Ensemble spread for mean sea level pressure / Mean sea level pressure
Thursday 12 Jul, 00 UTC T+0 Valid: Thursday 12 Jul, 00 UTC
These charts represent forecasts of Mean Sea Level Pressure (MSLP) and Wind speed at 850 hPa, or geopotential height at 500hPa and temperature at 850hPa. all from the ECMWF high resolution forecast (HRES). Select which using the parameter drop down menu (grey box).
is the surface pressure reduced to sea level. Solid lines are isobars (every 5 hPa), that is lines of equal MSLP. These charts show surface pressure patterns - areas of high and low pressure which are associated with different weather types. Usually low pressure systems (cyclones or depressions) bring unsettled weather whilst high pressure systems (anticyclones) are associated with settled weather. In the northern hemisphere the air rotates anti-clockwise around the low pressure centres and clockwise around the high pressure centres (the opposite applies in the southern hemisphere). Wind speed is roughly proportional to the distance between isobars: so closely packed isobars mean strong winds, and vice versa.
Wind speed at the 850 hPa level
is shown in metres per second (m/s) using colour shading. Shading starts at 15 m/s. So this field highlights areas of strong winds approximately 1.5 km above sea level.
500hPa Geopotential Height
The geopotential height of the 500 hPa pressure surface shows approximately how far one has to go up in the atmosphere before the pressure drops to 500 hPa (i.e. 500 millibars). On average this level is around 5.5 km above sea level, and it is often referred to as a steering level, because the weather systems beneath, near to the Earth's surface, roughly move in the same direction as the winds at the 500 hPa level. Height contours are labelled in tens of metres (=decametres, ="dam") with an interval of 6 dam. The contours effectively show the main tropospheric waves that "control" our weather - low heights indicate troughs and cyclones in the middle troposphere whilst high heights indicate ridges and anticyclones.
850 hPa Temperature
Colour shading indicates temperature at the 850 hPa level in degrees Celsius (oC), in 4oC colour bands. This is the temperature approximately 1.5 km above sea level, usually just above the boundary layer. At this level the diurnal (daily) cycle in temperature is generally negligible. Therefore temperature at 850 hPa can be used to indicate frontal zones (i.e. areas of large temperature gradient, where the isotherms are more closely packed together), and naturally also to distinguish between warm and cold air masses. Sometimes temperature at 850 hPa can be used to roughly assess the maximum temperature at sea level by adding 10 to 15oC, and for higher ground one can interpolate. However there are situations when this method does not apply, particularly in winter.
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