Why does a low pressure system bring cold weather?

A question just asked on the sssf facebook page.

Generally it brings wind and thunderstorms because it implies upward movement of hot air ( which results in condensation and rain). Technically the low pressure is an effect of the rising air.

dv said:

Generally it brings wind and thunderstorms because it implies upward movement of hot air ( which results in condensation and rain). Technically the low pressure is an effect of the rising air.

The lows tend to pull in cold Southern air & the highs bring hot inland air.

Tamb said:

dv said:

Generally it brings wind and thunderstorms because it implies upward movement of hot air ( which results in condensation and rain). Technically the low pressure is an effect of the rising air.

The lows tend to pull in cold Southern air & the highs bring hot inland air.

Depending on where you live, the inland could be to your south…

dv said:

Tamb said:

dv said:

Generally it brings wind and thunderstorms because it implies upward movement of hot air ( which results in condensation and rain). Technically the low pressure is an effect of the rising air.

The lows tend to pull in cold Southern air & the highs bring hot inland air.

Depending on where you live, the inland could be to your south…

The FB thread originated in Melbourne.

I see

On east coast Oz approaching and central high brings the coldest weather.

Better question is : Why does a low bring bad weather?

Ian said:

On east coast Oz approaching and central high brings the coldest weather.

Better question is : Why does a low bring bad weather?

Rising warm air causes the low.

Rising warm air causes an in-rush of surrounding air, and also causes condensation and rain.

The low per se does not cause the storms.

Here’s a quick graphic that I knocked up.

This doesn’t help, surly if a high pressure system forces air down then it would be relatively cold air.

dv said:

Ian said:

On east coast Oz approaching and central high brings the coldest weather.

Better question is : Why does a low bring bad weather?

Rising warm air causes the low.

Rising warm air causes an in-rush of surrounding air, and also causes condensation and rain.

The low per se does not cause the storms.

No. There are many factors.. upper atmospheric disturbance.. but mainly the position of the systems.

Bureau of Meteorology
High and Low Pressure Systems

ABC Science
How do high and low weather systems work?

from
http://scied.ucar.edu/shortcontent/highs-and-lows-air-pressure

Pressure varies from day-to-day at the Earth’s surface – the bottom of the atmosphere. This is, in part, because the Earth is not equally heated by the Sun. Areas where air is warmed often have lower pressure because the warm air rises and are called low pressure systems. Places where air pressure is high are called high pressure systems.

A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses forming clouds and often precipitation too. Because of Earth’s spin and the Coriolis Effect, winds of a low pressure system swirl counterclockwise north of the equator and clockwise south of the equator. This is called cyclonic flow. On weather maps a low pressure system is labeled with red L.

more…

I’m trying to figure out the key points, feel free to add more or correct mine

The sun does not warm the earth evenly.

Air pressure measured in hectare-pascals is denser towards earths surface, and gets less dense in the upper atmosphere.

Low pressure systems circulate clockwise, (southern hemisphere) circulate anti-clockwise northern hemisphere.

High pressure systems circulate anticlockwise, (southern hemisphere) circulate clockwise northern hemisphere.

A cold front is the delineation between cold polar air moving towards the equator and undercutting warm tropical air moving towards the poles.

In a high pressure region, warm sinking air spirals outward in an anticlockwise direction and is stable (southern hemisphere)

A high pressure system covers a greater area, moves more slowly and has a longer life, will be typically dry and free of cloud.

In a low pressure region, warm rising air spirals inward in a clockwise direction and is unstable (southern hemisphere)

A low pressure system covers a smaller area, moves more rapidly, has a shorter life, will produce clouds wind and rain.

As air rises it cools, which causes water vapor in the air to condense into water droplets, forming clouds and sometimes rain.