Tag Archives: met office

A weather-related ‘Did you know?’ post

Did you know that wintry weather is more likely in March than December in the UK? On average, snow falls on five days in December. In March, it falls on six days.

That makes no difference to my wish for this seemingly endless winter to begone, though. Honestly, it seems to have been going on forever. And I’m not being fooled: look at this picture. This is my back garden.

Don't be fooled. It may look lovely and sunny and warm in my back garden, but it's brass monkeys out there.

Doesn’t it look lovely and Spring-like?

I just went outside to put the washing out, and my fingers froze solid. I’m typing this with my (freaky monkey) toes.

So, anyway. My point was this: it’s not particularly unusual weather for March. It’s just really annoying.

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The wrong type of rain; or, why there are floods during a hosepipe ban

This country is in a recession. Are we not all miserable enough without the recession spreading to our weather, dammit? We do not have enough water. But how can this be, people cry despairingly, when there is all this wet stuff pouring out of the sky constantly?

My favourite explanation comes from an old schoolfriend – Ashwin – who is a gentleman, a scholar, and a wit. He posits that “the first 22% of rainfall goes to Her Majesty’s Revenue and Customs, unless there is lots of rain, in which case, Her Maj takes 40%”.

The popular press (and by this I mean the Daily Mail and its ilk) will, inevitably, go for the most attention-grabbing, frothworthy headlines they can. They will scream about flood warnings while moaning about a hosepipe ban, shouting: “Why why why? Look at all this water – how can there possibly be a drought on?”

Often, they then go on to blame the Met Office for the droughts, then the floods. Which is funny in itself. Unless these pesky meteorologists really are sitting in a secret volcano lair somewhere controlling the weather. In which case, the Daily Mail has been right all along and life as we know it is coming to a soggy end.

There are floods during a drought. And there are very good reasons why. They are also interesting reasons, so it’s a real shame that most people aren’t given the facts.

In Warwickshire, it’s been raining. A lot. More on than off for the past two weeks, and almost continuously for the past three days. That’s quite a lot of rain – 50mm in some places on Wednesday alone – and it has followed two unusually dry winters.

Several factors cluster together to pose a flood risk. But first things first: where did all this rain come from in the first place?

Water, water, all around…

A particularly strong jet stream is currently separating warm, moist air in the south (an area of high pressure) from cool air in the north (low pressure), creating a front. Jet streams are fast-flowing, narrow currents of air high in the atmosphere (near the top of the troposphere – the tropopause) which blow from west to east. Their locations are not fixed; they tend to meander around, but their position helps with weather prediction.

At the moment, the warm, moist air from the Mediterranean is flowing north (air travels from areas of high pressure to areas of low pressure) into the low pressure zone over the UK. Because warm air is less dense than cool air, it is forced upwards above the cool air.

As the warm, moist air rises, it cools and its water content condenses into rain-clouds (cool air can carry less water vapour than warm air). If the front stays steady, the rain will persist as long as the clouds are being fed by a flow of warm, moist air from the south.

Unfortunately for us (but fortunately for our gardens) the front is persisting, as is the seemingly endless precipitation…

But we’re suffering from a drought, aren’t we?

But why then, you may ask, is there a hosepipe ban across much of the country? As mentioned above, the UK has experienced two unusually dry winters. These two dry winters mean that the UK is now suffering from a water shortage. Reservoirs of water have been depleted and it takes time – and a lot of rain – to refill them.

Almost everyone knows about freshwater reservoirs – man-made lakes created specifically to provide stores of drinking water. These are replenished by rain and by rivers and streams. Fewer people realise that an awful lot of our fresh drinking water is stored as groundwater, in the rocks themselves. Just one per cent of all freshwater is accessible to humans, and of that, four million cubic kilometres is located underground. In the pores and cracks in the bedrock, and between the grains of rock themselves.

Water in the ground

Rock that is porous and permeable enough to store water and allow it to flow through easily are called aquifers, and most of the UK’s aquifers are located in the southeast of England – notably the Chalk (which is a soft, white limestone with a network of fine cracks, giving it high permeability). Individual boreholes in the Chalk can yield more than 10 million litres of water each day, which can provide for around 70,000 people.

That’s a lot of water.

Groundwater is not only accessed via boreholes, it also helps to replenish rivers. If rivers were only fed by runoff from the land, they would quickly dry up after the rain stopped. Rainwater that passes through the surface of the land soaks down through the underlying soil and rock until it reaches the water table – a level when all the pore spaces in the rock are full of water. The water table separates the unsaturated zone from the saturated zone – and the saturated zone is called groundwater.

The drought persists even after prolonged heavy rain because recharging the saturated zone takes time. There’s an awful lot of ground for the water to soak through, and the water table level will recover only slowly. And the first bout of heavy rain after a prolonged dry period, such as that the UK experienced in March, won’t do much to help – parched, hard ground won’t absorb water easily, so it simply runs off the top. It will, of course, gradually soak in, but not straight away…

Which leads us to why there are flood warnings

Most river floods result from extremely heavy and/or prolonged rain. Several factors can affect the intensity of flooding – area, slope and altitude of the river catchment, soil type, geology, vegetation cover, the nature of the drainage basin network, and the shape of the river channel.

However, in this instance there are two main reasons. Firstly, the sheer amount of water being pushed into the rivers by the rain itself and the runoff from the land surface. There is only so much extra water a river can take in a relatively short space of time before it bursts its banks.

Secondly, if there is a large volume of water dumped on the ground, the unsaturated zone will become saturated – it won’t have time for the water to soak down to the saturated zone before all the upper pore spaces are filled. So there will be even more runoff than usual.

So although drought and floods are both caused by precipitation (or the lack of it), different mechanisms are at play. Drought is a long-term problem that cannot be solved by a lot of rain in a short period of time. Floods are a short-term problem that are often caused by a lot of rain in a short period of time.

There are really cool (and by that, I mean nerdy) ways to predict how severe a flood is likely to be. In fact, the whole topic is pretty interesting – aquifers are marvellous things.

It’s really not that complicated at this level (of course, there are other aspects to consider, such as the deforestation of the British Isles, the amount of concrete and tarmac laid down, and the effects of vegetation on water movements, but in a nutshell, this is how you get floods in a drought). Yes, you could dig a little deeper and find an entire aquifer full of interesting scientific details to do with groundwater supplies, but a basic understanding of what causes droughts and floods is not out of the grasp of most people, so it’s a real shame that journalists don’t have the time to research and explain this. And it’s more of a shame that the press releases from the Met Office and the BBC don’t include simple, easy-to-follow explanations (and if they do – then why don’t the papers use them?).

I keep hearing wits on the radio commenting that this is a very wet drought, isn’t it? Ha ha ha. Well yes. Yes, it is a very wet drought. Two weeks’ worth of rain will not, sadly, make up for two winters’ worth of unusually dry weather. So put your hosepipes away and start conserving your water. It’s a precious natural resource, and deserves to be treated as such.

Cool scientific instruments

Of these, there are many; this much is true. However, by far the coolest scientific instrument I have seen recently is this: the sunshine hours recorder.

Coolest scientific instrument of the day

I came across this during my virtual fieldtrip to the Teign Valley, where I am having a crash course in climate in the local area. We’re trundling around a meteorological station and poking about in the instrumentation.

This device is a thing of beauty. It’s a crystal ball, for goodness’ sake. What’s not to like? And it is simplicity itself. The glass ball focuses sunlight onto the paper chart, and burns a small hole in it. When the sun is behind a cloud, no hole is burned.

Useful things don’t have to be ugly. Here endeth today’s (very short) lesson. I’m off to get me a crystal ball.