If you get to know the different types of cloud, you can start to predict when it might rain. However, if you want to know how sunny it is going to be then you need to know the amount of cloud.
Cloud amount is measured in oktas.
Think of some other words that start with the same sound…
Octagon. Octapus. Octave.
The spelling is different but the sound is the same and they all come from the same Greek word meaning eight.
So cloud amount is given by how many eighths of the sky are covered by cloud.
No cloud at all is zero oktas and completely cloudy is 8 oktas.
It’s judging the ones in between that can be a bit tricky, especially as clouds usually come in very irregular shapes.
Unfortunately there isn’t some nifty measure, that you can wave in the air, that will tell you the answer. You have to crane your neck, look around and give it your best judgement. And if your friend has a slightly different opinion, there is no easy way of proving who is right.
Last week we were looking at clouds and you may have been puzzling over how to tell your cirro- from your alto- as it is difficult to judge height. This week is easier as we are looking at clouds at ground level.
Or is it? What is the difference between mist and fog? And what is smog?
Both mist and fog are basically cloud at ground level. The difference between them is how thick the cloud is: how far you can see through it. The boundary is usually taken as 1 km. So if you can’t see something 1 km away, then you are in fog.
If you can see through the cloudiness for more than 1 km, then it’s mist. So you aren’t judging height now but distance, which tends to be easier as there are more things around for scale.
The word smog comes from the two words smoke and fog. It was originally used to describe just that: smoke particles mixed with fog. This was common in cities, when open fires were the main form of household heating.
The most famous example of smog in Britain occurred in 1952, when London was wrapped up in it for 5 days. Breathing the polluted air resulted in many deaths.
Today the word smog is used to refer to the hazy effect that is caused by air pollution. Again it is usually found in cities, where there are lots of vehicles, releasing exhaust fumes, as well as gases from industries.
Smog still indicates poor air quality. Breathing it in can result in sickness. So since 2008, London has operated a low emission zone to try to reduce this type of smog. Vehicles that don’t meet the required standards have to pay to drive in London. The minimum is £100 per day. At that price you would avoid London if you could, thus reducing the pollution in the city.
Of course to find out the likely answer you can simply look up the latest weather forecast and, whereas in the past forecasting was known for being a bit hit and miss, today it tends to be quite reliable. Consequently, people no longer need to deduce their own forecast from the state of the sky – the clouds.
It is hard to photograph clouds. The eye does a much better job of seeing the bigger picture. And the more sky you can see at once the better – for comparison and so you can judge the amount.
If you want to impress your friends with a bit of name dropping then here is what you need to know:
Cloud in sheets = stratus
Fluffy ones = cumulus
Prefix with cirro- if they are very high, so cirro-cumulus and cirro-stratus
Prefix with alto- for medium height, so alto-cumulus and alto-stratus
Judging height is not easy. It helps when several layers are visible at once or if you can see any aircraft.
High wispy ticks = cirrus
Cirrus, followed by increasing cloud amounts and decreasing cloud height, usually means rain is on the way.
So that’s the basics. If you want to find out more, check out the gallery at the Cloud Appreciation Society. It is worth a look, just to admire the beautiful pictures.
If you have used it recently, it may have had to cope with some of this stuff!
Some parts of the world only have rain in the summer, because temperatures are below freezing for the rest of the year. So just measuring rain doesn’t give the whole picture. You need to count all types of precipitation. That’s the overall name for all the ways in which water falls from the sky – rain, snow, sleet, hail.
But snow is light and full of air spaces so you can’t just measure the depth of the snow and add it to the rain.
The amount of moisture in a snowflake depends on the temperature: the colder the temperature the drier the snow.
You have perhaps heard that 10mm of snow is equivalent to 1mm of rainfall.
Well for Britain that is about right, because that is the case when temperatures are just below freezing and that’s usually the conditions when Britain gets snow. If temperatures really plummet here, then the weather tends to be dry.
So next time it snows, venture out with a ruler. The maths is easy: just divide by 10 for the approximate rainfall equivalent.
So where in Britain has the highest temperature been recorded?
Go on, have a guess!
Ok, I’ll tell you.
Faversham in Kent recorded a high of 38½°C on August 10th 2003. Given that that is 1½°C hotter than body temperature, it must have been pretty uncomfortable.
Was your guess somewhere in the south of England? The hottest summer temperatures are usually in the south, where the sun is higher in the sky and so heating more intensely.
Right, now guess the location of Britain’s coldest recorded temperature.
There are actually two places that have recorded the same figure of -27.2°C; colder than your freezer.
Does that surprise you?
With the hottest place right down in the south, you perhaps picked somewhere right at the north of Britain for the coldest place. However, all places on the Shetland Islands are close to the sea, and the sea around them doesn’t freeze. So the northern extremities of the British Isles are warmer in winter than the mountains in the heart of Scotland.
When the weather hits the headlines, we are usually told that it is the hottest / coldest / wettest / driest “since records began”. That was just under 150 years ago with the founding of the International Meteorological Organisation, in 1873. Over the next few weeks Blog About Britain will be taking a look at some of the evidence for temperatures prior to when records began. Join me again next Monday.