How the wind blows

It's generally a given, in Australia, that a southerly wind is cool, and a northerly wind is warm. 

But this is not always the case... and here's why.

First of all, let's clear up what a 'northerly wind' means. The terms northerly or southerly indicate where the wind is blowing from. A northerly blows air down from the north, a southerly blows it up from the south. 

Simplified maps use an arrow to indicate this, but you may also see a wind barb: 

Each half barb (short line) equals 5 knots, each full barb (long line) 10 knots, and each flag (triangle) 50 knots. A rough guide is to double the knots to get km/h.

To determine how hot or cold the wind will actually be, its helpful to look at the bigger picture, and where the wind has originated.

For example - in Victoria:

A northerly could have originated over the Tasman Sea, so it will be warm and a bit humid.

But a northerly that starts over the outback is generally hot and dry (if the skies over the outback are clear).

A southerly that loops around a high over the Bight is cool.

But a southerly that comes up from near Antarctica brings much colder air with it.

If we want to get more technical we have a look at the thickness. Thickness tells us how short or tall a column of air is. Warm air expands so it's tall, cold air is short:

Image: psu.edu

Thickness (from 1000 to 500, two different heights in the atmosphere, labelled THK) is represented on these maps using grey lines. mb stands for millibars, a measure of pressure, and it decreases as you go up, because the air is less dense with height (gravity pushes air towards the ground).

There are three important ones, marked red, blue and purple.

Red = 5760 = if that line is down over Victoria its likely to get to 40C in Melbourne. This line indicates extremely hot air.

Blue = 5400 = "the infamous 540 line" talked about in snow communities. This indicates that the air is cold enough for it to snow on the mountains, down to about 1500 metres.

Purple = 5200 = this is the really exciting one. If this line is over Tasmania or Victoria, it represents snow to sea level to the south of that line, and some bitterly cold air. 

So, we look at thickness and where the wind originated from to gauge how hot or cold the wind will actually be.

And this last map really puts it into perspective - a warm and humid southerly(!):