Meteorology

Meteorology

The great thing about studying meteorology (weather) is that it’s going on around you all of the time. Look out of the window, there it is.

Meteorology is a big subject and much of it is abstract and complex. Don’t worry, you don’t have to be a physicist to understand weather. Just get a grasp of the basics (below), download weather information from the web, and tie it together by looking out of the window.

The basics

Fundamentally, weather is caused by the sun. Heat energy from the sun warms the surface it shines on. That surface, whether land or sea, warms the air above. The sun doesn’t warm the air directly.

The warmed air rises because it has become less dense than the surrounding air. Think of a hot air balloon, it gets carried upwards because the hot air within wants to rise. As the air is trapped it takes the balloon up with it.

That rising air is replaced at deck level by an in-flow of surrounding air. That air may become heated and will also rise. You may get a circulation going, where warmed air rises and cools and then descends to flow back into the hot spot on the ground.

Sea Breeze

That is the basics behind a sea breeze. Consider, - a still day, sunny, no breeze at all early on, by mid morning it picks up ‘till by lunchtime it’s blowing quite nicely - around force 3 – 4, more or less onshore.

At the same time you might notice that a line of cloud has formed along the coast. From offshore it looks like it’s a couple of miles inland.

That line of cloud is formed by water that has been picked up as vapour by the in-flowing air that has blown across the sea, (the air that has been propelling your boat), and then carried aloft by the warmed air. But why cloud?

Dewpoint

One interesting characteristic of air is that the warmer it is the more water it can carry as vapour. Warm air can carry lots of vapour, cold air carries virtually none. Have you ever put cheese in the ‘fridge without wrapping it up? Goes all rubbery because it’s dried out, because the air in the ‘fridge is so dry, it just draws the moisture out of it.

When a parcel of air is cooled it will reach a temperature at which it can no longer carry the water vapour. The vapour will condense to form droplets. That temperature is the dewpoint. Hence clouds, or fog if it happens at deck level.

So far, in a fairly un-scientific way we have grasped a couple of fundamentals. Warm air rises - cool air descends. And the idea that warmer air can carry more water vapour than cooler air, and that when our parcel of vapour laden air is cooled beyond its ability to carry vapour, the vapour condenses to form water droplets.

Fog

There are two sorts of fog, land fog and sea fog, known officially as, respectively, radiation and advection.

Fog occurs when a parcel of air, laden with water vapour is cooled below it’s dew point.

The difference between the two types of fog is how that wet air got to wherever it became fog.

Land fog. A winter time, still air, phenomenon. Goes like this…

Wet, winter time land is warmed by the sun, water evaporates and forms a shallow layer of wet air over the ground. The sun disappears by mid to late afternoon and the ground temperature rapidly falls. Our parcel of air is cooled by the underlying cold ground, eventually reaching it’s dew point. The vapour within condenses, forming water droplets, bingo! – fog.

Sea or advection fog is much the same except the moisture laden air wasn’t formed on the spot. It has blown in over the sea. Rather than being cooled by the underlying cold ground, as with land or radiation fog, it is the underlying sea that chills the air above. As the air travels up from the tropics it blows over ever colder seas until it cools to its dew point and fog forms.