How is convection formed?

Convection, literally, refers to the relative flow inside a fluid due to the different temperatures of various parts. It is also the abbreviation of convective weather, which mainly refers to thunderstorm, squall line, hail, tornado and other weather. So what do you know about convection? Let me give you a detailed introduction to the relevant knowledge of convection.

How is convection formed? First of all, it is impossible to produce convective weather without water vapor. As we all know, it won't rain without clouds, and water vapor is the most basic condition for forming clouds and causing rain. The more water vapor, the greater the air humidity and the greater the precipitable water. Therefore, when continuous water vapor transport flow occurs, an area will be in a state of air saturation for a long time.

Secondly, the atmosphere must be in an unstable state, that is, unstable stratification conditions. It sounds hard to understand, but it's easy to explain. As we all know, water is denser than oil. When you put them in a bottle, the bottom must be water and the top must be oil. If the bottle is turned upside down, the oil and water will first mix and roll, and finally the oil will be on top and the water will be on the bottom. Having said that, I believe everyone will know that there is oil on the ground and water under it is stable. If there is water on the top and oil on the bottom, they will mix and roll.

The atmosphere is similar. The density of cold air is greater than that of warm air, so the stable atmospheric stratification is that cold air is below and warm air is above. However, due to various reasons, the opposite situation appeared, and the atmospheric stratification was unstable. Once there is a trigger mechanism, cold and warm air masses collide violently, resulting in convective weather such as thunderstorms and strong winds.

? Trigger mechanism? It is the last condition of convective weather. What is its scientific name? Promotion conditions? . Take the simplest example: in the summer afternoon, the solar radiation is strong. Under the strong sunlight, the surface temperature rises rapidly, and the surface temperature is much higher than the air temperature.

Under the action of surface heating, the closer to the ground, the higher the air temperature and the lower the air density. Therefore, the closer to the ground, the lower the air density, and the easier it is to move to the upper layer. When this happens. Up? When the movement reaches a certain level, it will excite unstable air, and if there are good water vapor conditions, it will produce convective weather.

Cause analysis of weather phenomena in convective weather

When a thunderstorm occurs, there are usually weather phenomena such as thunder and lightning, rainfall and gust, as well as changes in meteorological factors such as air pressure, temperature and humidity. These phenomena mainly occur in the mature stage of thunderstorm clouds; The reason will be discussed below.

thunder and lightning

Lightning is caused by cumulonimbus clouds? Temperature difference electrification? And other electrification effects. Generally speaking, when the cloud top develops above the isotherm height of -20 degrees Celsius. There will be lightning and thunder. The first thunder shows that the cloud top has reached the isotherm height close to -20 degrees Celsius. As the cloud top rises, lightning and thunder become more frequent Generally speaking, the intensity and frequency of discharge in clouds are related to the height and intensity of thunderstorm clouds. Therefore, lightning phenomenon can be used to judge the intensity of thunderstorms.

rain

There is generally a water droplet gathering area near the strongest updraft in thunderstorm clouds. When the cumulative amount exceeds the supporting capacity of the updraft, it begins to rain. Because the water in the stagnant water area pours down, it causes showers or heavy rains. The duration of showers varies from a few minutes to an hour, depending on the intensity and water content of thunderstorm clouds. The precipitation area formed by thunderstorm group and thunderstorm belt is also flaky or banded. Because the intensity of each unit in a thunderstorm group (belt) is different, the precipitation distribution is very uneven. Moreover, because thunderstorm clouds often spread by leaps and bounds, precipitation also has a jump (interval) distribution.

special flavour

In the cumulus stage, the surface wind is generally weak. The convergence of low-level directional clouds promotes the development of updraft. In the mature stage of thunderstorm clouds, when the downward airflow generated in the clouds rushes to the ground, it spreads around, resulting in gusts. Generally speaking, before the gust, the wind is weak and the direction is uncertain, but it is mostly southerly. When a gust occurs, the wind direction tends to be cyclonic rotation, and then anti-cyclonic rotation. A slow-moving thunderstorm. The outflow airflow under the cloud is almost radial (that is, it spreads in all directions). But in most cases, the wind speed in the downwind direction of thunderstorm is greater than that in the upwind direction.

Changes in pressure, temperature and humidity

Due to the evaporation of water droplets in downward flow, downward flow is almost saturated. Therefore, the sinking air is heated by the wet insulation from the upper layer to the lower layer. Although the upper cold air will warm up during the sinking process, the heating rate is small, and it is still colder than the surrounding ground air when it reaches the ground. Therefore, a nearly saturated cold air mass is formed under the thunderstorm cloud. Because of the high density, the air pressure is higher. This high pressure is called. Thunderstorm high? .

When the thunderstorm cloud moves forward, the thunderstorm high under the cloud also moves forward. When it passes through the station, meteorological factors such as temperature drop, sudden increase of air pressure, increase of relative humidity, drop of dew point or absolute humidity appear in the station. The range of change depends on the intensity of thunderstorm clouds and the position of weather station relative to thunderstorm clouds. The area where the thunderstorm center passes changes obviously, but the edge area changes little.

The influence range of convective weather

Convective weather is very strong and prone to disasters. Its influence range is small, its duration is short, and it is usually local disastrous weather. But sometimes there will be a large-scale strong thunderstorm weather process, which can affect dozens of counties to several provinces and last for about one day.

For example, on June 8 1962, there were heavy hailstorms in twenty or thirty counties in Shandong, Jiangsu, Anhui and other provinces. Another example is1974 June 17, which starts from Shandong Peninsula in the north, passes through Shandong, Jiangsu, Anhui and other provinces, and reaches the vast areas in northern Zhejiang, northwestern Jiangxi and eastern Hubei in the south. Severe weather such as 8- 12 gale or hail occurred from north to south. There are similar situations abroad.

For example, 1974, from the evening of March to April 4, 100, many tornadoes hit the United States 12 states and parts of Canada. These large-scale strong thunderstorms will cause large-scale serious disasters.

Convective weather has a great influence not only on all sectors of the national economy, but also on military activities. For example, there are strong disturbances, icing and discharge phenomena in cumulonimbus clouds, which pose a great threat to flight safety. So even ordinary thunderstorm weather will be dangerous to it. Therefore, it is of great significance for disaster prevention, national economy and national defense construction to make a good forecast of convective weather and prevent sudden attacks of convective weather.

In view of the fact that convective weather generally has the characteristics of small scale and rapid development, besides weather map method, it is best to cooperate with mesoscale weather analysis, radar and satellite detection in forecasting. The following introduces the basic knowledge and forecasting methods of convective weather.

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