Various forms of rain (From Worcester News)

Various forms of rain

9:09am Tuesday 17th July 2007 in News By Paul Damari

Rain is merely one phase in the perpetual recycling of the world's constant supply of water. Vapour in the air condenses as cloud, from which falls rain, snow or hail. Precipitation forms in various ways, the main one's being frontal, convectional and orographic rain.

Frontal rain is caused by different air masses, they do not mix easily so there is a boundary between those two contrasting air masses. When both bodies of air are moving in the same direction, usually at different speeds, the front slopes as warm air rides over cool or colder air ahead of it. If air behind the front is warmer than that ahead of it, it is a warm front. If the air behind the front is cooler, it is a cold front. Rain along an active warm front falls for hours well ahead of the frontal source. Rainfall along a cold front can be heavy, even violent with squally winds but only last for a shorter period.

Rain caused by convection is very local and showery in nature, inside a cloud air rises rapidly by convection, then it cools and sinks again, in "convection cells". If conditions are unstable they will continue to develop and expand until they produce showers or storms.

Orographic rain occurs when moist milder air is forced to rise up slope, in the British Isles that's often along western hill slopes or mountain ranges. The air rises, cools and then condenses into precipitation, the rain, or if it's cold enough snow, can become more intense over these western up slopes.

Next Week.....Air Masses

The answer to last week's question.....16.400 per cubic inch.

This week's question:

Orographic rain is formed over:A/ Hills and Mountains.

B/ Valleys and Plains.

Good Luck.

Glossary of Common Weather Terms: Rain: Liquid precipitation, the individual droplets measuring up to 6mm (0.24 inches) in diameter.

Weather for period Monday 16th July-Friday 20th July

No sign of any real prolonged summer weather with low pressure again near to the British Isles.

A week of sunshine and local storms, hail, thunder and squally winds in some of them. But having said that, longer drier, brighter spells will occur between them through mid week. Risk of more general rain later Friday.

Maximum day temperature 20-22c 68-72f.

Minimum night temperature 13-15c 55-59f.

Last Week's Observations (Monday-Friday)

Highest day temperature......24.9c 77f on Thursday 12th July.

Lowest night temperature ...10.9c 52f on Monday 9th July.

Wettest day........................8.0 mm on Friday 13th July.