Why does hot weather tend to give high pressure readings ?
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Why does hot weather tend to give high pressure readings ?
Wasn't sure where to put this so please feel free to move it if necessary.
From what I know, hot weather means there will be less dense air in an area and so wouldn't that mean a lower reading for atmospheric pressure ? How comes it usually gives readings above 1013 hpa though ? I'm equally confused by cold weather and how it tends to give readings below 1013, wouldn't the thicker air mean more air molecules and so a high atmospheric pressure ? Thanks !
From what I know, hot weather means there will be less dense air in an area and so wouldn't that mean a lower reading for atmospheric pressure ? How comes it usually gives readings above 1013 hpa though ? I'm equally confused by cold weather and how it tends to give readings below 1013, wouldn't the thicker air mean more air molecules and so a high atmospheric pressure ? Thanks !
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Hi Officer Kite,
I think your referring to general pressure systems rather than altimeter readings?
In which case, high pressure weather is often associated with fair weather, hence the temperature can be warm. But in the high pressure system the air is sinking from the upper atmosphere down into in the high pressure system, the weight of descending air compresses the air below causing higher pressure (as I understand it) this descending motion also prevents weather like thunderstorms from building hence the stable weather often associated with high pressure. Whereas in a low pressure system the air is travelling into the system from the lower part of the atmosphere and then rising, causing lower pressure.
The high or low pressure is just relative to the what's around it. You can have a high pressure of 998 if the pressure of the air outside the high pressure system is 988 hpa for example.
The highest pressures recorded are actually in polar regions due as you refer to very cold dense air.
Local pressure patterns can occur due to temperature, a sea breeze is a good example. The land warms and air rises, cold air from the sea comes in from below to fill the lower pressure area. As the air rises towards higher levels it moves out to sea to replace the air that has been moving from the sea to the land, so you end up with a cycle of air. I guess this makes more sense to what you understand of warm and cold air.
I hope this helps, it's not a perfect explanation for sure!
I think your referring to general pressure systems rather than altimeter readings?
In which case, high pressure weather is often associated with fair weather, hence the temperature can be warm. But in the high pressure system the air is sinking from the upper atmosphere down into in the high pressure system, the weight of descending air compresses the air below causing higher pressure (as I understand it) this descending motion also prevents weather like thunderstorms from building hence the stable weather often associated with high pressure. Whereas in a low pressure system the air is travelling into the system from the lower part of the atmosphere and then rising, causing lower pressure.
The high or low pressure is just relative to the what's around it. You can have a high pressure of 998 if the pressure of the air outside the high pressure system is 988 hpa for example.
The highest pressures recorded are actually in polar regions due as you refer to very cold dense air.
Local pressure patterns can occur due to temperature, a sea breeze is a good example. The land warms and air rises, cold air from the sea comes in from below to fill the lower pressure area. As the air rises towards higher levels it moves out to sea to replace the air that has been moving from the sea to the land, so you end up with a cycle of air. I guess this makes more sense to what you understand of warm and cold air.
I hope this helps, it's not a perfect explanation for sure!
Hi,
The following may seem silly and/or unhelpful, but the examples you are offering are simply not clear enough either to verify or to comment.
For example, what do you mean by "a reading for atmospheric pressure"? If you are a pilot, are you talking about a sea-level reading on a synoptic pressure chart (QFF), or a QNH at an airfield? If the latter, are you comparing cold and hot readings at the same airfield, or an airfield in a cold country with a different airfield in the tropics? And are you dealing with sea-level airfields or high-altitude airfields?
In short, please be more specific!
The following may seem silly and/or unhelpful, but the examples you are offering are simply not clear enough either to verify or to comment.
For example, what do you mean by "a reading for atmospheric pressure"? If you are a pilot, are you talking about a sea-level reading on a synoptic pressure chart (QFF), or a QNH at an airfield? If the latter, are you comparing cold and hot readings at the same airfield, or an airfield in a cold country with a different airfield in the tropics? And are you dealing with sea-level airfields or high-altitude airfields?
In short, please be more specific!
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Thanks for the replies,
As it seems I may have been a bit unclear I'll try explain my issue further. So far I've established that high pressure readings (QNH at sea level airfield) tends to lead to hot (sunny at least) weather and low pressure usually means low cloud and wind, rain etc.
What I don't understand is how the above is true, when on a hot summers day you would expect the air to be very thin and not really dense which to me would equate to low atmospheric pressure (yet I know I'm wrong and somehow it's actually high pressure). How is it "high" pressure when there aren't that many air molecules ?
I was using the ISA 1013 as a measuring tool for what is high or low, though I may be wrong in doing that from seeing peacekeeper's reply.
As it seems I may have been a bit unclear I'll try explain my issue further. So far I've established that high pressure readings (QNH at sea level airfield) tends to lead to hot (sunny at least) weather and low pressure usually means low cloud and wind, rain etc.
