🌦 Weather forecast

The variables #

In meteorology, three variables are used: pressure, humidity and temperature. Air is a very bad thermal conductor and mixes badly. We speak of an air mass; a volume of air where the temperature and humidity are uniform.

Pressure #

It is the weight of the air column above a surface. It is therefore expressed in Newton per square meter or Pascals. 100 Pa = 1 hPa = 100 N / m2 = 1 mb.

Pressure is measured with a barometer. The standard and average pressure is 1013.25 hPa. There are other units:

• The millimeter of mercury: 760 mm Hg = 1013, 25 hPa (Anglo-Saxon system: inch of mercury).
• The atmosphere: 1 atm = 1013.25 hPa.

Above 1015hPa we speak of high pressure or anticyclone and below low pressure or depression (except in some cases: relative depression and anticyclone). The lines of equal pressure at a given altitude are called isobars. Note: at our latitudes, the time of day hardly changes the pressure.

What is the average pressure of the atmosphere at sea level?

Answer

1013.25 hPa

You read a pressure of 980hPa on the barometer, what do you deduce from this?

Answer

We are currently in the low pressure regime

Temperature #

The temperature is measured with a thermometer. The standard and average temperature is 15°C. For the same volume, warm air is lighter than cold air. Cold air descends while warm air rises.

Humidity #

Humidity is the quantity of water vapor (water in a gaseous state) contained in the air. We distinguish two humidities:

• The absolute humidity: mass of water vapor in 1kg of air. Expressed in g water vapor/kg dry air.
• Relative humidity: The relative humidity is that of “every day”. It is measured with a hygrometer. It is the quantity of water vapor that a mass of air contains compared to the maximum quantity that the same mass of air (same temperature) can contain (until saturation of this quantity of air). It is expressed in %.

Two air masses with different properties (temperatures and humidities) do not mix or mix only slightly. Depending on the temperature of the air mass, the maximum allowable amount of water vapor before saturation varies. This variation is represented by the Mollier diagram. Finally, the dew point is the temperature for which the air mass must be cooled (at constant pressure) for it to saturate (100% relative humidity).

The atmosphere #

There are different layers in the atmosphere (from the ground to space):

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere

• 78% nitrogen
• 21% oxygen
• 1 to 4% water vapor
• Less than 1% carbon dioxide, ozone, sulfur or nitrogen oxides, rare gases, suspended particles

The troposphere is the only atmospheric layer that interests us. In fact, it alone represents 90% of the mass of the atmosphere. The characteristics of the troposphere are :

• Variable thickness around 12km. Varies according to the season and location: 8 km at the poles to 15 km at the equator.
• Regular drop in temperature: 6.5 ° C per km.