Where should anemometers and automatic weather observation stations be located?
Observation sites across the country are set some distance from buildings and have lots of open space around them. Why are meteorological observation sites established in open areas?
Because various weather phenomena such as rain, snow, frost, fog, etc., are caused by changes in air flow and temperature, which cause the water vapor contained in the air to condense. Therefore, to understand the changes in weather, we must first understand the properties of air. The properties of air are greatly affected by the environment. For example, indoors, the temperature in winter is higher than that outdoors, and in summer the temperature is lower than outdoors; similarly outdoors, the temperature in the south of the house is higher than in the north; when the wind blows, the temperature between buildings In the passages between places, the wind force is often stronger than in open areas.
An anemometer is a speed measuring instrument that converts flow velocity signals into electrical signals. It can also measure fluid temperature or density. The principle is that an electrically heated thin metal wire (called a hot wire) is placed in the air flow. The heat dissipation of the hot wire in the air flow is related to the flow rate. The heat dissipation causes the temperature of the hot wire to change, causing the resistance to change. The flow rate signal is converted into electric signal. It has two working modes:
①Constant current type. The current passing through the hot wire remains constant. When the temperature changes, the resistance of the hot wire changes, so the voltage at both ends changes, and the flow rate is measured from this;
② Constant temperature type. The temperature of the hot wire remains constant, for example, 150°C, and the flow rate can be measured based on the current required to be applied. The constant temperature type is more widely used than the constant current type. The length of the hot wire is generally in the range of 0.5 to 2 mm, the diameter is in the range of 1 to 10 microns, and the material is platinum, tungsten or platinum-rhodium alloy, etc.
Why are meteorological observation sites established in open areas?
This special situation caused by environmental influences is only useful when studying microclimates. In order to obtain accurate meteorological element data and make accurate weather forecasts, we must first understand the original appearance of the air, so that environmental influences must be avoided. How can we avoid environmental impacts?
This requires that the weather station observation site be surrounded by open space so that air can circulate freely. Secondly, in the observation site, it is necessary to measure sunshine, ground temperature, wind and rainfall, etc. In order to avoid sunlight being shaded and to avoid wind and rain being blocked by surrounding buildings, the surrounding area of the site also needs to be open. Third, in the observation field, it is necessary to observe clouds and visibility, as well as distant weather phenomena. If there are buildings around, which blocks the line of sight, it will be impossible to observe these phenomena.
