Why are there clouds?

Cloud is the foundation of precipitation and the middle link of the earth's water cycle. The occurrence and development of cloud is always accompanied by energy exchange. The shapes of clouds are ever-changing, and specific clouds often appear in specific weather, so clouds have certain indicative significance for weather changes.

(A) the formation conditions and classification of clouds

In the atmosphere, the important conditions of condensation are the existence of condensation nuclei and the supersaturation of air. For the formation of clouds, its supersaturation is mainly caused by adiabatic cooling caused by vertical rise of air. The form and scale of ascending motion are different, and the state, height and thickness of the formed cloud are also different. There are four main ways for the upward movement of the atmosphere:

1. Thermal convection

Refers to the convective upward movement caused by uneven ground heating and unstable atmospheric stratification. Clouds formed by convection are mostly cumulus clouds.

2. Dynamic uplift

Refers to the large-scale upward movement of warm and humid airflow caused by the action of front and convergent airflow. The clouds formed by this movement are mainly layered clouds.

3. Atmospheric fluctuations

Refers to the fluctuation caused by the atmosphere flowing through the uneven ground or below the inversion layer. Clouds produced by atmospheric fluctuations mainly belong to fluctuation clouds.

4. Topographic uplift

Refers to the upward movement caused by the forced upward movement of the atmosphere when it is blocked by the terrain. Clouds formed by this movement include cumulus clouds, wavy clouds and layered clouds, which are usually called topographic clouds.

Although the shapes of clouds vary widely, there are always certain rules for their formation. According to the formation height and shape of clouds, China classifies clouds into 4 families 10 genera. 1972 The Cloud Map of China published by China divides clouds into three families, 1 1 genus (see Chapter 5 of Meteorology and Climatology Practice for table 3.3).

(B) the formation of various clouds

1. Cumulus formation

Cumulonimbus clouds are vertically developing clouds, mainly including light cumulonimbus clouds, thick cumulonimbus clouds and cumulonimbus clouds. Cumulus clouds are mostly formed in the summer afternoon, with isolated and scattered appearance, flat cloud bottom and raised top.

The formation of cumulus clouds is always related to upward convection in unstable atmosphere. Whether cumulus clouds can be formed by convection depends not only on condensation conditions, but also on the height of convection energy. If the maximum height of convection (upper limit of convection) is higher than the condensation height, cumulus clouds will be formed, otherwise cumulus clouds will not be formed. The stronger the convection, the greater the difference between the upper convection limit and the condensation height, and the greater the thickness of cumulus. The horizontal range of convection rising area is wide, and the horizontal range of cumulus is large.

Light cumulonimbus, dense cumulonimbus and cumulonimbus are different stages of cumulonimbus development. Cumulus generated by thermal convection in air mass is the most typical. In summer, the ground is strongly radiated by the sun, and the ground temperature is very high, which further heats the near-surface gas layer. Due to the inhomogeneity of the earth's surface, the air in some places is seriously heated, and the air in some places is humid, so gas blocks (hot bubbles) slightly different from the surrounding temperature, humidity and density are produced in the atmosphere near the ground. The internal temperature of these air blocks is relatively high, and they float with the wind due to the buoyancy of the surrounding air, constantly living and disappearing. The larger air mass rises at a higher height, forms a convective monomer when it reaches the condensation height, and then gradually develops into an isolated, dispersed cumulus with a flat bottom and a convex top. Because the air movement is continuous and compensates each other, the air in the rising part condenses into clouds due to cooling, and the air around the clouds is supplemented by sinking, so the sinking air heats up adiabatic quickly and does not form clouds. As a result, the accumulated clouds dispersed and the blue sky was exposed among the clouds. For a certain area, at the same time, the horizontal distribution of air temperature and humidity is almost the same, and its condensation height is basically the same, so the bottom of cumulus is flat.

If the upper limit of convection is slightly higher than the condensation height, generally only light cumulus clouds are formed. Because the cloud top is generally below the 0℃ isotherm height, and the cloud body is composed of water droplets, the velocity of the ascending airflow in the cloud is not large, generally less than 5m/s, and the turbulence in the cloud is also weak. At the height where light cumulus clouds appear, if there is strong wind and strong turbulence, the clouds of light cumulus clouds will become broken, which is called broken cumulus clouds.

When the upper limit of convection is much higher than the condensation height, the cloud body is tall and the top is cauliflower-like, forming cumulus clouds. Its cloud top extends to a height below 0℃, and the top is composed of supercooled water droplets. The updraft in the cloud is strong, reaching 15-20m/s, and the turbulence in the cloud is also strong.

