By Claire

Weather chart for Thursday 1st March.

I don’t know what the weather has been like over the last week or so where you are, but here in Canberra, it has been bucketing down. Since last Thursday (1st of March), Canberra Airport recorded a total of 163.2mm of rain. Now that already sounds like a lot of rain to me, but let’s put that into perspective.

The total average rainfall for March at Canberra Airport is only 50.3mm.

That total was exceeded on the 1st of March, when a daily total of 59.2mm was recorded.

Now that really is a lot of rain! But where did it all come from?

If you look at a weather map of Australia for last Thursday, you’ll see a long dotted blue line across Australia, from the north-west, down to the south-east. This feature is known as a trough. A trough is a band of low pressure, lower than the air surrounding it. Think of a horse’s drinking trough – the low pressure being the base of the trough, with the higher pressure around it, forming the sides.

Sea surface temperature around Australia for March 2012.

In a trough, the low air pressure allows air to rise, forming clouds. If the air in the trough is moist enough, you end up with a large volume of clouds being formed, which leads to a lot of rain. This particular through originates off the north-west coast of Australia. If we look at the sea surface temperatures (SST) in this region, we can see that they are warmer than average, and hence, are shown in orange/yellow. These warm SST encourage evaporation, which feeds moist water into the trough and leads to large amounts of rainfall over Canberra.

So that’s where all the water came from!

Thankfully, the trough is slowly moving out to sea and the relentless rainfall we have experienced over the last few days is over, for now.