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Cheviotranger

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  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    Hi

    If I lived at a higher altitude where the pressure is lower, would the weather be wetter ( as low pressure is synonymous with wet weather)?

    I am quite confused about the reasons for wet weather when there is low pressure.

    Regards

    There are other reason why it might be wetter, like warm air being lifted from sea level to a higher altititude and forming clouds but strictly speaking low pressure does not cause wet weather.

    Wet weather tends to form at the boundary between warm and cold air with warm air being lifted up so it condenses out into clouds. These boundaries occur between a low pressure system and a high pressure system, so low pressure systems tend to sweep in wet weather ahead of them. If high pressure sits on top of you then there is no boundary and no wet weather. Anywhere were air rises, due to temperature differences between low and high levels (thunderstorms etc) or due to colder air sweeping in underneath warmer air (fronts) and you will get wet weather. A good indicator for where wet weather will be is the location of the Jet stream (high level- 300hPa strands of wind).

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    There are other reason why it might be wetter, like warm air being lifted from sea level to a higher altititude and forming clouds but strictly speaking low pressure does not cause wet weather.

    Wet weather tends to form at the boundary between warm and cold air with warm air being lifted up so it condenses out into clouds. These boundaries occur between a low pressure system and a high pressure system, so low pressure systems tend to sweep in wet weather ahead of them. If high pressure sits on top of you then there is no boundary and no wet weather. Anywhere were air rises, due to temperature differences between low and high levels (thunderstorms etc) or due to colder air sweeping in underneath warmer air (fronts) and you will get wet weather. A good indicator for where wet weather will be is the location of the Jet stream (high level- 300hPa strands of wind).

    Hi

    Thanks for replying and I understand it a lot better now!

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    • 1 month later...

    My first post. I am interested in pressure too and a complete beginner. I have searched the web but can not find a good intro to reading pressure maps like the one found here at the Met Office. I understand that the basic contour lines indicate pressure, but can't make sense of the coloured lines with triangles or circles on them. Can anyone help here?

    A good indicator for where wet weather will be is the location of the Jet stream

    I have found a forecast map of the jet stream here. Is there any way to correlate the stream with the lines on the air pressure map?

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    Posted
  • Location: Caterham-on-the-hill, Surrey, 190m asl (home), Heathrow (work)
  • Location: Caterham-on-the-hill, Surrey, 190m asl (home), Heathrow (work)

    I understand that the basic contour lines indicate pressure, but can't make sense of the coloured lines with triangles or circles on them. Can anyone help here?

    Coloured lines with triangles or circles are weather fronts, see here for the key:

    http://www.metoffice.gov.uk/weather/uk/guide/key.html#pressure

    I have found a forecast map of the jet stream here. Is there any way to correlate the stream with the lines on the air pressure map?

    The jet stream is fairly high up, roughly about 30,000ft (though can vary depending on time of year)- and the winds at the jet level are not always/often not blowing in the same direction as the surface winds, so you can't really correlate jet stream lines with surface level pressure which charts you refer to. Though the jet stream can aid in the development of low pressure, i.e. the left exits and right entrances of a jet stream, where upper winds diverge, creates a vaccum and forces air upwards from the surface to create low pressure near the surface.

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    Coloured lines with triangles or circles are weather fronts, see here for the key:

    http://www.metoffice...y.html#pressure

    The jet stream is fairly high up, roughly about 30,000ft (though can vary depending on time of year)- and the winds at the jet level are not always/often not blowing in the same direction as the surface winds, so you can't really correlate jet stream lines with surface level pressure which charts you refer to. Though the jet stream can aid in the development of low pressure, i.e. the left exits and right entrances of a jet stream, where upper winds diverge, creates a vaccum and forces air upwards from the surface to create low pressure near the surface.

    Thank you.

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    • 4 weeks later...
    Posted
  • Location: Bexley (home), C London (work)
  • Weather Preferences: Thunderstorms
  • Location: Bexley (home), C London (work)

    Where I tend to get a little confused, is the difference between normal Lows and surface lows!

    My understanding, which is rather limited, is that on hot days (normally), air rises due to being heated up. Is it the case therefore, that this area of rising air is what causes a surface low to develop - i.e the rising air causes a vacuum beneath, air moves in to counter the displacement, thus setting the cycle for a developing surface low?

    I understand that these can develop quite commonly under areas of High Pressure too, hence the reason why you shouldn't get too dispondent if you are T'storm fan if High pressure is close by. We had a pretty awesome plume two years ago, with High pressure over the continent, which triggered a surface low over France that led to a pretty decent MCS over parts of the SE and EA.

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