11 May 2015

Ever wondered how cloudy earth is at any one time?

As a nation obsessed with the weather, we have a strange fascination with the spectrum of clouds that drift across our skies.

From the cuteness of a fluffy white cumulus to the scary, menacing aura of a towering dark cumulonimbus, each cloud paints its own picture in the heavens above.

But typically, just how cloudy is our planet at any one time?


Well, a joint study by Nasa and Colorado University’s Laboratory for Atmospheric and Space Physics, looked at a decade of satellite data and carried out such research.

Earth – cloudier than you might think

Interestingly, it estimated that typically, 67 per cent of earth’s surface is covered by clouds – although with notable variations over land and ocean.

Over the oceans, it was estimated that less than 10 per cent of the sky is completely clear of clouds at any one time, rising to 30 per cent over land.

However, there are notable seasonal variations over land – give that the weather patterns change throughout the year.

cloud_cover_NASA_wpAverage cloud cover over a decade: dark blue (no clouds) to light blue (some clouds) to white (frequent clouds)

Zones of cloud and no cloud

The way in which weather patterns work on our planet dictate as to where clouds are most likely to form and where they are not.

To get cloud to form, air needs to rise and cool, so the water vapour in the air can condense into tiny water droplets.

On our planet, this tends to happen in three main zones. Firstly, there’s the equator, where the inter-tropical convergence zone (ITCZ) consists of a band of thunderstorms around the globe.

Then there are two zones at mid-latitudes in the southern and northern hemisphere, where the jet streams move from west to east, spawning cloud-bearing weather systems.

earth_cloud_EUMETSAT_wpAt the other end of the spectrum, the cloudless zones tend to be at around 30 degrees either side of the equator (where the world’s deserts lie) and at the north and south poles. In these areas, the air is sinking – the opposite of what is required for mass cloud formation.

Why is cloud cover important?

Clouds play a huge role in transporting moisture from one part of the planet to another, as well as determining how much of the sun’s energy reaches the surface or is reflected back into space.

The latter of these two points is important in the context of climate change, because cloud cover is very hard for computer models to deal with.

This is because cloud cover is parameterised – which means it is calculated based on a number of assumptions and simplifications of pressure, temperature and humidity of the atmosphere.

These difficulties of dealing with cloud in climate models can introduce uncertainties in projections, because if the cloud cover is incorrect, it can have a notable impact on how much energy enters and remains in earth’s atmosphere, and thus how much warming potentially takes place.

So, the next time you stare up at the sky and look at the clouds, you’ll know that they a play a much greater part in the future of our planet than just looking pretty.

Images: Nasa, Eumetsat

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