By Claire

I’m not sure where you are reading this blog from, but today (Monday), in Canberra, Australia, it’s windy! We have also had a few days of really nice and warm weather, but today, it’s much cooler.

Why is that? Well, here is a quick guide to interpreting weather maps that should help you understand why the weather has been so variable over the last few days.

Mean Sea Level Pressure map for Australia as of 11am 22nd October 2012.

So, just briefly, before I go into the specifics of interpreting this weather map, I just want to cover some basics.

When you see a “H” on a weather map, this indicates a high pressure system (“average” pressure is 1013 hPa, so when pressure is either side of this number, you get a high, or low pressure cell). Similarly, “L” indicates a low pressure system.

In a high pressure system, air is sinking and in a low pressure system, air is rising. It is these high and low pressure systems that create the weather we see each day. When you have low pressure, air is drawn in to replace the air that is rising. Similarly, when you have sinking air, air is pushed out as more air sinks on top of it. So now, we have wind. Wind is simply moving air.

“Wind will blow from high to low”. Air from an area of high pressure needs to go somewhere and it will move towards a low pressure system.

But, it’s not as simple as that (things never are).

The Earth is spinning, and so because of that, wind doesn’t move in a straight line between high and low pressure cells. It is deflected to the left in the Southern Hemisphere (to the right in the Northern Hemisphere). So what we see is wind “spinning” out of a high pressure system, into a low pressure system.

Ok, lets get back to the weather map for Monday morning.

The lines on the weather map are called isobars, and these connect areas of equal pressure.

Wind moves parallel to these isobars, spiralling out from the high pressure system near Tasmania, being deflected to the left (did you get all that?).

Air moves out from the high pressure cell, parallel to the isobars, being deflected to the left.

So now we can see that we in Canberra, have a southerly wind. (NB. Wind is named from the direction that it’s blowing FROM. So a northerly wind is blowing from the north etc). The closer together the isobars are, the stronger the wind. That’s because the pressure is changing more over less distance, so we have a greater pressure gradient force to drive the wind. Make sense?

But why is it so cold?

Well, now that we can see where the wind is coming from, answering that question is easy. The wind is coming from the south, and we know that the Southern Ocean (the ocean to the south of Australia) is cold. So, our wind today is blowing up from the Southern Ocean, brining cool air with it, making the temperature in Canberra a brisk 17 degrees Celsius.

Simple!