Atmospheric Pressure, Weather Dynamics and the Foehn Effect

Atmosphere and Atmospheric Pressure

Atmospheric pressure: The weight of the air exerted on the Earth's surface. Air pressure is measured in millibars, which is why atmospheric pressure decreases rapidly with altitude. It also varies horizontally with latitude.

Geothermal variation: Variation of temperature, noting two points situated at an altitude difference of 100 meters.

Anticyclones: Regions of high pressure where air circulates, due to the Coriolis force, in the direction of clock hands in the Northern Hemisphere, and in the contrary direction in the Southern Hemisphere.

Depressions: Areas where air circulates in the opposite direction of clock hands in the Northern Hemisphere, favored by the Coriolis force. They usually bring bad weather, called low-pressure systems (borrascas), cloudy skies, and precipitation.

Coriolis force: A force caused by the rotation of the Earth that causes the trajectories of objects moving on the Earth's surface to curve.

Air Quality and Pollution

Air quality: Standards set to manage the frontier between clean and contaminated air.

Emission level: The amount of each contaminant released into the atmosphere in a determined period of time.

Susceptibility level: The maximum limit of contaminants tolerable for humans, animals, plants, and soil.

Air contaminants: Air pollution occurs when its composition presents one or more foreign substances harmful to living beings.

Acid rain: The incorporation of acidic substances into rainwater. This pollution is caused by nitrogen and sulfur oxides that increase the acidity of precipitation.

Erosion of the ozone layer: The decrease of the ozone layer in the stratosphere due to the action of certain contaminant gases, such as chlorofluorocarbons (CFCs). The stratospheric ozone layer prevents dangerous UV rays from reaching the surface intensely. This erosion forms holes in some areas.

Structure of the Atmosphere

Atmosphere: The gaseous layer that surrounds the Earth and protects it, being in contact with the hydrosphere, geosphere, and biosphere.

Composition: Formed by a mixture of gases called air and particulates in suspension, such as pollen and dust. The most abundant gases are nitrogen, oxygen, carbon dioxide, argon, methane, hydrogen, ozone, and water vapor.

Structure: It is divided into three different layers based on temperature and composition.

The Troposphere

Troposphere: The region closest to the Earth's surface and the densest zone of the atmosphere. The initial 500 meters are known as the dirty layer due to dust in suspension from deserts, pollution, and eruptions. It acts as a condensation nucleus, facilitating the transition of atmospheric water vapor into liquid water.

Atmospheric or meteorological time: The characteristics of the atmosphere at a specific moment.

Dew point: The temperature to which a determined volume of air must be cooled so that the water vapor it contains saturates.

The Foehn Effect

Foehn effect: A curious and well-known phenomenon of marked local character, also known as the Foehn, Fogony, or Fagueño, depending on the region. This atmospheric mechanism can have significant consequences, distorting thermal comfort in valleys. Valley temperature conditions can be very capricious, depending on valley orientation, depth, and morphology (whether valleys are of glacial or fluvial origin). Additionally, stable meteorological situations can produce temperature inversions that break the normal thermal behavior of the atmosphere.

The Foehn effect can rapidly melt snowpacks on the leeward side or cause sudden drops in humidity in affected valleys. Let's see how it works. The term Foehn comes from alpine valleys, where it is very well known in mountains where the wind produces this phenomenon. In our regions, we know it as the Fagueño or Fogony in the Pyrenees.

It is a very surprising meteorological situation for anyone who experiences it, consisting of a sudden and abnormal rise in temperatures and a sharp drop in relative humidity at low levels (sometimes down to 10%) of the valley's depth. To explain how this phenomenon works, we can look at a diagram:

• The Foehn effect is always linked to a humid wind flow directed towards the windward slopes of a mountain.
• After reaching the windward skirt, the forced flow ascends slowly.
• The cooling rate of moist air with height, known as the moist adiabatic gradient, is 1°C per 100 meters.
• For example, if our air is at 15°C at the base of the mountain range (located 200 meters high), when going up to 600 meters, the air cools to 11°C.
• This temperature drop reaches the dew point, where water condensation begins in the moist air, forming clouds and eventually precipitation.
• Once saturated, the air continues its journey towards the summit, but now cools at a lesser rate of 0.5°C per 100 meters.
• Upon reaching the summit at 1,000 meters, condensation processes stop because the air is no longer saturated, but rather dry (having dropped its moisture load during condensation).
• As it descends the leeward slope of the valley, its temperature increases by 1°C per 100 meters.
• Its temperature will rise, reaching 17°C at the valley floor (at the same altitude as the base on the windward side, 200 meters), having warmed by 2°C during its journey and lost its moisture burden.

This proposed scheme simplifies more complex differences, but in specific Pyrenean valleys—such as the Val d'Aran—when a northerly wind blows strongly, temperatures can rise by up to 10°C while humidity drops to almost unbelievable values in a very short time.

This meteorological situation is especially dangerous because the rapid Foehn effect has occasionally been responsible for dangerous ice melting on leeward slopes in the presence of persistent windward flows.