Air pressure and winds

Wow, it's windy on the beach today. Do you know where the wind comes from? Ehr, from there? Yes, but why does it get windy? What is wind?

Let's look closer at the beach ball to understand it better. Philip pumped his beach ball really full of air. Then he pumped in a bit more. Loads of air molecules crammed inside the ball. When the air molecules are crowded together, we say that the air pressure is high.

Now, the air pressure inside the beach ball is higher than the air pressure outside. What happens if Philip removes the pump, and opens the valve? The air molecules don't like it crowded. They flow out. That is, air runs out.

It becomes a gust of wind. When the pressure inside the ball is equal to the pressure outside the ball, the wind through the valve ceases. This is an area on earth, where the air pressure is high. We write an H, for high pressure. Here, there is a higher air pressure, compared to the areas around.

The molecules of the air are crowded, just like in the pumped beach ball. And do you remember what happened in the beach ball? When it's crowded, the molecules are pushed away from the dense area. They move from the high pressure area, to the low pressure area. And air moving, that's the same thing as wind blowing.

The air wants to get away from the area with high pressure. Since the earth spins, the winds will not blow straight from the high pressure area. The winds are bent and spiral out from the centre of the high pressure. What happens if the air pressure is low? Then there is more space for the air molecules here, than in the surrounding area.

There is more space, the pressure is low, and we write an L. The same thing happens. The air molecules want to move towards the area where it's more sparse, so it starts to get windy. If the pressure is low, the wind blows toward the centre of the low pressure. So, when it's windy, difference in air pressure is the cause.

And the larger the difference, the stronger the winds. So, wind is caused by difference in air pressure. But how does the difference arise? How are high and low pressures created? There are several ways.

This is an example. The sun shines on a coastal area. It shines equally on the water and on the land, but the water takes a longer time to warm up. So, above land, the air is heated. When air warms up, the molecules spread out.

The warmer air is therefore lighter than the surrounding cooler air. Lighter air rises, like when a hot air balloon floats up to the sky. Now the air molecules are quite sparse in this area. A low pressure area has been created. This is one way a difference in air pressure can arise.

And now, the surrounding air wants to even out the air pressure. The air molecules start to move. It becomes windy. Low pressure often brings cloudy weather with precipitation: rain, snow or hail. And if it's high pressure, the weather often gets sunny and beautiful.

If it's high pressure in the summertime, the days get warm. The kind of days that Philip and Kim like to spend on the beach. Alright, you ready? Nooooo, stupid air pressure difference...