Pressure Level: Sea

One of the things that we are often asked is ‘why does the reading from the WeatherSignal app differ from the reported pressure on the weather forecast?’ Ok, maybe it’s not all that often, but it still gets asked and is a very good question.

To understand the difference between the barometric pressure reported by WeatherSignal and the pressure reported on the forecast, we have to first understand a concept called Sea Level Pressure – which is (as you might have guessed) the pressure at sea level.

Pressure is essentially a measure of the mass of air above you – and so the higher you go the lower the pressure will be in your location. To get around this and to make showing pressure across countries and regions comparable, pressure readings are transformed into sea-level pressure readings – so that the isobars showing fronts of pressure don’t end up simply reflecting changes in the elevation of the earth. Put simply, WeatherSignal simply shows you the raw pressure reading taken at your location – it does not account for altitude. If you live somewhere high up (Hi WeatherSignal users in Denver!) then WeatherSignal will consistently give you a different pressure reading to the forecast. For those of you who live by the sea or in low-lying cities (like us in London) then WeatherSignal will return a pressure value almost identical to the forecast.

What is most interesting about pressure as an atmospheric variable capable of telling you about the weather is, however, not the absolute value – but rather the rate of change. Pressure change pretty much tells you the speed at which the weather is changing (and also gives you information about wind – which is essentially air flowing between areas with a pressure differential). So, even if WeatherSignal is not reporting an accurate absolute value, it can still tell you that the weather is improving or getting worse!

For more information on how to calculate sea level pressure from a raw pressure this [pdf] page may prove instructive.