How the wind blows

Its 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, lets 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

Image: noaa.gov

Image: noaa.gov

Image: eoas.ubc.ca

Image: eoas.ubc.ca

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

You can see this in action here (with Obs Rose turned on): http://www.weatherzone.com.au/radar/vic 

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 started over the Tasman Sea, so it will be warm and a bit humid.

A northerly could have started 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.

But a northerly that starts over the outback is generally hot and dry.

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

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.

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 its tall, cold air is short:

Image: psu.edu

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 mighty hot air.

Blue = 5400 = "the infamous 540 line" talked about in snow communities. This indicates that it should snow 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 southerly(!): 

warm southerly