Monday, February 19, 2007

The Hydrological Cycle and Gardeners

Up early this morning and into my workboots before 7am. That it was tipping with rain only added to my excitement. It's going to be a "soft day" as we'd say in Ireland, meaning rain falling softly on your head, constant and cleansing from clouds so low it looks like the mist has rolled off the Dales and into my garden, just for my delight. The first time I flew into Ireland, I was stunned by the vast rolling greenness, hardly surprising when you remember your geography lesson about the water cycle, properly called the hydrological cycle.

There are three main types of rainfall - convectional rainfall, frontal rainfall and relief rainfall. Convectional rainfall may occur in Britain in the summer, after a long hot day, but is most commonly found in places with warmer climates. The ground or water in lakes or seas is warmed by the sun, throughout the day. The air above the land becomes heated. This makes the air less dense, so it rises. As the air rises it cools. Cool air can not hold as much water vapour as warmer air, when the air becomes too cold for the amount of water it holds condensation occurs. The point where this occurs is called the dew point. These drops of condensation form into clouds, gradually becoming tall thunderstorm clouds, called cumulonimbus. When these clouds become too big, containing too much water gravity forces them to release the water in a huge downpour.
Frontal rainfall is more common in the UK than convectional rainfall. Frontal rainfall gets its name because it occurs when two air masses of warm and cold air meet, causing a front. When they meet, the less dense, light warm air is forced to rise above the denser, heavier cold air. As the warm air is forced upwards it cools. When air rises above the dew point, when it can no longer hold all its water, the water starts to condense and form clouds. Precipitation falls over a wide area.

Relief rainfall or orographic rainfall is common in the west of Britain. The
prevailing winds blow moist air from the Atlantic Ocean to the west of Britain onshore. As the air hits higher land, such as the hills and mountains in Wales and the north-west of England, the air is forced to rise. When air rises above the dew point, it can no longer hold all its water, which starts to condense and form clouds. This type of "rainfall" is called relief because it is affected by the lie; or "orographic" because it is affected by mountains. And this explains why Ireland (or West Britain if analysing 17th century Irish history), is so green.

Hydrological cycle revised, I headed out into the garden to check the winter standing brassicas. Cavolo nero looking magnificent, one or two red cabbages still hanging on in there, but the purple sprouting broccoli has at last come into its own. Thigh high, these vegetables are just calling out to be picked, their leaves squeaky and stiff, their magnificent, glaucous leaves holding the rain drops and looking just so enticing.

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