Understanding 500hPa Pressure Charts

[lisach]

Can I please ask what values are indicative of HP and what are LP?

Is there a hard and fast rule that decide that 1025 or 1030 will bring any trend of weather?

Thanks

[johnholmes]

short answer to both your questions is no

Values of pressure are relative to one another although the standard average world wide pressure is looked upon as around 1013mb(in the aviation world).

so values below are usually shown as low and those above are shown as high.

differences of 5 mb in the regions of 1025 and 1030mb are impossible to give any figure or weather to, you have to read the charts as to what has just gone and what values are being predicted.

Generally an area shown as a low on a surface chart will give ppn of some kind with that showing high pressure, at least in the middle of the area, is usually dry, that is not to say it will be sunny with it!

welcome to NW enjoy all there is, forum, weather charts, forecasts etc.

Can I ask a favour please?

pop your nearest town in your avatar so we all know where you are when you report some interesting weather.

do ask questions, visit our Weather Guides and learners area. as well.

Edited August 3, 2008 by johnholmes

[Nick F]

One has to remember to look at what's going on aloft with height tendencies aswell as the pressure at the surface. I've quite often seen situations where the surface pressure has been 1025mb, when one would expect there to be fine settled weather in summer, however - if there is low heights aloft at 500mb - or the height tendency is falling rapidly, then showers or even thunderstorms can sometimes be expected. Such a situation occured during May 2006, when thunderstorms broke out across central southern England -despite sfc pressure being around 1025mb. An upper low was present that day.

[johnholmes]

very true Nick, the penalty in my answer of trying to be as simple as I could keep it!

[Rob]

very true, it can tip it down when the pressure is very high and can be sunny and warm with low pressure

Edited August 3, 2008 by rob28081991

[BrickFielder]

There are a number of different charts to look for each level in the atmosphere.The 500hPa level is about the mid level of the troposphere and is often used as a general guide to what weather to expect. Very commonly amateur forecasters will start by looking at 500hPa temperature or height charts which will give some idea about likely locations of High and Low pressure. Typically low pressure will tend to bring wetter windier weather and high pressure drier weather.

What happens at mid levels in the atmosphere does not always correspond to what happens at the surface. Another chart to look at may be surface pressure which when combined with 500hPa heights will tell you much more about the weather. Take the next chart for example and look at Greenland where you can see a surface high pressure where 500hPa heights may not tell the whole tale.

Equally wind charts can be usefull to identify where high and low pressure and may give hints about where rainfall may be likely.

Looking at Cloud and Precipitation charts should show how accurate our estimates are about likely weather conditions.