Its raining, it’s pouring!

As you have probably seen we have witnessed some of the worst weather on Ascension Island in many decades. In fact our records at the Airhead (which go back to mid 1984) indicate we have had the wettest period in 26 years. We have so far this month recorded 164.2mm of rain in the official Airhead rain gauge, which beats the rainfall total from April 1988 (but not April 1985) making it the second wettest April on record. In an average year, only 85 mm would be expected to fall at the Airhead! There have been many impacts of this heavy rain such as flooding.

Despite all this rain, long term rainfall records (CSR site from 1962 & Georgetown since 1924) show that this month is becoming drier, possibly because these extreme events are becoming rarer.

What was the reason for all the wet weather?

The Inter Tropical Convergence Zone is at is seasonal southernmost limit now. This is a band of heavy showers and thunderstorms. It usually sits a little to north of the island but on this occasion it moved slightly further south to affect the island since Friday 15^th April onwards. On top of that we saw a band of low level cloud and moisture (an old cold front) move up from the south at the same time. Thirdly, we have been witnessing a weakening of a temperature inversion (a natural 'lid' on the atmosphere that stops the cloud developing here too much) at around 8,000 to 10,000 feet above us, this allowed the clouds to grow to 45,000 feet and create thunderstorms and heavy showers. The situation developed quickly but has since has been slow moving leading to large single point rainfall totals. It is the right time of year for this type of weather event. The three main heavy rain event periods were Friday 15^th afternoon and night, Monday 18^th afternoon and evening and Tuesday 19^th night to Wednesday 20^th lunchtime. This last event led to only light rainfall at the airhead as the wind direction changed slightly, but giving heavier rainfall this time over Travellers Hill, Georgetown and Two Boats.

Ascension Island April Weather Facts

• The hottest time of the year on the Island is from mid March to mid April, with the hottest days of the month occurring before the 11^th April. Since 1985, the month of April has gradually become warmer by around 0.5ºC.
• April is the most likely month for a thunderstorm on the Island, most storms that have occurred at the Airhead since 1984 have happened in this month.
• On average in April we can expect a total of 38.6 mm at the Airhead. The wettest April on record was in 1985 when 344.4mm of rain was recorded. The driest was in 2001 when just 3.3 mm of rain was recorded.
• In a 12 hour spell overnight on the 29^th-30^th in 1934, 209.6 mm of rain fell in Georgetown.

Compiled by: Weather Witch

Crown Copyright 2011

Met Office Ascension Island base