If the updraft is strong, the cloud top of cumulus can extend upward, and the cloud top can extend to the upper air below-15℃. Therefore, the cloud top freezes into ice crystals, and a filamentous structure appears, forming cumulonimbus clouds. The top of cumulonimbus cloud, blown by high-altitude wind, spreads horizontally into an anvil, which is called anvil cloud. In the direction of high-altitude wind, the cloud anvil can extend very far, so its extension direction can be used as a judgment of the moving direction of cumulonimbus clouds. The thickness of cumulonimbus clouds is very large, which can reach 5000 ~ 8000 meters in mid-latitude areas and 10000 meters in low-latitude areas. The descending airflow rises rapidly in the cloud, and the ascending airflow can often reach 20-30m/s, and the rising speed of 60m/s has been observed, and the sinking speed is also10-15m/s. The turbulence in the cloud is very strong.

Cumulus formed by thermal convection has obvious diurnal variation. It is usually light cumulus clouds in the morning. With the strengthening of convection, it gradually developed into cumulus clouds. Convection is strongest in the afternoon and can often develop into cumulonimbus clouds. At night, convection weakens, cumulonimbus gradually dissipates, and sometimes it can evolve into false clouds, cumulonimbus clouds and cumulonimbus stratocumulus. If there are only faint cumulus clouds in the afternoon, it means that the air is relatively stable, cumulus clouds can no longer develop and grow up, and the weather is better, so the faint cumulus clouds are also called clear cumulus clouds, which is a sign of continuous sunny days. In summer, if cumulonimbus clouds appear in the early morning, it means that the air is unstable and may develop into cumulonimbus clouds. Therefore, the appearance of cumulus clouds in the morning is a sign of thunderstorm. At night, the stratocumulus evolves after the cumulus dissipates, indicating that the air stratification is stable and the clouds will disperse at night, which is a sign of fine weather. It can be seen that the diurnal variation characteristics of cumulus formed by thermal convection are helpful to directly judge short-term weather changes.

2. Formation of stratiform clouds

Stratiform clouds are uniform curtain clouds, often with a large horizontal range, including cirrostratus, Cirrus, Stratosphere and nimbostratus.

Stratiform clouds are caused by the ascending motion of large-scale air systems, mainly by the ascending motion of fronts. This systematic upward movement usually has a large horizontal amplitude, and the upward speed is only 0.1-1m/s. Because of its long duration, the air can rise for several kilometers. For example, when warm air moves to the cold air side, due to the different densities, the stable warm and humid air slowly slides upward along the cold air slope and is adiabatic cooled to form a layered cloud. The bottom of the cloud is roughly the same as the inclined plane (also called front) where cold and warm air meet, and the top of the cloud is approximately horizontal. The thickness of the cloud varies greatly in different parts of the slope. There are cirrus clouds and cirrostratus in front, and cirrostratus is the thinnest, usually between several hundred meters and 2,000 meters. The clouds are composed of ice crystals. Located in the middle is a high-level cloud, the thickness is generally 1000-3000 m, the top is mostly composed of ice crystals, and the main part is mostly composed of ice crystals and supercooled water droplets. Finally, the rain layer stone, generally 3000-6000 m thick. Its top is composed of ice crystals, the middle is composed of supercooled water droplets and ice crystals, and the bottom is composed of water droplets because the temperature is higher than 0℃.

It can be seen from the formation of layered clouds in the above system that some clouds can be used as omens before the arrival of precipitation. Cirrostratus, for example, usually appears in front of the layered cloud system, and its appearance is often accompanied by solar and lunar halos. So when you see a halo in the sky, you know that cirrostratus has moved, so will nimbostratus, and it may turn rainy. The agricultural proverb "the sun is dizzy in the middle of the night, the rain is dizzy at noon, and the wind is dizzy at noon" refers to this sign.

3. The formation of wave clouds

Wave clouds are wave cloud, including cirrocumulus, altocumulus and stratocumulus. The rising speed in the cloud can reach tens of centimeters per second, which is second only to the rising speed in the accumulation cloud.

When the air fluctuates, it rises at the peak and sinks at the trough.

Due to adiabatic cooling, clouds will form where the air rises, but not where the air sinks. If there is a layered cloud with uniform thickness before the wave is formed, the cloud will thicken at the peak, thin or even disappear at the trough, thus forming a parallel cloud with a small thickness and a certain distance, showing rows or rows of wave clouds.

