoming radiation during the day.

In conclusion, the amount of insolation across the Earth’s surface varies greatly over both space (altitude and latitude) and over different periods of time (a day, the seasons and a year).  However, I feel that it is the way in which the Earth moves around the sun, the tilt of the Earth and the length of day and night that have the greatest impact of Insolation levels.  Altitude, aspect, cloud cover and urbanisation can have local impacts, but our insolation levels are generally under the influence of forces operating at planetary scales.

Using the image above answer the following questions;

1. Describe the balance of energy received at the Earth's surface?

2. How does this balance of energy link the the Tri-cellular model of atmospheric circulation?

3. Think of as many reasons as possible to explain
a) Areas of energy surplus
b) Areas of energy Deficit

Click on the image to get very complicated notes on this issue.

Heat Budget

Below is a satellite image of the lower troposphere, the blue and purple colours represent areas of lower solar energy and the red areas higher energy.  What pattern is evident on the map and why?