The colour gradient you see behind your town or city forecast depicts the amount of cloud cover expected during daylight hours - ranging from clear blue to dark grey (overcast). If rain or snow is expected, raindrop or snowflake symbols will also appear in the background. The further down the screen the symbols appear, the heavier we expect the rain or snow to be.
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The amount of water vapour present in a cloud, and the strength of the rising air ('updraft') in that cloud, determines the intensity of rain it can produce. The depth, or thickness, of cloud determines how much water is available.
The heaviest rain is usually associated with very deep clouds such as thunderstorm cells, which may extend from about one kilometre above the Earth’s surface to over 10 kilometres high. These clouds also contain very strong updrafts, which means large drops of rain can be held up in the cloud until they grow so big that the rising air can no longer support them.
Prolonged heavy rain can also fall from very thick, slow-moving stratiform clouds known as nimbostratus.
In New Zealand, the heaviest rain usually falls on the western side of the mountain ranges, where moisture-laden winds are forced upwards by the mountains, creating very deep clouds.
For rain to fall, the water vapour that makes up clouds needs to combine and grow into droplets heavy enough to counteract rising air (‘updrafts’) in the cloud. But the process relies on more than water alone. Tiny specks of dust and other microscopic substances, known as ‘condensation nuclei’ are required. Condensation nuclei are everywhere in the atmosphere. Water vapour wraps itself around them, which begins the process of raindrop growth. As the drop gets bigger, it collides with more and more droplets and growth accelerates until the drop is heavy enough to fall.
Forecasters use a plastic or metal open-topped cylinder called a rain gauge to capture rain. Rain gauges have standard dimensions so that measurements can be compared from place to place. The amount of rain collected is measured over a certain timeframe – usually hourly or accumulated over 24 hours. Measurements can be made manually or electronically. In New Zealand we measure rain in millimetres. Ten millimetres of rain in Wellington means that if all the rain that fell within the city was spread out over hard flat ground (so it couldn’t drain away), it would form a puddle 10 millimetres deep.
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