Rain

Rain

For those of you who have been following our preparations for the Heritage weekend Friday was full of heavy showers which took me to seek refuge in the text books! Here is what they tell us about …..

Rain plays a role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into drops, precipitates (falls) from the sky, and eventually returns to the ocean via rivers and streams to repeat the cycle again.  

A major scientific explanation of how rain forms and falls is called the Bergeron process. More recent research points to the influence of Cloud condensation nuclei released as the result of biological proces

The fine particulate matter produced by car exhaust and other human sources of pollution form cloud condensation nuclei, leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over the course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States’ Eastern Seaboard, the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays.

When classified according to amount of precipitation, rain can be divided into:]

  • Very light rain — when the precipitation rate is < 0.25 mm/hour
  • Light rain — when the precipitation rate is between 0.25 mm/hour – 1.0 mm/hour
  • Moderate rain — when the precipitation rate is between 1.0 mm/hour – 4.0 mm/hour
  • Heavy rain — when the precipitation rate is between 4.0 mm/hour – 16.0 mm/hour
  • Very heavy rain — when the precipitation rate is between 16.0 mm/hour – 50 mm/hour
  • Extreme rain — when the precipitation rate is > 50.0 mm/hour.

Falling raindrops are often depicted in popular culture as “teardrop-shaped” — round at the bottom and narrowing towards the top — but this is incorrect. Only drops of water dripping from some sources are tear-shaped at the moment of formation. Small raindrops are nearly spherical. Larger ones become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes. He found that small raindrops (less than about 2 mm diameter) are approximately spherical. As they get larger (to about 5 mm diameter) they become more doughnut shaped. Beyond about 5 mm they become unstable and fragment. On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid.

Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm drizzle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.

Cultural attitudes towards rain differ across the world. In the largely temperate Europe rain metaphorically has a sad and negative connotation — reflected in children’s rhymes like Rain Rain Go Away — in contrast to the bright and happy sun.  Though the traditional notion of rain in the Western World is negative, rain can also bring joy, as some consider it to be soothing or enjoy the aesthetic appeal of it. 

Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or is extremely heavy (as in a monsoon). Rain may be harvested.

Many people find the scent during and immediately after rain especially pleasant or distinctive. 

 

 

 

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