In this chapter, basic information on the measurement of meteorological variables is provided, including examples and recommendations. The variables include wind speed and direction, rainfall, intensity of solar radiation, etc.
The basic examples present solutions, which are suitable for the measurements done by the average users; these are not professional meteorological measurements, although methodological rules of the Czech Hydrometeorological Institute are respected, wherever possible.
Meteo sensors should be installed in an open area, which is not overshadowed by trees and buildings, if possible.
The outdoor temperature and air humidity is measured at the height of 2 m above the ground, the ground minimum temperature is measured 5cm above the ground. The sensors should be placed so as not to be affected by the radiation component (the sensor must not be exposed to direct sunlight) - the small shields of cheap meteo sensors are generally unsuitable and when they are lit by the sun, the error of the measurement is extensive. A precondition for accurate measurement is also adequate air flow around the sensor (the installation should avoid various nooks, alcoves, etc.). When you are measuring in the open area, it is recommended to utilize the Stevenson screen (also called the instrument shelter). It is a white wooden or plastic box, with double louvered walls, a roof and perforated bottom, which allows natural ventilation. It is painted both from the outside and the inside with a white glossy paint).
The speed and direction of wind is measured at 10 m above the ground (the so-called surface wind). The speed of wind measured lower than 10 m above the ground must be recalculated with a correction coefficient for meteorological purposes.
If the required height of measurement above the ground cannot be met, a correction factor is used for the calculation of the speed of wind as per this formula:
V10/Vh = 1/(0.233 + 0.656 * log10(h+4.75))
V10/Vh the correction factor, by which the measured wind speed is multiplied.
h the height of your sensor above the ground in meters (e.g. if your anemometer is 5 meters above the ground, V10/Vh will be 1.134).
Precipitation is usually measured 1m above the ground.
Precipitation is classified according to its rate:
very light rainfall < 0.25mm/hour
light rainfall > 0.25 mm/hour and < 1.0mm/hour
moderate rainfall > 1.0 mm/hour and < 4.0 mm/hour
heavy rainfall > 4.0mm/hour and < 16.0mm/hour
very heavy rainfall > 16.0mm/hour and < 50.0mm/hour
torrential rainfall > 50.0mm/hour
For more information on temperature sensors, see Chapter 10.
Conversion of expressing temperature:
Absolute temperature (Kelvin scale):
Temperature can also be expressed in Fahrenheit scale:
The air pressure is measured absolutely, and subsequently it is converted into pressure which is relative to the sea level.
The unit of pressure used in meteorology is hPa (a hectopascal, before it was a millibar, mbar).
1 Pa=1 N.m-2. (a Newton per square meter), or 1 hPa=100 N.m-2.
Absolute air humidity :
indicates the amount of water vapour in grams in 1 m3 of humid air (g.m-3). In temperate latitudes and in lower layers of atmosphere, the values of absolute humidity fluctuate around 5 g.m-3, in summer up to 15-20 g.m-3.
It is the ratio of the actual content of water vapour in a particular volume of air to the maximum possible water vapour content at a given temperature. Relative humidity is expressed in %. 100% RH means that air is saturated with water vapour (the temperature at which the vapour contained in the air becomes saturated and is called the dew point).
It represents a bundle of virtually parallel rays coming from the sun. The basic phenomenon in the description of the direct solar radiation is its intensity I, which is defined as the quantity of radiant energy that during a time unit hits an area unit oriented perpendicularly to the sun rays.
The rainfall shows how many mm of rain will fall in an hour, if the current intensity of rain is maintained.
The wind chill
Mathematically, it is possible to express the perceived temperature on the surface of the body at a certain air temperature and wind velocity by the so-called wind chill factor. Wind chill expresses the cooling effect of wind acting on the body surface. E.g. at the external temperature of 10 °C and the wind speed of 30km/h, the perceived temperature on the surface of human body is only 3 °C. If the outdoor temperature is -10 °C and the wind speed is the same, the perceived temperature on the body surface is as little as -26 °C. Of course what applies here is a direct correlation between the wind speed and the loss of heat.
This value also takes into account the effect of wind on our perception of the outside temperature. When the temperature is below 37 °C, human body heats up the surrounding air. If there is no wind, the heated air does not move and thus creates a kind of insulating layer around the body. Once the wind starts blowing, it blows away the warm air and the feeling of cold increases.The effective (perceived) temperature is calculated on the basis of the actual temperature and the wind speed according to the following formula: :
WCT = 13.13 + 0.62 * T - 13.95 * V0,16 + 0.486 * T * V0,16
WCT=effective temperature, T=real temperature, V=the speed of wind
Thermal comfort is a relative notion. Thermal comfort depends on the physical conditions and human activity. If a person is not too warm and does not feel cold, it can be said that s/he is in a state of thermal comfort. A basic condition for thermal comfort is adequate air temperature in the room, but it is not the only condition. The temperature of the room utilities, walls and humidity of air are also important factors. If the temperature of air is e.g. 20 °C, the surface temperature of the walls should not drop below 18 °C. At a lower temperature of surfaces, the air temperature would have to increase; this would cause condensation of the water vapours on the walls, and the thermal comfort would deteriorate. Insufficient thermal insulation of walls results in a low surface temperature. A recommended relative humidity in rooms is from 30 to 50%. When the humidity is lower, evaporation from human bodies increases, which cools them down; on the contrary, at higher humidity levels water evaporates badly, which results in sweating. During ventilation, the relative humidity is increased by cooling the air. Heating the air decreases the relative humidity, so it is therefore advisable to increase it by evaporation of water from e.g. a vaporizer.
The difference of the surface temperature (walls, floors, windows, doors and equipment of the rooms) and the air temperature should not be higher than 4 °C. The sum of these temperatures should be around 38 °C.