For those interested in the topic or curious about Meteorology in general
Meteorology
It is the study of the movements and phenomena of the Earth's atmosphere in their relations with weather and climate, with the aim of forecasting the weather, by measuring temperature, precipitation, atmospheric pressure, wind speed and direction, etc.
Clouds are a collection of tiny water particles and/or ice crystals in the air.
When two different air masses come into contact, they don't mix. They push against each other along a line called a front. When a warm air mass meets a cold air mass, the warm air rises since it is lighter.
Through water vapor condensation processes due to humidity saturation, gives rise to the formation of clouds or nebulosities of different types and shapes.
Cloud classification:
High clouds - 6000 to 12000 meters
· Cirrus (Ci) - Isolated clouds - white and delicate filaments - white or almost white banks or narrow bands. Fibrous or silky appearance.
· Cirrocumulus (Cc) - Sheet or thin layer of white clouds, without shadows, made up of very small elements, connected or not and arranged more or less regularly.
· Cirrostratus (Cs) - Transparent and whitish nebulous veil, with a fibrous or smooth appearance, that totally or partially covers the sky.
Medium level clouds - 2000 to 6000 meters
· Altocumulus (Ac) - Sheet or layer of white or gray clouds, generally with their own shadows, made up of sheets. Sometimes partially fibrous or diffuse, connected or not.
· Altostratos (As) - Sheet or layer of grayish or bluish clouds with a striated, fibrous or uniform appearance, which totally or partially covers the sky, and has sufficiently faint proportions for the sun to be seen.
Low clouds - Ground at 2000 meters
· Stratocumulus (Sc) - Sheet or layer of gray or whitish clouds, almost always with dark portions, with a non-fibrous appearance, connected or not.
· Stratos (St) - Cloudy layer, generally gray, with a very uniform base.
When you see the sun through the layer, the outline is clear.
When there is precipitation it's in the form of drizzle.
· Nimbostratos (Ns) – Gray nebulous layer, often shadowy.
The appearance becomes diffused by the more or less continuous fall of rain or snow.
It is thick enough to hide the sun.
Below the layer there are often low, ragged clouds, whether or not connected to it.
Documentary that I always played in my courses
Vertically developing clouds - 500 to 12000 meters
· Cumulus (Cu) - Isolated clouds, generally dense and with clear outlines.
They develop vertically in the form of mounds, domes, towers, which upper region looks like a cauliflower.
The sunlit proportions are almost always a bright white, while the base is really shadowy, and roughly horizontal.
These clouds (Cu) are sometimes tattered..
Cumulus can be further divided into:
· Cumulus humilis
· Cumulus mediocris
· Cumulus conjestus
· Cumulonimbus (Cb) - Dense and strong cloud, of great vertical extent, in the shape of a mountain or huge towers.
The upper region, at least in part, is, as a rule, smooth, fibrous or striated, and almost always flat.
This part often spreads out in the shape of an anvil.
Short movie with beautiful images of clouds by Kym Fielke
Rising air is a key process in the production of clouds and precipitation
Air flows in the atmosphere tending to maintain a certain pressure balance.
Wind is formed by pressure differences in the Earth's atmosphere, which are in turn due to temperature differences.
In actual fact, the Sun heats the Earth's surface unevenly, creating areas of high and low pressure.
Air always moves from areas of high pressure to areas of low pressure, thereby generating wind.
Visual representation of how the wind is created using colored water, activate the video subtitles.
As atmospheric pressure decreases with altitude, there is a force that tends to move gas molecules from high pressures (on the ground) to low pressures (at altitude).
If our planet has an atmosphere, it is because the force of gravity counterbalances this force and prevents molecules from escaping freely into space.
In the absence of other processes that favor the rise of the air, these two forces balance each other (it is called hydrostatic equilibrium) and there is no resulting force that makes the air rise or fall.
It is the processes that break the hydrostatic balance and lead to the rise of humid air that give rise to the formation of clouds.
What origins the rise in air that leads to the formation of clouds?
The rise of air can be produced by convection, by air convergence, by topographic elevation or by frontal elevation.
1. Convection – air bubbles that rise
As the Earth is heated by the Sun, there are bubbles of hot (and less dense) air that rise up like hot air balloons.
They will continue to rise as long as there is instability (as long as their temperature is higher than the air above them).
As they cool and lose their lifting power, they become diluted in the surrounding air.
But there are other bubbles that form next and follow the same path, generally always rising a little higher than the previous ones until they manage to rise enough for their cooling to correspond to the so-called dew point, temperature at which the air saturates.
The moisture inside it (water vapor) then begins to condense into droplets that become visible in the form of a convective cloud (characterized by its rapid vertical development).
Convection implies a transfer of heat from the surface to the atmosphere - the so-called latent heat flow (based on the evaporation and condensation of water).
Every time water changes state there is an exchange of energy - called latent heat.
