What is so nice about ‘nice weather’? The temperature? But what if it’s raining while the temperature is 30 degrees? So precipitation is also important. And what if the sun isn’t shining? The cloudiness is also an important factor then. What if a sharp wind is blowing? The weather is the ensemble of phenomena, like temperature, humidity, clouds, precipitation, wind and more extreme phenomena such as thunderstorm, whirlwind and freezing rain. The combination of these factors and many more determines the weather. Here comes the description of some of these factors.
Precipitation is any kind of water (solid or fluid) that falls from the sky. This includes rain, snow, hail, sleet and freezing rain. Precipitation can’t occur in all clouds. The diameter of a raindrop is 2 mm, while the diameter of a vapour drop in a cloud is only 0,02 mm. Logically, about one million cloud drops form one rain drop. To explain what sort of precipitation comes from what sort of cloud, we need first more information about the process of precipitation forming.
For the forming of precipitation, it’s very important to have a nucleus which gathers cloud drops. Very fine rain, like drizzle, falls from clouds hanging low, but real rain has to fall from clouds situated more high. The forming of such clouds requires something for the vapour to cling to, the so-called condensation nuclei. A condensation nucleus is a tiny particle suspended in the atmosphere, like dust or salt crystals.
In mild and cold areas, it’s important that the cloud contains as well cold drops of water as ice crystals. If the drops of water evaporate around an ice crystal, the crystal starts to grow and it gets so heavy that it falls to the earth due to gravity. During their fall, these crystals take more drops of water and ice crystals down.
If the temperature of the area where the precipitation falls is around zero degrees or less, the crystals reach the ground as snow. If the temperature is higher, it rains. In the Netherlands, 775 mm rain falls averagely per year.
Beside precipitation, wind is a very important meteorological phenomenon as well. Wind is created by a difference in barometric pressure between two air masses (caused by the uneven heating of the Earth’s surface) and tends to flow from the area of high pressure to the area of low pressure until the two air masses are at the same pressure. Wind has a speed and a direction. The speed of the wind is measured by an anemometer and can be given in meters/second, kilometres/hour or in knots. In weather reports, the speed of wind is measured according to the Beaufort wind force scale. The wind can change its direction and speed very quick, which can be dangerous for navigation and aviation.
The temperature expresses how warm or how cold is the air. Even if you don’t have anything to measure temperature, you just feel it. Warm air flows always towards cold air. Temperature can be measured by a thermometer and given according to the Fahrenheit, Celsius or Kelvin scale. The thermometer is based on the principle saying that two substances being in a long-term contact get the same temperature. The substance in a thermometer (e.g. mercury) expands if it stays long in contact with warm air. The expansion of that substance in a column leads to an altitude difference so that the temperature can be easily read off (after a calibration).
Humidity is the concentration of water vapour in the air. The humidity of the air is very important for humans. Just like the temperature, the speed of wind and the strength of sunlight, humidity determines as well if weather is found nice or not. The weather is for example said to be muggy when the relative humidity is 100% and the temperature is 20 degrees, or 80% and 25 degrees, or 55% and 30 degrees or when it’s 30% and 35 degrees. Wind brings generally cooling while sunbeams make it warmer.
Air-conditioning in cars, offices and schools assure an air-humidity of 50%. This percentage is medically too low. This can be unpleasant and can cause a so-called sick building syndrome. Humidity can be also too low in rooms with central heating. If humidity is really low (under 20%), nails and lips can break.
A cloud is a visible mass of condensed droplets or ice crystals suspended in the atmosphere. Cloudiness is the ensemble of clouds covering the sky. If it’s between 12,5% and 25%, it’s lightly cloudy, between 25% and 50% it’s partly cloudy and above 50% it’s mostly cloudy. Clouds can be of many kind and size, and they change constantly.
Basically, we can distinguish ten cloud types:
1. Altocumulus: A middle cloud, usually white or grey, often occuring in layers or patches with wavy, rounded masses or rolls, often called ’fleecy clouds’.
2. Altostratus: blue or grey veil.
3. Cirrocumulus: made of thin, white patches (cloudlets)
4. Cirrostratus: clouds of high altitude forming a transparent white veil.
5. Cirrus: clouds composed of ice crystals, formed at an altitude of 6000 metres and shaped like hair like filaments.
6. Cumulonimbus: huge grey dense clouds often involved in thunderstorm.
7. Cumulus: white clouds often indicating nice weather for the coming several hours.
8. Nimbostratus: a dark grey layer of clouds hanging low, indicating bad weather.
9. Stratocumulus: large dark, rounded masses, usually in groups, lines, or waves.
10. Stratus: horizontal layering with a uniform base, essentially fog that is above ground level.