Weather Glossary

The wind speed can be displayed in different measurement units:

- Kilometre per hour (km/h)
- Metre per second (m/s)
- Knot per hour (Kt/h):
- Note: 1 knot per hour = 1,852 km per hour = 0,515 m per second

- Beaufort (from 1 to 12; ie details in Beaufort)

BAROMETER
The barometer is the basic meteorological instrument. Its main function is to measure atmospheric pressure. Atmospheric pressure and especially its foreseeable evolutions are the determining factors for generating short- and medium-term weather forecasts. Even if atmospheric pressure remains the indispensable measure, other natural events must be observed and measured to generate high accurate trends.

Barometer operating principle is based on a physical experiment: the pressure at a specified point of a fluid (like air, water, or mercury) reveals the intensity of the force exerted by the fluid on a specific surface, whatever the orientation of this surface in space is. If we decide to position this surface horizontal, the force, for a quiescent vacuum fluid, is nothing but the backbone weight of the fluid overcoming the surface. So, measuring the force exerted by the fluid means measuring this weight...that is to say the backbone weight of the fluid overcoming the horizontal reference surface. Of course, this applies to the pressure exerted by the air at each point of the atmosphere and/or of Earth Planet.

In mercury barometers, a vertical glass tube extends the quiescent mercury volume in contact with free air. The air pressure on a horizontal level associated to the tube surface is the same everywhere, and this pressure is atmospheric pressure. Thus, the mercury column weight inside the tube at this level is the same as the air column weight that would overcome a surface with a section equals to the one of the tube, through the whole atmosphere. So, measuring atmospheric pressure means measuring this weight...that is to say the mercury column height from the surface level, since this height is independent of the tube section.The standard atmospheric pressure, fixed at 1 013,25 hPa, is equivalent to 760 mm of mercury height.

Invented in 1643 by the Italian scientist Torricelli, mercury barometer figures among the oldest instruments used in meteorology. Mercury was used (before being considered as dangerous) for it was the heaviest liquid ever tested: therefore shorter glass tubes could be used.

Later aneroid barometers were widely used, they were so called because its functioning was based on capsules movements amplification (unique or stacked up for more precision) after vacuum inside those capsules was made.

BEAUFORT
This measurement unit was created by the British Admiral Francis Beaufort (1774-1857) and has been used since 1874, especially by the navy. It indicates in a simple way the wind power at a standard height of 10m above a flat and covered soil.

The Beaufort unit has its equivalents in km/h and knot (kts).

WIND VANE

The wind vane was invented by Leonardo da Vinci. It positions itself in the wind direction, then mechanically or electronically indicates it on a compass graduated North, North-West, West, etc. The wind direction also expresses itself in degrees on this same compass graduated from 0? to 360?. Then the North is 0? as well as 360?. The graduation turns clockwise, so that the East is at 90?, the South at 180? and the West at 270?.

HUMIDEX OR COMFORT INDEX

This index is calculated from the combination of the measured relative humidity level and the effective temperature: the humidex or comfort index allows evaluating the comfort of your environment.

If the indoor temperature is between +20?C and +26?C and the relative humidity is between 45% and 65%, then the comfort level is satisfying. Outside these ranges, the comfort level becomes mediocre then bad and finally dreadful...the further off these ranges it moves.

HUMIDITY

Searching for humidity value to evaluate the quantity of water in the air. So electronic devices try to determinate the relative humidity level. Talking about humidity means also talking about the humidity felt at a specific time, at a certain place, by one or several people.

We will talk about a humid environment where the air is almost at saturation, even saturated, and about a dryness feeling where on the contrary the relative humidity level reveals an appreciable steam deficiency compared to what would be acceptable at the same place (and at the same temperature) before condensation starts.

Thus, humidity is a quantifiable meteorological unit, but also a feeling, rationally subjective.

HYGROMETRY
The humidity measurement is hygrometry and its measurement unit is the percentage (%)...of the relative humidity of ambient air.
This relative humidity gives, in percentage, the ratio between the quantity of steam effectively absorbed by the air and the maximum quantity that could be absorbed at the same temperature. However, it is advisable to distinguish:

Absolute humidity (corresponding to the quantity of water (in weight) contained at a specific time in a certain volume of air)

Relative humidity (percentage corresponding to the ratio between the existing water weight and the maximum water weight that could contains the air mass)

METEOROLOGY

Meteorology is the science which aim is to study and experiment atmospheric phenomenon and its laws. This discipline is based on accurate scientific observations resting on physical laws.

It belongs to our day life since weather forecast, the most famous branch of meteorology, has an influence on our behaviour in many ways. From the choice of what to wear to influences on human activities (agriculture, aeronautics, etc.), meteorological phenomenon are very important for the day life.

Despite being able to modify them when they are detrimental, the human being has been trying to study and forecast them. The climate is defined by the same parameters as " what is the weather like?": the temperature measured with the thermometer, the air humidity level (hygrometry) measured with the hygrometer, the precipitations measured with the rainmeter, the wind force and speed measured with the anemometer, etc...

RAIN

The measurement unit of pluviometry is the millimetre (of rainfall...liquid water).

The device designated for measuring the precipitation height is the rainmeter. It is a receptacle with a specific surface collecting a certain quantity of water that felt during a specific period of time: a mathematic operation which is sometimes inaccurate allows graduating simple rainmeters, so called funnels. Electronic rainmeters use the reference units counting to upgrade the rainfalls.

Rainmeters must always be placed in non-sheltered areas, away from trees, houses or walls.

DEW POINT

During the night, with a generally clear weather and no major disturbance, the soil becomes colder and transmits this cooling to the immediate air layers, which temperature is step by step dropping. If the very low layers night cooling goes on with a sufficient persistence, the saturating vapour pressure contained inside these layers will not stop dropping as the temperature decreases. The steam will start condensing and deposit dew on the soil: the achieved temperature level is called dew point

ATMOSPHERIC PRESSURE

The measurement unit of atmospheric pressure is hectopascal (hPa) : 1hPa = 100 Pa (100 Pascal).

Atmospheric pressure changes accordingly with the localisation and the temperature.The standard pressure is 1013,25 hPa at 0 m altitude. At 500 m altitude, 954,5 hPa and at 1000 m altitude 899 hPa, etc....The pressure changes from 1 hPa every 8 meters.

That is to say moving a barometer from the ground floor of a building to the 10th floor can immediately modify a forecast. When the pressure increases the weather should get better, when the pressure decreases the weather should get worse. Improvement and degradation are always referred to the former situation...

TEMPERATURE

The temperature measurement units are firstly the degree Celsius (°C) then the degree Fahrenheit (?F). Thermometers allow measuring the temperature, but also temperature changes. The European measuring system is the Celsius system, where water freezes at 0°C (freezing point) and boils at 100°C (boiling point).

Anglo-Saxon countries use the Fahrenheit scale, where the freezing point is at +32?F and the boiling point is at 212°F. The conversion factor between °C and °F is calculated as follow:

°F = (1,8 x °C) + 32 OR °C = 0,56 x ( °F - 32)

WINDCHILL

Windchill is the temperature such as we feel in certain conditions, especially in low temperature and strong wind. So windchill is a different temperature (generally lower) than the real measured temperature.

Around the skin, a thin air layer warmer than ambient air is generated, because the body supplies a certain amount of heat. The wind sends away this warm air layer and then the skin is not protected anymore. Therefore the stronger is the wind, the colder we feel...And the temperature felt by Mister X will then be different than the one felt by Mister Y, depending on what clothes they are wearing.