Evaporation occurs when a molecule is freed from all of its neighbors, by increasing its kinetic energy at the expense of energy extracted from the environment (around 600 calories for each gram of liquid water evaporated).
Condensation occurs when a molecule becomes slow enough to bind to a group of neighboring (liquid) molecules and results in the release of latent heat into the environment.
If the upper layer of the atmosphere is not very unstable, vertical growth will be restricted and only fair-weather cumulus or stratocumulus will form.
If the layer is more unstable, vertical growth may continue, forming cumulus congestus or cumulonimbus, which may then give rise to rain.
When the feeding of new bubbles stops, the cloud will dissipate.
© Stephen J Gledhill
2. Convergence – the rising of layers of air
When there is a convergence of air arriving horizontally in a region, the air is forced to rise because it cannot go downwards.
This is what happens in regions with low pressures, to whose center the air converges from the surrounding regions, with higher atmospheric pressures.
Layers of air can rise over hundreds of kilometers.
This phenomenon tends to result in the formation of clouds - so-called “dynamic clouds”.
The upward movement of air is weaker than that associated with convection and therefore the clouds that form are generally less vertically developed than those generated by convection (forming, for example, cirrostratus).
Note:
In anticyclones (high pressure centers), air flows outward, spiraling away from the center.
This ends up resulting in a downward movement of air that counteracts any rise in air that could lead to cloud formation.
This is why anticyclones are generally associated with clear skies.
3. Topography - which produces orographic clouds
When horizontal winds are faced with a mountain, the air is forced to rise.
If the rising air cools to dew point, water vapor condenses and an “orographic cloud” can form.
The exact type of cloud depends on the height of the topographic obstacle and the humidity and stability of the air.
At small elevations, cumulus or stratocumulus may form;
Altocumulus may form at higher elevations;
Note:
on the other side of the obstacle, the air descends, becomes compressed and heats up, which prevents clouds from forming.
The Foehn effect occurs in regions with mountains.
The term Föhn, in German, means hair dryer.
Therefore, the dry, hot air flowing down the mountain has been described as a "hair dryer" wind.
During descent, pressure increases and temperature increases due to air compression.
The air that comes down on the other side of the mountain (leeward) has a higher temperature than the air before going up the mountain, at the same altitude, because it has lost its moisture.
Drier air becomes warmer than humid air at the same altitude, as humidity reduces the air temperature.
4. Front Lifting
At a front, the meeting between air masses at different temperatures and humidity causes warmer air to rise above the cold air which, as it is denser, tends to remain close to the ground.
The rise of air ultimately results in the formation of clouds that appear just in front of the frontal surface on the ground, in the case of a cold front, and much further in front of the frontal surface on the ground in the case of a warm front (see fronts blog post).
Instability in the atmosphere
The atmosphere is said to be stable when rising air is unlikely (a rising air bubble descends again because it is colder than the ambient air).
The atmosphere is said to be unstable when rising air is likely (a rising air bubble may continue to rise because it remains warmer than the ambient air).
When an air bubble rises, it passes from an altitude where atmospheric pressure is higher to one where it is lower.
As the external pressure decreases, the air bubble expands, increasing its volume.
As air is a good thermal insulator, we can consider that all the energy spent on expansion («pushing the ambient air around it») comes from the molecules inside the air bubble itself, in other words, that expansion is an adiabatic process.
Adiabatic transformation occurs when a gas undergoes expansion or compression very quickly, without sufficient time for heat transfer.
We can ignore leaks to the outside and consider that the air cools only through decompression: the temperature decreases if the pressure is reduced and vice versa.
The air molecules will lose some kinetic energy and the air will cool.
The cooling rate is approximately constant: about 9.8°C/km for dry (unsaturated) air or 1°C per 100m.
When the air descends, it is compressed and also heats up at the same rate (9.8º C/km).
Documentary about Earth atmosphere
Suppose that an air bubble with an average temperature of 9.8ºC is at the surface and the dew point temperature is 0ºC.
If the air rises, it will be saturated at an altitude of 1 km.
If it continues to rise, it will continue to expand and cool but now the air will be saturated.
There will then be condensation of water vapor which will release latent heat, slightly counteracting the associated cooling.
Therefore, the adiabatic cooling rate for humid (saturated) air is slightly lower: around 6º C/km.
The atmosphere is said to be absolutely unstable (a not very common situation) if the rate of cooling of the ambient temperature with altitude is greater than 9.8ºC/km.
In this situation, a rising air bubble will always be hotter than its surroundings.
If this rate is less than 6ºC/km, the atmosphere is said to be absolutely stable.
In this situation, a rising air bubble will always be colder than its surroundings.
If the rate of cooling of the ambient temperature with altitude falls somewhere between 9.8 and 6ºC (a very common situation), the atmosphere is said to be conditionally unstable.
In this situation, a rising air bubble will continue to rise or not, depending on whether or not the air becomes saturated somewhere along its upward path.
Man on constant adventure around the Planet
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