Tuesday, 28 June 2011

Extreme Weather (Lightning) 28th June 2011

Today was the last hot day in South East England with temperatures peaking at 27.3 degrees at noon recorded at Bethany. Yesterday was even warmer with a high of 30.2 degrees, lower than Gravesend's 33.1 degrees which was the warmest temperature recorded in the UK for 5 years. Due to its rarity we can describe this as 'Extreme Weather'.  Which meteorological processes combined to produce this exceptional event?  This blog post attempts to answer this question!

A Cold Front - the cause of today's thunder and lightning
This mini heatwave was caused by a battleground between two contrasting air masses - hot humid 'tropical continental' air from the warm European continent which delivered very warm and humid air to South East England, and cooler, denser polar maritime air from the Atlantic Ocean.  These air masses met at the cold front.  It's called a cold front because the cooler air is advancing; the front is the start of the cold air.  As the cold front crossed the region from west to east, colder heavier polar maritime air undercutted the warm air.  This created rapid uplift, forming heavy frontal rain, towering cumulonimbus clouds and lightning (see figure above).

The band of thunderstorms on the Met Office's radar
The weather chart to the right shows the cold front mapped and the satellite image shows the band of cloud associated with the cold front.   The rain radar image shows heavy bursts of frontal rainfall and the lightning bolt map (from tonight only) shows the remains of these thunderstorms as they move out into the North Sea.  The BBC reported this lightning storm as unprecedented.  Lightning struck the control tower at Gatwick Airport, caused flights to be grounded for 25 minutes and also disrupted parts of the rail network in the South East.  Surface flooding also caused delays on the M2 Motorway near the Medway Towns.  All in all it was an amazing day of weather action!

Continued in part 2. Click here to read Extreme Weather (Lightning) - Part 2

The cold front (blue) can be seen on this weather chart

The satellite image reveals today's band of thunderstorms