Why is it that most of the time on the map, there are a few low pressure systems, and some high pressure systems on the BOM maps.

http://www.bom.gov.au/australia/charts/4day_col.shtml

The cold fronts circle in towards the “low”, and it’s cold there, and if you are in the “high” it’s usually nice and warm.

Why does the cold air rise, and the warm air descend?

Kingy said:

Why does the cold air rise, and the warm air descend?

I thought it was the opposite. Cold air is more dense, so it sinks to the surface. A cold front moves along the surface and underneath the warmer air and lifts the warmer air upwards. As the warmer air gets lifted it gets cooled down too. Any moisture in the warm air quickly forms clouds and falls as rain.

We think of cold fronts “bringing rain”, but really they just trigger rain, the moisture being already held in the warm air. The cold front shakes the moisture out of the warmer air as it slides underneath.

My basic understanding of weather is that low pressure zones are warmer than the surrounding atmosphere, low pressure are warmer , warmer air has more moisture in it. “Colder” air is denser , open a fridge door and you’ll feel cold air running out of the bottom of the cabinet, warm air rushes in from the top.

A cold front will hug the ground pushing warmer air upwards, where the cold air and warm air with more moisture in it meets that’s where clouds form ( rain), the back end of the front has warm air pushing over colder air with the same effect.

Colder air is dryer compared to warmer air that has more moisture.

Warm and cold being relative you can have a dry “colder” zone that’s actually quite nice temperature wise.

Warmer air being forced to rise over mountains cools the air and forces the moisture out of the air.

When you drive from or innisfail up to Kuranda and out to Ravenshoe it’s all relatively green, just past Ravenshoe is where it all starts going very brown because all the moisture was dumped in the mountains.

Warmer air drags moisture into it, cold air pushes outwards with the same moisture in it making it dryer.

party_pants said:

Kingy said:

Why does the cold air rise, and the warm air descend?

I thought it was the opposite. Cold air is more dense, so it sinks to the surface. A cold front moves along the surface and underneath the warmer air and lifts the warmer air upwards. As the warmer air gets lifted it gets cooled down too. Any moisture in the warm air quickly forms clouds and falls as rain.

Yes, warm air rises, cooler air sinks. Basic thermodynamics.

But when a cold front starts to rotate, all the cold air is drawn to the centre, and there is a Low on the map.

The cold air is dense, it should be sinking and making a high pressure area, but it is a low. Why?

wookiemeister said:

My basic understanding of weather is that low pressure zones are warmer than the surrounding atmosphere, low pressure are warmer , warmer air has more moisture in it.

Around here, each cold front rotates around a low pressure area, with the cold air being drawn into the centre. With the air, being cold, in the centre. Cold, not warm. Still rising, creating a low pressure area.

Cold, dense air is heavy, and should create a high pressure area under it, but it doesn’t.

Why not?

I think more about wind direction and cloud cover..

Here, it’ll be be about 3C just after dawn.. stable a f.. no cloud cover.. no wind. Fine warm day. I’m charging up my spray back pack.
.

The situation is completely reversed in WA

HTH

Ian said:

Kingy said:

Why is it that most of the time on the map, there are a few low pressure systems, and some high pressure systems on the BOM maps.

http://www.bom.gov.au/australia/charts/4day_col.shtml

The cold fronts circle in towards the “low”, and it’s cold there, and if you are in the “high” it’s usually nice and warm.

Why does the cold air rise, and the warm air descend?

In the Southern Hemisphere wind moves clockwise around the centre of a low pressure system.

Cold fronts form when cooler air meets warmer air. The cooler air being denser forces the warmer air upwards. That warmer air cools as it rises, water vapour condenses and clouds are formed. This is why cold fronts so often bring rain.

Looking at the map Ian provided, with reference particularly to the low pressure system and its associated cold front on the far right middle which is moving from west to east. The warmer air is to the east. Air moves towards the low pressure centre but is affected by Coriolis forces, so rather than heading straight for the centre it begins rotating clockwise around the low pressure system, spiraling in so to speak. This means air to the south of the centre of the easterly moving low pressure system is actually heading west, away from the warmer air, so no cold front forms there. The air to the north is moving east, towards the warmer air and so forms a front.

Please note I’m not an expert on this. If someone comes along confidently giving a different answer go with them instead of me.

esselte said:

Ian said:

Kingy said:

Why is it that most of the time on the map, there are a few low pressure systems, and some high pressure systems on the BOM maps.

http://www.bom.gov.au/australia/charts/4day_col.shtml

The cold fronts circle in towards the “low”, and it’s cold there, and if you are in the “high” it’s usually nice and warm.

Why does the cold air rise, and the warm air descend?

In the Southern Hemisphere wind moves clockwise around the centre of a low pressure system.

Cold fronts form when cooler air meets warmer air. The cooler air being denser forces the warmer air upwards. That warmer air cools as it rises, water vapour condenses and clouds are formed. This is why cold fronts so often bring rain.

Looking at the map Ian provided, with reference particularly to the low pressure system and its associated cold front on the far right middle which is moving from west to east. The warmer air is to the east. Air moves towards the low pressure centre but is affected by Coriolis forces, so rather than heading straight for the centre it begins rotating clockwise around the low pressure system, spiraling in so to speak. This means air to the south of the centre of the easterly moving low pressure system is actually heading west, away from the warmer air, so no cold front forms there. The air to the north is moving east, towards the warmer air and so forms a front.

Please note I’m not an expert on this. If someone comes along confidently giving a different answer go with them instead of me.

Actually, wait, I may have got that all wrong. Forgot what different types of lines mean on weather maps. Please ignore. ;)

Just to simplify things, in the absence of any Coriolis effect or heating/cooling.. Gravity makes it warmer at the surface there being more air molecules bouncing around.

I have to go spray some weeds… :)

Where’s Steve when you need him¿

Ian said:

Just to simplify things, in the absence of any Coriolis effect or heating/cooling.. Gravity makes it warmer at the surface there being more air molecules bouncing around.

I have to go spray some weeds… :)

Where’s Steve when you need him¿

I was thinking this one needs Steve.

Ian said:

Just to simplify things, in the absence of any Coriolis effect or heating/cooling.. Gravity makes it warmer at the surface there being more air molecules bouncing around.

I have to go spray some weeds… :)

Where’s Steve when you need him¿

on the FB sssf page. he could be tagged.