What I don't understand is how the above is true, when on a hot summers day you would expect the air to be very thin and not really dense which to me would equate to low atmospheric pressure (yet I know I'm wrong and somehow it's actually high pressure). How is it "high" pressure when there aren't that many air molecules ?
I was using the ISA 1013 as a measuring tool for what is high or low, though I may be wrong in doing that from seeing peacekeeper's reply.
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To get warm weather in the mid-latitudes, it is usually necessary to have clear skies.
To have clear skies one must have a stable airmass.
Although stability is measured in terms of temperature lapse rate, in practice a stable airmass is one which has subsidence: air moving down from above.
You'll get relatively high pressure anywhere you get wide-scale subsidence.
Now it's true the warm air is less dense than the cooler air, but pressure is increased by the mass of air above trying to descend onto it. Think of it this way: air that is warmed tries to expand, reducing both density & pressure. But on a synoptic scale, the air cannot expand much: it is surrounded by other air that resists being pushed aside. So the warmed air expands just a little (reduced density), but not enough to bring the pressure back to reference pressure.
So: subsidence tends to prevent cloud from forming, which leads to sunny conditions, which leads to warm conditions at ground level. Warm air expands, but not as much as it would like to. Pressure remains high.
Different effects apply in the tropics (where you can get warmth in the middle of an intense low pressure system) and in the polar regions (where there is little surface heating by the sun). But I guess you're posting from a temperate zone!
To have clear skies one must have a stable airmass.
Although stability is measured in terms of temperature lapse rate, in practice a stable airmass is one which has subsidence: air moving down from above.
You'll get relatively high pressure anywhere you get wide-scale subsidence.
Now it's true the warm air is less dense than the cooler air, but pressure is increased by the mass of air above trying to descend onto it. Think of it this way: air that is warmed tries to expand, reducing both density & pressure. But on a synoptic scale, the air cannot expand much: it is surrounded by other air that resists being pushed aside. So the warmed air expands just a little (reduced density), but not enough to bring the pressure back to reference pressure.
So: subsidence tends to prevent cloud from forming, which leads to sunny conditions, which leads to warm conditions at ground level. Warm air expands, but not as much as it would like to. Pressure remains high.
Different effects apply in the tropics (where you can get warmth in the middle of an intense low pressure system) and in the polar regions (where there is little surface heating by the sun). But I guess you're posting from a temperate zone!
Last edited by Oktas8; 17th Jan 2015 at 19:06.
Hi Chris, could waffle on but these boys and girls sum it up nicely. Phil Avery was my METO when I was serving so, since he now works for the Met Office, I now trust what they say! 
See High and low pressure - Met Office. It seems back-to-front (to use the polite phrase) but it works. However, if you are on a coastline, localised sea breezes are generated as you suggest (land warming the air leading to an upwelling which draws cooler sea air in over the land (during the day)) - at night it all reverses. Cyclonic and local thermal effects "appear" to be 180 degrees out. It does make sense - usually after a few beers!!!! 
See High and low pressure - Met Office. It seems back-to-front (to use the polite phrase) but it works. However, if you are on a coastline, localised sea breezes are generated as you suggest (land warming the air leading to an upwelling which draws cooler sea air in over the land (during the day)) - at night it all reverses. Cyclonic and local thermal effects "appear" to be 180 degrees out. It does make sense - usually after a few beers!!!!
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Now it's true the warm air is less dense than the cooler air, but pressure is increased by the mass of air above trying to descend onto it
Thanks to all who helped !
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Why does hot weather tend to give high pressure readings ?
The answer to your question is actually in reverse. It is the high pressure which gives rise to warmer surface temperatures.
High-pressure areas at ground level are normally caused by air above that is moving downward from the troposphere. This sinking air, called subsidence is warmed by compression of the gas molecules causing the air mass to heat up – hence warmer weather – it also prevents condensation, meaning less or no clouds & more sun.
High-pressure areas at ground level are normally caused by air above that is moving downward from the troposphere. This sinking air, called subsidence is warmed by compression of the gas molecules causing the air mass to heat up – hence warmer weather – it also prevents condensation, meaning less or no clouds & more sun.
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is the mass of air that's keeping this warm air near the surface in its place a warm or cold mass of air
Warm (less dense) air is prevented from rising on a synoptic scale by a huge mass of cool, dense, air at tropopause level trying to sink onto it. This is the driver of the whole system.
But on a local scale air will rise unless, in rising and cooling at 2*/1000', it encounters air that is warmer & less dense than itself. Perhaps the air above is 2.5*/1000' cooler, so for every 1000' of height rise the surface air becomes 0.5 degrees cooler than the new air it encounters. Being relatively cooler and relatively denser, the rising air immediately tends to sink again. It is kept in its place by warm air immediately above it. But this warm air isn't driving the whole system; it is part of the subsidence & stable airmass. You have to go very high to find the subsiding, denser, air.
What causes high level air to descend, driving a surface high pressure system? It could be Hadley effects (see "Hadley cell") for example. But there are many other effects...
Last edited by Oktas8; 19th Jan 2015 at 21:09.
