High pressure

[PompeyPaul]

I've often wondered. Why does high pressure in winter lead to cold temperatures but in the summer lead to blisteringly hot barbecue weather?

A QNH of 1040 is over us which, during summer, would have me inside and the aircon on full. In the winter it means we shiver in 0 degrees.

Is it because it limits cloud cover? In the longer sunlight days of summer we get more heat, but in the short days of winter we get more clear night skies to loose any heat we got during the day?

[englishal]

Because the air rotates clockwise around an anticyclone and hence it sucks continental air in. IN the winter this is typically very cold and in the summer hot and dry.

[PompeyPaul]

I originally thought it was Coreallis and pulling air from the north which is colder. If that were the case though you would expect summer high pressure to bring colder weather too, since it'd pull cold northern air down. However, as observed, the opposite seems true.

[Lord Spandex Masher]

Paul, have a look at these.

Azores High (http://en.wikipedia.org/wiki/Azores_High)
Icelandic Low (http://en.wikipedia.org/wiki/Icelandic_Low)
North Atlantic Oscillation (http://en.wikipedia.org/wiki/Azores_High)

Goes in hand with Englishals explanation.

[IO540]

Much more important is to hit the right buttons on the right website and get the weather before flying somewhere :)

You'd be amazed how many people don't do that.

There was me thinking it is cold because it is the WINTER :)

[FlyingForFun]

You mean like yesterday, when if you pressed all the right buttons you'd have found that the cloud at Bournemouth was going to be scattered at 3500', and the actual weather was Cavok? So you went flying, and when you came back half an hour later it was broken at 300'?

FFF
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[ProfChrisReed]

Ask a glider pilot about the weather!

The normally positioned high doesn't pull air in from the continent - it pulls it in from the N Atlantic/lower Arctic regions. In the winter this is very cold.

In the summer this air is cool as well, so to start with in summer highs we have cool air but bright sunshine = feels warm in the sun, cool in the shade. These are the days glider pilots live for.

If the high stays in place in the summer, the inversion gradually lowers (don't ask me why, but there is met theory to explain it). The inversion traps the air close to the ground and stops it mixing with the upper atmosphere, thus dissipating its heat. That air is warmed each day by the sunshine, and gets hotter and hotter. A few days into a summer high it's hot in the shade as well as in the sun.

As an example, on the first day after a cold front during a summer high the convection will go well above 5,000 ft, with gliders to the thermal tops. If the high sticks and no cold front comes along, the inversion can drop to as low as 1,000 ft of warm sticky air - thermals don't go above it, glider pilots become bad tempered along with the rest of us.

[Jan Olieslagers]

FWIW: the chap's name was actually Coriolis, (stress the LAST vowel!) and he must have been a good deal cleverer than my poor self.
Coriolis effect - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Coriolis_effect#Meteorology)

[flybymike]

The inversion traps the air close to the ground and stops it mixing with the upper atmosphere, thus dissipating its heat.

This suggests that warm air is being trapped at low level. In fact, in an inversion there is a temperature increase with altitude.

[IO540]

You mean like yesterday, when if you pressed all the right buttons you'd have found that the cloud at Bournemouth was going to be scattered at 3500', and the actual weather was Cavok? So you went flying, and when you came back half an hour later it was broken at 300'?:)

That does happen, because forecasting is very imprecise much of the time, especially when it comes to transient / coastal fog/haze conditions, but anybody who thinks they can become amateur weathermen, and beat the people who get paid to do this all day long, with any statistical significance, are kidding themselves.

I know it is tempting to try to understand weather, but it doesn't work. For starters you can't get your hands on the data. The minimum package from the UKMO is a few grand a year, AFAIK. Most of the commercial weather packagers make heavy use of the free US-run GFS model but it isn't any good for small scale effects like coastal fog popping up.

What might help is using the satellite imagery. I use this (http://www.metoffice.gov.uk/satpics/latest_IR.html) for a "cloud tops METAR" (and it is excellent) and you could use this (http://www.metoffice.gov.uk/satpics/latest_VIS.html) to see stuff like fog building up offshore.

in an inversion there is a temperature increase with altitude. Indeed; I went for a flight to FL100 a couple of days ago and the SFC temp was about 0C and at FL100 it was +6C. That is one helluva inversion...

[24Carrot]

IO540, the satellite picture is very interesting. Have you ever found any kind of scale for it?
Ideally a chart of shades of gray vs temperature, or even numeric pixel brightness vs temperature, but any rule of thumb would do fine.

[IO540]

I have found such sites in the past - some notes here (http://www.peter2000.co.uk/aviation/tops/index.html).

But currently none of them seems online. This site (http://nwcsaf.inm.es/SAFNWC_CTTH2_Europe_Open.html), which has been down for months, used to show the IR images with an altitude scale in metres - nice...

What I find however is that, in the context of an aircraft which can climb to say FL160 plus, the accuracy is not an issue providing you avoid fronts. With non-frontal weather, the tops are rarely above FL140 and if they are it is really obvious on the IR image. And frontal weather has tops usually well above that kind of operating ceiling so this facility is not useful.

Nowadays I plan my IFR/airways flights almost entirely with MSLP charts, tafs, metars, and the IR image on the morning of the flight. I have stopped using the GFS-data-plotting sites like Meteoblue because one just sits there worrying about stuff which doesn't exist on the day.

[24Carrot]

Thanks, the notes are excellent.

[ProfChrisReed]

Quote:
The inversion traps the air close to the ground and stops it mixing with the upper atmosphere, thus dissipating its heat.
This suggests that warm air is being trapped at low level. In fact, in an inversion there is a temperature increase with altitude.

What happens is that the inversion traps a layer of air below it until the sun's heating is sufficient to warm that air enough to break through the inversion. In a summer high the inversion can be really strong - I've flown on a day when the temperature had to reach 28 degrees before the thermals broke through the inversion.

Air warmed at the ground cools as it rises at about 2.5 degrees per 1,000 ft. So if you have an inversion at, say, 2,000 ft where the air is at 20 degrees, it needs to get to 25 on the ground before convection goes above the inversion and cooler air is drawn down from above the inversion. If it only gets to 24 it stays hot and sticky on the ground.

In winter highs the inversion is weaker, and there is less solar energy to heat the surface (and thus the air). Thus more thermic activity (mixing the warmed air with cooler air from above), and the air is warmed less anyway.

[englishal]

But I think the main thing that causes warm / cold weather is the airflow and the direction it comes from.

From a low typically we get tropical maritime airmasses over the UK from the Atlantic - so in the winter warmer and wetter (than it should be) and in the summer cooler and wetter (than say the continental Europe). The sea acts as a big regulator in other words.

Around a high, due to the clockwise spin, this typically drags in colder arctic air in the winter, from Siberia direction. This air tends to be dry and cold. In the summer, the Azores high may build up and due to the position of the high tends to bring the air in from the continent (from the E or SE), which at that time of year is typically hot and dry. This is why the SE corner of the UK is typically hottest in the summer. In the winter the SW corner of the UK is typically warmest as the airmass hasn't had time to 'cool' over land after passing over the relatively warm Atlantic. We tend to get masses of snow when these two air masses bump into each other, so you get the warm, humid air from the Atlantic meeting the cold dry air from Siberia....

Low pressure areas tend to be further south in the winter due to the quasi stationary polar front moving south (where the Jet Stream tends to exist). In the summer when the QSPF moves north there is the chance for the Azores high to build and hence the possibility of a run of stable warm weather. Highs have the effect of blocking the lows, which then tend to move around to the north.