Weather Explanations

On this page is will explane weather meaning

UV Index

The UV Index is a forecast of the amount of skin damaging UV radiation expected to reach the earth's surface at the time when the sun is highest in the sky (around midday). The amount of UV radiation reaching the surface is primarily related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amounts of cloud cover. However, thick cloud can greatly reduce ultraviolet radiation levels and, surprisingly, certain types of thin cloud can magnify the ultraviolet radiation strength. 
The peak daily ultraviolet radiation level changes over the year. The strongest being at the Summer solstice (21st June) and the weakest at the Winter solstice (21st December).

The UV Index can range from 0 (at night) to 11 or 12. It might even be higher in the tropics and/or at high elevations under clear skies. The UV Index does not exceed 8 in the UK (8 is rare; 7 may occur on exceptional days, mostly in the two weeks around the summer solstice). However, indices of 9 and 10 are common in the Mediterranean area

CAPE - Convective Available Potential Energy

Convective Available Potential Energy (CAPE) is measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather.


Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on a sounding diagram by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area). However, when estimating thunderstorm probability one should also always have a look at the Lifted Index.

850hPa temperature fields

850 hPa level is roughly at 1.5 km, usually above the atmospheric boundary layer. That means there is no diurnal temperature variation, and the underlying surface such as cool sea doesn't affect it's temperature. That is why 850 hpa temperature is used to distinguish air masses and thus to locate cold and warm fronts. 

Because the models have had several problem in surface parametrisation, 850 hPa temperature forecasts have been more accurate than those for lower levels. So we used to bring the 850 hPa to surface by adding 15 (or 10) degrees, and use it instead of surface maximum temperature. [***]
WARNING: all this works only at low altitudes, not in the mountains. It works only when the sun is heating the ground: not at the sea or windward coast, not at night-time, not at winter. 15 is the continental value (for dry adiabatic lapse rate); in Ireland they use 10 (for moist adiabatic lapse rate) and I guess at the Alps the 850 hPa temperature is more or less equal to the surface temperature.

You can see from all these warnings, that using the 850 hPa temperature to infer the screen temperature is problematic at best!

[ *** To a first approximation, you can also use T850 to assess the surface temperature in persistent precipitation, provided the layer 850 - surface is near-saturated: in this case, add ~7 degC 

500 hPa patterns

Level of 500 hPa is roughly dividing the mass of the atmosphere in two. It lies near 5 km, and it's height is typically analysed at intervals of 40 or 80 m, corresponding to MSLP isobars at intervals of 5 or 10 hPa. 

The shapes of 500 hPa isohypses are similar to those at the MSLP isobars: even here you can find lows, highs, ridges and troughs, though the latter two are more usual than closed circles. Formations are much smoother because the underlying surface (except in the high mountain areas of the world) doesn't have an effect at these high altitudes (and because observations are sparse). 

If you compare 500 hPa and MSLP charts, you often find a family of small surface lows below an upper level low. Also, the weather under a 500 hPa low or trough tends to have more precipitation, at least showers, even is there are no fronts present. 

For numerical models, 500 hPa height (and especially the stream pattern it reflects) is much easier to get right than the exact location of individual surface lows and fronts. So we look at it to get a general outlook of weather, especially to talk about 5-10 days ahead.

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