Rain, wind and (perception of) temperature

The weather is probably one of the most discussed subjects in the Netherlands. Furthermore, it is often a ground for complaint. The elements that govern the weather are rain, wind and temperature. How are they generated and how do we experience them?

Rain

Around the equator, hot damp air goes up and spreads at great heights, comparable to the way smoke does. Once the air drifts farther away from the equator, it cools down and drops again. As hot air can retain more moisture than cold air, reaching cooler regions may cause the air to release the moisture and rain to start falling.

The ability of hot air to retain a great deal of moisture accounts for many immensely fierce and  heavy rains in the tropics in midsummer, once the limit has been reached and the air is cooling down.

Wind

However unlikely it may seem, air has a considerable weight. Yet we do not feel pressure because our body is mainly made up of substances that are not compressed and counterbalance this pressure.

However, as soon as air starts moving, it suddenly becomes clear that it has a considerable mass. There are about 15.5 tons of air over each square kilometre of the earth. When those millions of tons rush past at a speed of 50 to 60 kilometres (31 to 37 miles) an hour, it is not a great surprise that slates are blown off roofs and branches snap off.

Air begins to move when there are differences in atmospheric pressure, which in turn are a result of the uneven distribution of solar heat over the earth. As air seeks to be in balance, it floats from high-pressure areas to low-pressure areas. The greater the differences in pressure, the windier it is.

Temperature

Heat is generated by the activity of molecules. Sunlight activates the air molecules of the earth, causing collisions of small particles resulting in friction and heat. At sea level air molecules are close to each other, which causes a lot of collisions. However, air at greater heights is thinner and contains fewer molecules. At a height of 80 kilometres (50 miles), air is so thin that the distance between two random molecules may be as great as a few kilometres (1.24 to 2 miles), resulting in lower temperatures up in the air than below. On average, temperature drops by 6.5° Celsius (11.7° Fahrenheit) every kilometre (0.62 mile) higher up in the air.

Perception of temperature

Apart from the actually measured temperature, there is also a personal perception of temperature. It is a well-known phenomenon that when there is a wind blowing, temperature feels colder than the thermometer indicates. Damp air, too, can make the weather feel colder under a specific temperature or, conversely, warmer above a specific temperature. There are various methods to assess perception of temperature, which are considerably divergent mutually. The most used methods are Steadman's Wind Chill and Heat Index. The Wind Chill is of use especially under 5° Celsius (41° Fahrenheit) and the Heat Index particularly in temperatures above 25° Celsius (77° Fahrenheit).

In low temperatures the body will increase its metabolism to release energy. Shivering with cold and chattering teeth are activities of the body to boost metabolism. Furthermore, the pores of the skin are closed to retain body heat, resulting in goose pimples.

In high temperatures the opposite happens: the blood vessels widen and muscular activity is reduced, arousing a lazy and sluggish feeling. Furthermore, the pores of the skin open as far as possible to get rid of heat through perspiration.

The atmospheric humidity has a double influence on heat or cold: it prevents the evaporation of perspiration, so the drainage of body heat and damp air feels colder or, conversely, warmer than the drainage of dry air.

As long as ambient temperatures are lower than that of the skin (33° Celsius or 81.4° Fahrenheit), damp air will feel colder for an uncovered body. Because clothing hampers the exchange of heat and moisture with the surroundings, the turning point is considerably lower, viz at 25° Celsius (77° Fahrenheit).

Commentaar

Hoewel dit stuk al een keer is gekeurd, zou ik het op prijs stellen als de fouten alsnog met rood worden aangegeven omdat de markering door een technische fout de eerste keer niet werd weergegeven. Ik weet dus nog steeds niet waar de fouten zitten of welke kritiek er is gegeven. Ik hoop dat het niet teveel gevraagd is...  

Sources: www.todio.nl