What is thickness?

Thickness is shown on a weather model projection (showing pressure, thickness and rainfall) as the fainter grey lines, including the purple, blue and red ones.

See: http://www.weatherzone.com.au/models/

Have a look at the US 8 day and see how the thickness lines change over a few days.

The thickness shown on those maps is how far it is between 1000 hPa and 500 hPa, two important levels in the atmosphere. Warm air expands so a warmer column of air would show as a larger number, and a cold column of air shows as smaller.

So, thickness is telling us how warm or cold the atmosphere is.

• When we are south of the blue line (5400) any precipitation is able to fall as snow above about 1500 metres. This level lowers the further south of the line we are.
• When we are south of the purple line (very rare), snow could fall near sea level.
• When we are north of the red line (in summer) it is a very hot day.

To get snow you need the moisture to still be there when the colder air is overhead. Some of our weather systems have the main precipitation part in the warm zone and just a little moisture in the cold part, so they bring a fair bit of rain and then a little snow. Others have the cold air arriving early and produce a lot of snow.

If your not interested in snow, thickness is a good indicator of how cold or warm the day will be, or how much of a change in airmass we will get after a cold front moves through.