Ascension : Met Office - Weather Report
Submitted by The Islander (Met Office) 31.01.2013 (Article Archived on 14.02.2013)

Ascension: Mainly dry but isolated light showers affected some parts, mainly around Green Mountain with sunny spells developing away from the showers.

Statistics for the week ending Monday 28-Jan-13

	Max (Celsius)	Min (Celsius)	Rainfall (mm)
AIRHEAD	30.0	23.1	Trace
TRAVELLERS	30.8	19.2	0.0
RESIDENCY	25.8	17.9	Trace
GEORGETOWN	31.0	22.9	0.0
ST. HELENA	24.3	15.8	6.2
FALKLANDS	23.9	3.8	7.9
UK (Brize Norton)	10.6	-3.7	30.0

Past Week’s Weather

UK

The cold spell continued into the start of the week with 8 cm of snow on the ground at Brize Norton but milder air pushed across the country from the west, bringing strong winds and rain which has caused some flooding, especially when linked to the melting snow.

Falklands

After a warm start to the week the wind swung more southerly bringing cooler conditions from near the Antarctic. Showers and more persistent fronts brought spells of rain, low cloud and strong winds. Over the weekend the wind, occasionally gale force, became more westerly bringing warmer weather.

Ascension

Mainly dry but isolated light showers affected some parts, mainly around Green Mountain with sunny spells developing away from the showers.

St. Helena

Sunnier than of late but with some showers, mostly in the mornings, with cloud covering the hills at times. Some heavier showers developed later in the week, clearing somewhat over the weekend.

Rain, compiled by Rayne N Katzendogz

I mentioned that the UK has had rain this week which, combined with melting snow, has caused some flooding, but how much rain is 30 mm? How much water is there in 8 cm of snow?

Another way of expressing rainfall is that 1 millimetre of rain is 1 litre (or 1 kg) of rain per square metre. So if we have 2.5 mm per hour of rain falling (this is classed as moderate rainfall) that means every hour the rain continues, a square metre of ground will have 2.5 litres of rain fall on it. If you work out how big a roof you have, you can see why gutters matter! On one day in April 1985, 145.5 mm of rain was recorded as falling at the Airhead, so in that day over 145 litres of rain per square metre fell.

As for snow, the crystals of snow take up more room than liquid water, so much so that in the UK 1 cm of snow depth contains approximately 1 mm equivalent of liquid water on average. This means that when Brize Norton’s 8 cm of rain melted, it added around 8 litres of water per square metre.

When forecasters speak of light (or in the correct form, “slight”), moderate or heavy rain, and when we talk about showers instead of just rain, we mean specific things. “Rain” technically falls from layered cloud and generally lasts for longer periods, “showers” fall from convective (column) clouds. Slight rain is up to 0.5 mm per hour, moderate from 0.5-4.0 mm/hr and heavy is over 4.0 mm/hr. A shower of rain is generally a short event but with larger amounts of rainfall and bigger rain drops so slight showers are up to 2 mm/hr with moderate being between 2 and 10 mm/hr and heavy 10 to 50 mm/hr.

“Drizzle” is a different beast again and is defined by the small size of the individual drops and how much it reduces visibility. “Drizzly showers” are not supposed to be able to exist, but (thanks to the wonderfully humid air here) are something we experience here quite a lot!

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Met Office  Ascension Island Base