Typical phenomena of various weather conditions
Fog: When there is sufficient moisture, gentle breeze, and a stable atmosphere, if the air close to the ground cools to a certain extent, the moisture in the air will condense into fine water droplets suspended in the air, causing Reduced visibility at ground level is a weather phenomenon called fog. Fog occurs more often in spring from February to April. Whenever the visibility is lower than 1,000 meters due to the condensation of suspended water vapor in the atmosphere, meteorology calls this weather phenomenon fog.
Conditions for fog formation: First, cooling, second, humidification to increase water vapor content. Water vapor liquefies when it encounters cold
Snow: Two conditions must be met to form snowfall:
One condition is saturation of water vapor. The maximum amount of water vapor that air can contain at a certain temperature is called the saturated water vapor amount. The temperature at which air reaches saturation is called the dew point. When saturated air cools to a temperature below the dew point, excess water vapor in the air turns into water droplets or ice crystals. Because the saturated water vapor content of the ice surface is lower than that of the water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. In other words, water droplets must grow when the relative humidity (relative humidity refers to the ratio of the actual water vapor pressure in the air to the saturated water vapor pressure of the air at the same temperature) is not less than 100%; as for ice crystals, the relative humidity is often less than 100% It can also grow over time. For example, when the air temperature is -20°C and the relative humidity is only 80%, ice crystals can grow. The colder the temperature, the less humidity required for ice crystals to grow. In high-altitude and low-temperature environments, ice crystals are more likely to form than water droplets.
Another condition is that there must be condensation nuclei in the air. Someone has done experiments and found that if there are no condensation nuclei, the water vapor in the air can condense into water droplets only when it is supersaturated to a relative humidity of more than 500%. But such a large supersaturation phenomenon does not exist in the natural atmosphere. Therefore, without condensation nuclei, it would be difficult to see rain and snow on our earth. Condensation nuclei are tiny solid particles suspended in the air. The most ideal condensation nuclei are those material particles that absorb water the most. For example, particles of sea salt, sulfuric acid, nitrogen and other chemicals. That’s why we sometimes see clouds in the sky but no snowfall. In this case, people often use artificial snowfall.
Ice: Water turns into ice at 0 degrees. The associations of water molecules in natural ice are arranged according to the rules of the hexagonal crystal system. The simplest example of a so-called crystalline lattice is bricks that are tightly packed. If a hypothetical atom is replaced at the center of these bricks, a crystalline lattice is obtained. The crystal lattice of ice is a triangular prism with a top cone, and the oxygen atoms at the six corners are owned by the six adjacent unit cells. The oxygen atoms on the three edges are each owned by three adjacent unit cells. The two apex oxygen atoms are each owned by two unit cells. Only the central oxygen atom is unique to the unit cell. .
Clouds: Clouds floating in the sky are composed of many small water droplets or ice crystals, and some are composed of small water droplets or small ice crystals mixed together. Sometimes it also contains some larger raindrops and ice and snow particles. The bottom of the cloud does not touch the ground and has a certain thickness. Cloud formation is mainly caused by the condensation of water vapor. In this layer of atmosphere, which is more than ten kilometers upward from the ground, the closer to the ground, the higher the temperature and the denser the air; the higher you go, the lower the temperature and the thinner the air.
Rain: After the water on the earth is illuminated by sunlight, it turns into water vapor and evaporates into the air. When water vapor encounters cold air at high altitudes, it condenses into small water droplets. These small water droplets are very small, only 0.01 to 0.02 mm in diameter, and the largest is only 0.2 mm. They are small and light, held aloft by updrafts in the air. It is these small water droplets that gather into clouds in the sky. When these small water droplets turn into raindrops and fall to the ground, their volume will increase by about 1 million times. How do these small water droplets increase their size to more than 1 million times? It mainly relies on two means, one is condensation and sublimation increase. The second is to rely on the collision and growth of cloud droplets. In the early stages of raindrop formation, cloud droplets mainly rely on continuously absorbing water vapor around the cloud body to condense and sublimate themselves. If the water vapor energy source in the cloud body is continuously supplied and replenished, so that the surface of the cloud droplets is often in a supersaturated state, then this condensation process will continue, causing the cloud droplets to continue to grow and become raindrops. But sometimes the water vapor content in the cloud is limited. In the same cloud, water vapor is often in short supply, so it is impossible to make each cloud droplet grow into a larger raindrop. Some smaller cloud droplets have to be merged into larger raindrops. into the cloud droplets. If water droplets and ice crystals exist in the cloud, the condensation and sublimation growth process will be greatly accelerated. When the cloud droplets in the cloud increase to a certain extent, due to the increasing volume and weight of the large cloud droplets, they will not only catch up with the slower small cloud droplets during their descent, but also "swallow" more The small cloud drops make themselves stronger. When the big cloud droplets grow bigger and bigger, and finally become so big that the air can no longer support them, they fall straight from the clouds to the ground and become our common rain.
Dew: The liquefaction phenomenon in which water vapor in the air liquefies in the form of droplets on ground covering objects. The temperature drops at night, and the closer to the ground, the faster it cools, forming a temperature distribution that is cold at the bottom and hot at the top, opposite to that during the day. When the ground temperature cools to the point where the water vapor content in the air close to the ground reaches saturation, dew droplets begin to be observed on ground objects. . If the temperature continues to drop below 0°C, the dew drops freeze into ice beads, which is called freezing dew.
After sunrise, the ground temperature and humidity change to a distribution pattern that is completely opposite to that at night. The warming of the air close to the ground also makes the water vapor content of this air layer undersaturated. Various conditions will be conducive to the evaporation of ground moisture, and dew drops gradually disappear.