By Claire

The past two days have been very cold in Canberra, with minimum temperatures below 1 degree on both Monday and Tuesday. There was even snow falling in Perisher and Thredbo.

But why is it so cold all of a sudden?

Here is part two of your lesson on interpreting weather maps (and a link to part one in case you missed it).

Synoptic chart from Monday.

When we look at a synoptic chart for Australia for the last few days, you’ll notice that there is a large high pressure system sitting over most of the southern part of the continent. A high pressure cell is basically a mass of sinking air.

The atmosphere wants to even out these pressure anomalies, so air will move out of a high pressure cell, towards a low pressure cell.

“Wind will blow from high to low.” That’s how I remember it anyway.

But, wind can not move directly between high and low pressure cells. A whole lot of forces are acting on the air, including coriolis, and as a result, air moves anticlockwise out of a high pressure cell, and clockwise out of a low pressure cell. The air also travels parallel to the lines on the synoptic chart (again, as a result of a number of forces).

Looking back at the synoptic chart, we know that the air is moving either north or south over the ACT, because that’s the direction of the isobars (lines connecting areas of equal pressure) at that location. We also know that the wind will travel anticlockwise out of the high pressure cell. Therefore, the wind is moving from the south, to the north.

It has been so cold in Canberra over the last few days because the high pressure cell centred over The Bite is drawing up cool air from the Southern Ocean. Keep and eye out for this particular weather pattern over winter. Unfortunately, it will be back. Brrrrrrrrr.