Brrr….increasingly cold this week for Reigate!

November 25, 2012 — 1 Comment

Take a look below at the forecast for London temperatures through to the start of December.  Also, check the map: that blue colour approaching the UK is a cold pool of POLAR air which will make things increasingly chilly this week.  Some parts in the East and further north in the country may see snow by next weekend: this is more doubtful for Reigate but certainly cold rain!

Learning spot: We commonly see sea level pressure lines labelled 1016, 1020, 1024 etc.. measured in mb (millibars) / hpa (hectopascals) on charts (these measure the same).  These lines show air pressure at sea level and appear on most synoptic charts.  But what are the lines labelled “528”? They are atmospheric thickness lines!

Explanation: Cold air is more dense and has a low thickness (like a thin duvet!). Warm tropical air is thicker (like a fluffy duvet!).  Pressure decreases with height so that the 1000 hpa is usually the air pressure somewhere near the surface and 500hpa is usually half way up through the atmosphere (around 5000m). Thickness technically measures the difference in height between these two standard pressure levels in the atmosphere. Thickness is closely linked to the temperature of this layer of air.

So, for our purposes, cold, polar air has low thickness, and values of 528 dam (decametres 1dam=10m) or less frequently bring snow to the UK. Conversely, warm, tropical air has high thickness, and values in excess of 564 dam across the UK often indicate a heatwave.

This winter: watch those 528 dam thickness lines on weather charts!

One response to Brrr….increasingly cold this week for Reigate!

  1. 

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