A3.8 Natural Influences on Ozone Depletion


Natural influences on the amount of ozone in the stratosphere include:

Atmospheric Winds

A natural cycle in which prevailing tropical winds in the lower stratosphere vary over a time. A change from easterly flow to westerly flow can bring up to a 3% increase in ozone over certain locations, but it is usually cancelled out when the total ozone of the Earth is averaged.

Volcanic Eruptions

Large eruptions can potentially inject significant quantities of chlorine (via hydrochloric acid – HCl) directly in the stratosphere. However, the vast majority of volcanic eruptions are too weak to reach the stratosphere, around 10 km above the surface. In addition, there is no historical record that shows significant increases in chlorine in the stratosphere following even the most major eruptions.

However, as described above, neither is considered significant enough to have contributed to the depletion of the ozone layer.

Natural influences affect the defence against UV radiation. These include:


The sun’s rays are the most intense near the equator where they Impact the Earth’s surface at the most direct angle.


During winter months, the sun’s rays strike at a more oblique angle than they do in the summer. This means that all solar radiation travels a longer path through the atmosphere to reach the Earth, and is therefore less intense.

Time of day:

Daily changes in the angle of the sun influence the amount of UV radiation that passes through the atmosphere. When the sun is low in the sky, its rays must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components. The greatest amount of UV reaches the Earth around midday when the sun is at its highest point.


The air is thinner and cleaner on a mountain top; more UV reaches there than at lower elevations.

Cloud cover:

Clouds can have a marked impact on the amount of UV radiation that reaches the Earth’s surface; generally, thick clouds block more UV than thin cloud cover.


Rainy conditions reduce the amount of UV transmission.

Air pollution:

Like clouds, urban smog can reduce the amount of UV radiation reaching the Earth.

Land cover:

Incoming UV radiation is reflected from most surfaces. Snow reflects up to 85 per cent, dry sand and concrete can reflect up to 12%. Water reflects only five per cent. Reflected UV can damage people, plants, and animals in the same way that direct UV does.