ABC of Weather: Atmosphere

It's ABC Wednesday round 6 with a twist(er): I'm attempting to keep to a theme for 26 weeks rather than thinking of something at the last minute like I've done for the last 3 rounds ;)

We Brits are notorious for our obsession with the weather owing to its variability in our country. It's of interest to all gardeners and I'm also attempting to monitor what's going on in my garden. so it's got to be a sure-fire VP blogging winner*. I'm struggling with what will appear for X, but hey, that's ages away...

So without further ado, let's put the spotlight on A for Atmosphere.

Here's a nifty little diagram, showing how our atmosphere is divided into 5 layers above the earth's surface - you can click to enlarge it if needed:

Diagram adapted from The Weather Book.

Our atmosphere comprises a number of gases trapped by the pull of earth's gravity. The lowest layer, the troposphere contains about half of the atmosphere's total (though Wikipedia says around three quarters), and its height varies between the poles (where's it's at its thinnest) and the equator. The actual height of the troposphere on any day at any point on the earth's surface also varies slightly depending on what's happening with our weather. The name troposphere is derived from the Greek, tropos meaning turning or mixing. Most apt really because this is the layer where our very variable weather occurs, though it may also be influenced by the layers above it, particularly the stratosphere.

The gases in our atmosphere are about 78% nitrogen, 21% oxygen, 0.93% argon, 0.038% carbon dioxide, and small amounts of other gases plus a group of substances known as atmospheric aerosols, minute particles from volcanoes, desert dust whipped up into the atmosphere, plus human made ones from burning fossil fuels etc. These tiny particles can have a significant impact on our day to day weather e.g. dust from a volcanic eruption can increase cloud cover which in turn leads to atmospheric cooling and increased rainfall. The troposphere also contains almost all of our atmosphere's variable amount of water vapour, on average around 1%. The actual amount of water vapour is affected by what's known as the water cycle, particularly the effects of any evaporation or transpiration occurring at the time.

The earth's surface stores up the heat it receives from the sun and transfers it back into the troposphere, particularly at night. That's why the graph shows this layer as a relatively warm one (though the thermosphere is where it gets really hot - off the scale as far as my diagram is concerned - with a toasty high of 1,700 or 2,500 degrees centigrade depending on which reference we're looking at). Why the troposphere gets colder with altitude when we all know warm air rises, needs a post all to itself (in the meantime, see this link for an explanation).
The other layer I'll mention briefly is the stratosphere, because this is the one containing most of our atmosphere's ozone, the lessening of which has been of major concern with global warming. This reactive form of oxygen absorbs most of the sun's ultraviolet radiation, thus cushioning us from the harmful effects of these rays and preventing the troposphere from heating up to a temperature where we couldn't survive. If you're interested, here's some links to the layers I'm not telling you about: the middle layer aka the mesophere and the outer layer aka the exosphere.
So, to sum up: the atmosphere is complex (and I could say so much more about it) and very important for my themed ABC Wednesday round because it's where our weather actually happens.
How's the weather with you today? Here, it's snowing again!
* = I've also been rather obsessed with the weather since I was small: I invested my pocket money in a little book called The Weather Guide when I was 7 years old and I still have it. The Weather Book is similar, but published last year and I had a very happy half hour leafing through it at Bristol Temple Meads station last year whilst waiting for a train.
For more Adventurous As, do have a look at the ABC Wednesday blog.
Update: I'm also guest posting on The Guardian Gardening Blog today, where you can find out about Potato Days just like the one I'm going to on Saturday :)