Cold Fronts Shouldn't They Usher In Colder Air?

[damianslaw]

This will probably require a technical answer, but I always believed cold fronts by there name mark the boundary between warm and cold air and therefore when they move across the country they should usher in a change in airmass and thus cooler conditions.

However, why is it we have a cold front moving through tonight and tomorrow which if anything will bring slightly warmer air than we have right now? It doesn't seem quite right... is it simply the orientiation of the current low pressure which has sucked the life out of any cold air behind the cold front and this is the reason why we are not seeing any change in airmass or temperature? It is a bit of an odd situation it has to be said and it is frustrating to see the longfetch cold northwesterlies taking a loop into spain first only to then loop back northwards from this direction thus no longer bringing cold but conversely warm air - if this high pressure to our east was not on the scene we would be normally looking at a very pronounced change to cold polar air right now and the cold front would actually be doing what it is supposed too.

The cold front is acting like a warm front or occulded front - it is very confusing and not a great synoptic to put in a text book for those wanting to learn basic meteorology.

[mountain shadow]

I think the upper air is colder but as we get to 1000hpa levels the air is warmer due to the increased cloud level above. Could be wrong though.

[Marcus_surfer]

I think it depends on what time of year it is and which way the wind is blowing e.g. cold front tonight is pulling in very mild air from spain, which is also pulling in unstable air, this set-up would bring much more thundery outlook in summer.

[Andy_]

In my Geography A-Level we were taught that a cold front marks the boundary between Tropical Maritime and Polar Maritime air, meaning that as the low pressure passes over the country the warm front will firstly give us a taste of Tropical Maritime, and then as the cold front passes we are introduced to the cooler Polar Maritime airmass.

Of course in a real situation it is never this straight forward, and there are countless other factors affecting the positioning of the fronts of air and what weather conditions they may entail. But generally yes the cold front is meant to bring cooler air inevitably after a period of heavy rainfall as the front passes.

[damianslaw]

Thanks for posts so far . Will look forward to reading further responses tomorrow.

[summer blizzard]

A cold front just means that the air is cooler in the upper atmosphere. In relation to Britain, this can mean that if we have a continental flow outside summer first with warm uppers but not that warm at the surface, then the cold front drawing in a moist flow off the sea can produce higher temperatures.

In summer you can also get instances where because the 'cooler air' is drier, maxima is higher.

[davehsug]

I remember in 1975 I think, we had a particularly cold, grey frosty spell( probably under an inversion), which was broken by the approach of a cold front from the North West.

[ZONE 51]

I have always thought the same, that cold front bringing colder air-it does, but different layers holding the colder air, otherwise it would be an occluded front or a warm front, usually in autumn we get a weather system that brings a warm frontal rain belt with mild air which can last many hours, then a cold front which can be squally especially when we have a deep low pressure causing gales, behind the cold front there is cooler air and a showery flow of clear skies and convection, with troughs developing lines of heavy downpours.

i would think the cold front stalling/waving and pulling up warmer and moister air into the front would allow milder air at surface level but cold upper air at 500hpa? more potential for severe thunderstorms with the different airmasses having a bit of a battle!

[ZONE 51]

_{Take a look at thursdays fronts from the MetO}

cold front

develops a wave or wobble

now have a look at this chart,

http://okfirst.meson...ogy/Fronts.html - link

and then back to the cold front, also take a look towards the north its another wave developing,

So i think it absorbs some warmer moister air creating a lot of rainfall, this is the slow down of the front and a wave, so the effect is that part of the cold front changes into a warm front? also the system pushing up against the high pressure block to the east, as the front has plenty of energy it stays active, and as its not moving quick and waving it causes a lot of rainfall, and thunderstorms if other conditions allow this to happen, as we could see today/tonight.

I think we need some experts on this one!

[Thundery wintry showers]

I think it's much like some others have already suggested. Polar maritime air is inherently unstable, implying a larger-than-average temperature gradient between the surface and the upper atmosphere, hence the characteristic "sunshine and showers" conditions. Tropical continental and tropical maritime airmasses are both stable, implying a lower-than-average temperature gradient, hence generally stratiform clouds trapped under an inversion.

Also, the airmass that is coming in behind the cold front is essentially returning polar maritime, i.e. it has had to "return" a long distance to our south before heading up to us from the south, thus it is a somewhat modified form of polar maritime.

The phenomenon is most notable when continental air associated with "inversion cold" is replaced by unstable polar maritime air, when the 850hPa temperature may fall appreciably as the front passes over but the surface temperatures may rise by a similar amount.

[shedhead]

I think it's much like some others have already suggested. Polar maritime air is inherently unstable, implying a larger-than-average temperature gradient between the surface and the upper atmosphere, hence the characteristic "sunshine and showers" conditions. Tropical continental and tropical maritime airmasses are both stable, implying a lower-than-average temperature gradient, hence generally stratiform clouds trapped under an inversion.

The phenomenon is most notable when continental air associated with "inversion cold" is replaced by unstable polar maritime air, when the 850hPa temperature may fall appreciably as the front passes over but the surface temperatures may rise by a similar amount.

TWS has given a very good explanation there, but at the surface in Summer it's easy to see a 4-5c rise in temp behind a cold front, as heavy convective rain gives way to blue sky and strong sunshine.

[johnholmes]

hi

I hope the attached pdf will help understand a little more about cold fronts such as the one we are posting about today.

cold front 3 nov 2011.pdf

[Weather-history]

.

The phenomenon is most notable when continental air associated with "inversion cold" is replaced by unstable polar maritime air, when the 850hPa temperature may fall appreciably as the front passes over but the surface temperatures may rise by a similar amount.

The start of January 2009

Manchester Airport

0C max on the 1st

4C max on the 2nd

http://www.wetterzentrale.de/archive/2009/brack/bracka20090102.gif

http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00220090101.gif

http://www.wetterzentrale.de/pics/archive/ra/2009/Rrea00220090102.gif

[damianslaw]

Thanks for the very informative posts. I think the main reasn why temps have remained static is that we have a very mild returning polar maritime airmass which has drawn alot of moisture and warmth from Spain - the key those stubborn heights to our east preventing the low from taking a more normal path. If the high was not in its current position today should have been a cold day with convection.

[ZONE 51]

Thank everyone, interesting, and thanks JH for doing the pdf.

[Alan Robinson]

hi

I hope the attached pdf will help understand a little more about cold fronts such as the one we are posting about today.

cold front 3 nov 2011.pdf

That suggests to me John that writing a weather forcast from a surface pressure chart is in fact rather like playing the piano being guided by a sheet of music.

[johnholmes]

That suggests to me John that writing a weather forcast from a surface pressure chart is in fact rather like playing the piano being guided by a sheet of music.

I have not heard it described that way before Alan.

Any -ology including meteorology has to have a set of rules. In met' they are pretty flexible and I did not mention all the parameters that I as a forecaster would be looking at when analysing a chart; remember also that the forecaster uses data in 3-D, ie upper air as well, hence my comment about the skew-t data (t-phi's to Met O folk).