It is generally believed that there are two main reasons for fluctuation: first, there are interfaces with different air density and airflow speed in the atmosphere, which causes fluctuation. The second is the fluctuation caused by the airflow passing through the mountain (called topographic wave or lee wave). When fluctuations appear on the interface with low wind speed and high density in the upper layer and low wind speed and high density in the lower layer, because the wind direction and wind speed at each height often change, the direction of fluctuations changes with time, and the newly generated fluctuations are superimposed on the original fluctuations, thus forming a checkerboard cloud block. When the fluctuating gas layer is very high, cirrocumulus is formed, when it is high, it forms high cumulus clouds, and when it is low, it forms stratocumulus clouds.

The thickness of wave clouds is not large, usually tens to hundreds of meters, sometimes reaching 1000-2000m. When it appears, it often means that the gas layer is relatively stable and the weather changes little. Proverbs such as "Wayun will kill people" and "There are carp spots in the sky, so you don't need to turn over the grain tomorrow" mean that after the appearance of altocumulus or stratocumulus, the weather will be fine and there will be little change. However, systematic wave clouds, such as cirrocumulus, evolved after fluctuating on cirrus clouds or cirrostratus, so they are connected with large stratiform clouds, which indicates that there will be wind and rain coming. "Fish-scale sky, no rain but wind" refers to this omen.

4. Formation of special clouds

In addition to the formation of the above clouds, there are some special clouds, such as fort-shaped, flocculent, suspended spherical, pod-shaped, etc., which can often predict the changing trend of the weather. Therefore, understanding their causes and characteristics is helpful to use them to judge the future weather.

(1) Suspended spherical cloud: refers to the cloud mass hanging at the bottom of the cloud, which mostly appears at the bottom of cumulonimbus clouds. Sometimes it can be seen at the bottom of nimbostratus.

When there are a lot of water droplets in the cloud, if there is a strong updraft near the cloud bottom, the falling water droplets will be lifted up to form a cloud mass that seems to be suspended at the cloud bottom, which is a suspended spherical cloud.

The appearance of suspended spherical clouds usually indicates precipitation, because once the updraft weakens, the water droplets that were originally held will fall down and form precipitation.

(2) Fort Cloud and Flocculent Cloud: The bottom of the Fort Cloud is horizontal, and the top is a small cloud tower with bumps side by side, which looks like a castle in the distance. The formation of such clouds is often developed on the basis of wave clouds. When the wave cloud is formed under the inversion layer, if the inversion layer is not too thick, when the turbulence under the inversion layer develops, there will be a strong updraft passing through the inversion layer to condense the water vapor and form a cloud with an arc top, which is the castle cloud. Common fort clouds are fort-shaped altocumulus and fort-shaped stratocumulus.

Flocculent clouds are broken and shaped like cotton balls. They are usually formed by strong turbulent mixing in the humid air layer, mainly flocculent cumulus clouds.

In summer, if there are fort-shaped or flocculent cumulus clouds in the morning, it means that the upper air layer is unstable at this height. At noon, as soon as the low-level convection develops, the upper and lower unstable air layers will combine to produce a strong updraft, forming cumulonimbus clouds, thunderstorms or hail. The weakening of convection in the evening, such as the appearance of fort cumulus, indicates that unstable systems will approach at high altitude, and systematic thunderstorms may occur the next day.

(3) Lenticular clouds: Lenticular clouds are thick in the middle and thin in the edge, and the clouds are pod-shaped. Common lenticular clouds are mainly podiform cumulus and lentiform stratocumulus.

Lenticular clouds are formed by the convergence of local updraft and downdraft. When the updraft cools the air adiabatically to form a cloud, if it is blocked by the downdraft, its edge will gradually become thinner due to the downdraft, thus forming a lenticular cloud. In mountainous areas, the airflow will also form lenticular clouds under the influence of topography.

The physical processes of cumulus clouds, stratiform clouds, wavy clouds and some special clouds are introduced above. But they are not isolated and unchangeable. Due to the change of conditions, they can develop, dissipate, and transform from one cloud to another. For example, in cumulus clouds, light cumulus clouds can develop into thick cumulus clouds and finally form cumulonimbus clouds. Cumulonimbus clouds can evolve into pseudocumulus clouds, cumulonimbus clouds and cumulonimbus stratocumulus clouds after dissipation. For another example, when wave clouds develop, they can evolve into stratocumulus (shadow altocumulus can evolve into stratocumulus and shadow stratocumulus can evolve into nimbostratus). When the stratiform cloud dissipates, it will also evolve into a wave cloud (after the nimbostratus dissipates, it can evolve into a stratocumulus, a altocumulus or a stratocumulus). In short, the emergence, development and evolution of clouds are complex and regular.

References:

Zhou Shuzhen's Meteorology and Climatology (3rd Edition).