Weather Watching

By Simon Woodhouse

I suppose in its most basic form, weather watching is merely a matter of looking out the window to see if it's raining or not. Or you can let someone else do the hard work, and just watch the TV weather reports. But wouldn't it be good if you could understand and predict the local weather patterns, and therefore remove some of the need for guesswork, or relying on somebody else's high-tech guesswork.

Living on planet Earth as we do, we're in the lucky position of having weather that on the whole is usually quite bearable. If you look at our nearest neighbours, Venus and Mars, it's a very different story. I suppose if you're a sun lover, Venus would be your preferred choice, but you wouldn't last very long. Being so much closer to the sun means Venus is pretty hot, 900 degrees Fahrenheit in fact. It has also got an atmospheric pressure of 90 times that on earth. There's no rainfall as such on Venus, but there are clouds of sulphuric acid blown along by 180 mph winds. So on Venus you'd be burnt, squashed and melted all before you could get much of a tan. Mars on the other hand, is very cold (-145 Fahrenheit) and very windy, with gusts of up to 250 mph. Compared to these two, Earth is just about right.

From an historical point of view, weather watching has mostly consisted of simply looking up at the sky, and taking an educated guess based on previous experience. That changed in 1607 when Galileo (amongst others) built the thermoscope, a device used for measuring temperature. A few years later in 1643, a man called Evangelista Torricelli created the first barometer. Things moved forward in leaps and bounds after that, with discoveries about how atmospheric pressure behaves, and what it means in terms of prevailing wind patterns. In 1860 Robert Fitzroy was the first person to try and make daily forecasts based on national weather conditions. Using the new technology of the telegraph, he gathered reports from all around the UK, and his forecasts were published in The Times newspaper. Unfortunately people moaned a lot when he got it wrong (just like today), and after a while the pressure became too much and he killed himself. Other bits and pieces of technology came along, but in 1960 the first weather satellite, TIROS-1 was launched, and forecasting moved into a whole new era. But you don't need a satellite to make short-term, local predictions.

Most places in the world have prevailing weather patterns. This is due to circulations in the atmosphere trying to help us out, by balancing the temperature differences between the equator and the two Polar Regions. Most of us live in the mid-latitudes, where the wind flow tends to be west to east. A weather system, that's the areas of high or low pressure we're all used to seeing on the TV forecasts, are carried along on the wind flow, which means they generally move from west to east. Here in New Zealand this causes most of our weather to come across the Tasman Sea from Australia. Changes in the weather depend on the speed with which the wind flow is moving, and how soon it's going to bring something new our way.

Using a barometer, you can see the air pressure changing, and so predict what sort of weather is coming toward you. Rapidly falling pressure means it's time to get the umbrella out, as there's probably a storm on the horizon. But if the pressure is going up, then there's a high coming in and it's t-shirts and shorts weather. You can still tell what sort of weather is on its way even if you don't have a barometer. Areas of high and low pressure bring winds with them. Wind moves in a circular pattern (all be it on a very large scale), going anti-clockwise around a high and clockwise around a low, though you have to reverse these directions for the Northern Hemisphere. Bearing this in mind, I know that if I stand in my garden facing north, and the wind is on my back, that means it's coming up from the south and thus blowing in an anti-clockwise direction. Anti-clockwise means a high, and because I'm facing north, with west to my left, the high's going to pass over me and there's a good chance the sun will come out.

Fog is another handy indicator of what's to come, but it means different things at different times of the year. Fog is formed in one of two ways. First off there's a summer fog, a condition that comes into being when the temperature falls to what's known as the dew point (this value varies according to the location), and the humidity goes up to 100 percent. This tends to happen on very still, cloudless summer nights. So if you see fog in the summer, it means weather conditions are very settled. On the other hand, fog can also form when warm, moist air blows over a cold surface (a field in winter). And warm, moist air means there's rain to come.

As with most things to do with Mother Nature, none of these rules are set in stone. But on the whole they do have some relevance. You don't have to take my word for it though, as stepping outside your front door will probably be enough to demonstrate how predictable (or otherwise) the weather can be.