Green Mountain, Rain and the Dewpond; A Happy Family. Compiled by Weather Guesser
I think it would be a safe assumption to say that everyone on the Island knows of Green Mountain. It would also be fair to say that the vast majority of people will have been up there, a lot of these people would have walked up to the dew pond, and even a few brave or daft individuals will have gone for a dip in its stagnant waters. But how many of you have wondered how the dew pond stays in existence on a hot island, even though there appears to be no external source of water to replenish it?
“A dew pond is an artificial pond usually intended for watering livestock. Dew ponds are used in areas where a natural supply of surface water may not be readily available. Despite the name, their primary source of water is believed to be rainfall rather than dew or mist.”
So rainfall is the key here. Ascension, for such a small island, has an incredible variation in rainfall. Over the last couple of years Green Mountain received 5x as much rainfall than at the Airhead and over 8x as much than George Town, even though they are all less than 3 miles apart. Clearly the mountain has a part to play in this, which begs another question; why is it so much wetter around Green Mountain than across the rest of the island?
To understand why, a bit of boring meteorology is involved. Please bear with me here, I think if we try to keep your spirits high and my descriptions brief, we can get through this bit together…
In general the atmosphere cools as you get further away from the surface of the earth. However, here on Ascension we are special, if you were to go directly up from where you are standing (or sitting), the air would actually start to get warmer as you passed the 5000ft mark. This is known as an inversion. The inversion over Ascension is our friend; It sits there all year round minding its business keeping our rain generally light and infrequent. It is only when it disappears for a few days we miss it, as this can lead to torrential downpours and possible flooding, as was witnessed in April last year.
An inversion acts as a lid in the atmosphere; any warm air that rises from the surface will cool as it does so. When it reaches the inversion it is cooler than the air above it so it looses it buoyancy and stops rising.
So now we take Green Mountain – all 2800ft of the beast. When the winds hit the side of the mountain they are forced up and over the top of it, effectively squeezing the air between the top of the mountain and the bottom of the inversion. As a result a cloud is born (which is why even on largely cloudless days there is usually still some cloud loitering around the top of Green Mountain), this cloud can grow and produce rain over the mountain, topping up the dew pond as it does so. It rains only very locally to the mountain because as soon as the wind passes over the top of the mountain it is no longer being forced upwards, so the cloud looses its moisture source and kind of just ‘floats off’ down wind. This effect can be seen if you look to the north west of Green Mountain. On most days you will see a ‘stream of cloud’ that appears to never really move but can extend up to 70 miles down wind of the mountain itself.
So the next time you want to go to the beach and it starts raining, don’t be annoyed, be thankful that it is keeping legacy of the dew pond going – and the chances are it probably won’t be raining at the beach. But, more importantly, don’t blame the weatherman, blame Green Mountain!