Temperature is thequantity that objectively characterizes the subjective sensation of or cold left by the air.
Temperature measurement and thermometers
The temperature is measured using athat most often uses the of a body ( usually) placed in a thin tube (which amplifies the effect of dilation). The unit used in the international system is the degree (° C).
In this system, the difference between thelevel at the freezing point (physical) of the water and the boiling point of the water is 100 ° C (for standard ). The Fahrenheit scale is also used (0 ° C = 32 ° F and 100 ° C = 212 ° F).
Other types of thermometers exist, such as thethermometer, the bimetallic thermometer … The alcohol thermometer can also be at a minimum, at maximum or a minimum-maximum.
A weather station includes five thermometers:
- A first thermometer that measures the temperature.
- A second vertical thermometer whose tip is surrounded by fabric with a tip soaked in water and which measures the wet temperature. The difference between these two temperatures makes it possible to calculate the moisture content of the air.
- A third, oblique thermometer measures the maximum temperature: it is designed as a medical thermometer. It thus has a constriction of the capillary above the tank, which prevents the liquid from going down when the temperature drops after reaching its maximum value.
- A fourth thermometer, horizontal, measures the minimum temperature. It has an index embedded in the liquid. The index goes down with the liquid but does not go up.
- A fifth thermometer, at a minimum, measures the minimum temperature on the ground. It is placed out of the station, 5 cm from the ground.
These thermometers can be replaced by a recorder thermometer in the station, a recorder thermometer on the ground and a recorderin the station.
Moisture measurement and hygrometers
There are two definitions of moisture:
- Absolute humidity: number of grams of water vapor contained in one cubic meter of air.
- Relative humidity: the ratio of the effective pressure of the water vapor to the maximum pressure.
This second definition is of interest to the meteorologist. Relative humidity, expressed as a percentage (%), indicates the moisture content of the air. Schematically, at 0%, the air is dry. At 100%, the air is saturated with water: it cannot contain more water and the formation of water droplets is imminent.
We have seen in the section on the measurement of the temperature, that the humidity can be calculated from the difference between a temperature measured classically and a measured temperature called “wet temperature”: it is the method of the psychrometer. It is also possible to measure moisture directly, using hygrometers.
hygrometers use the property of human hair that lengthens when it becomes wet. The hygrometer measures the (physical) dew point of air humidity by cooling a metal surface.
Precipitation measurement (rainfall) and rain gauges
Rainfall is the waterfall on the ground: rain, snow, hail. Their measurement is called rainfall. Precipitation is measured with a rain gauge. The latter measures the thickness of water that has fallen into place, i.e., millimeters (the water must not flow, seep or evaporate). A thickness of one millimeter corresponds to one liter distributed over one square meter.
A rain gauge is a conical container (funnel-shaped) graduated in millimeters of water (the graduations are therefore closer in the lower and narrow part than in the upper and wider part).
precipitation (snow or hail) will be melted to be measured.
Outsideperiods, the recording rain gauge can be used. The latter memorizes each of water received. The study of the pluvio gram thus gives a precise representation of the precipitations, both regarding quantity and temporal distribution.
A simple rain gauge costs less than 10 euros. A larger model, and therefore more accurate, costs around $50. Be careful, the simplest models are afraid of frost and rarely last more than two winters. An electronic rain gauge costs a hundred euros.