Notes on weather observation composition

Fog: If the air near the ground is cooled to a certain extent under the conditions of sufficient water vapor, gentle breeze and stable atmosphere, the water vapor in the air will condense into tiny water droplets and be suspended in the air, which will reduce the visibility of the ground. This weather phenomenon is called fog. Fog is more common in spring from February to April. When the visibility in the atmosphere is lower than 1 km due to the condensation of suspended water vapor, meteorology calls this weather phenomenon fog.

Conditions for fog formation: cooling, humidifying and increasing water vapor content. Water vapor will liquefy when it is cold.

Snow: There are two conditions for snowfall:

One condition is that water vapor is saturated. The maximum amount of water vapor that air can hold at a certain temperature is called saturated water vapor. The temperature at which air reaches saturation is called dew point. When saturated air is cooled to a temperature below the dew point, the excess water vapor in the air becomes water droplets or ice crystals. Because the saturated water vapor content of ice surface is lower than that of water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. That is to say, water droplets can grow only when the relative humidity (relative humidity refers to the ratio of the actual vapor pressure in air to the saturated vapor pressure of air at the same temperature) is not less than 100%; As for ice crystals, they tend to grow when the relative humidity is less than 100%. For example, when the temperature is -20℃ and the relative humidity is only 80%, ice crystals can grow. The lower the temperature, the less humidity is needed for the growth of ice crystals. In high altitude and low temperature environment, ice crystals are more likely to be produced than water droplets.

Another condition is that there must be condensation nuclei in the air. Some people have done experiments, if there is no condensation nucleus, the water vapor in the air will only condense into water droplets if it is supersaturated to a relative humidity of more than 500%. But such a large supersaturation phenomenon will not exist in the natural atmosphere. So without condensation nuclei, it is difficult for us to see rain and snow on the earth. Condensation nuclei are tiny solid particles suspended in the air. The ideal condensation nuclei are those particles that absorb the most water. Such as sea salt, sulfuric acid, nitrogen and other chemicals. So we sometimes see clouds in the sky, but there is no snow. In this case, people often use artificial snowfall.

Ice: Water turns into ice at 0 degrees. The association of water molecules in natural ice is arranged according to the rules of hexagonal system. The simplest example of the so-called lattice is the close-packed bricks. If you replace an imaginary atom in the center of these bricks, you get a lattice. The crystal lattice of ice is a pyramid prism, and the oxygen atoms at six corners belong to the adjacent six units. The oxygen atoms on three sides belong to three adjacent cells, and the oxygen atoms on the top of two axes belong to two cells. Only one oxygen atom in the center is unique to this cell.

Clouds: Clouds floating in the sky are made up of many tiny water droplets or ice crystals, some of which are mixed together. Sometimes it also contains some big raindrops and ice particles. The bottom of the cloud does not touch the ground and has a certain thickness. The formation of clouds is mainly caused by water vapor condensation. More than ten kilometers from the ground, the closer to the ground, the higher the temperature and the denser the air; The higher the altitude, the lower the temperature and the thinner the air.

Rain: After the water on the earth is irradiated by sunlight, it turns into steam and is evaporated into the air. Water vapor condenses into small water droplets when it meets cold air at high altitude. These small water droplets are very small, with a diameter of only 0.0 1 ~ 0.02 mm and a maximum of only 0.2 mm ... They are small and light, and are held up in the air by the updraft in the air. It is these small water droplets that gather in the air to form clouds. These small water droplets will become raindrops and fall to the ground, and the volume will increase by about 1 10,000 times. How did these small water droplets increase their volume to more than 654.38+00,000 times? It mainly relies on two means, one is condensation and sublimation. The second is to rely on the collision and increase of cloud droplets. In the initial stage of raindrop formation, cloud droplets mainly condense and condense themselves by constantly absorbing water vapor around the cloud. If the water vapor in the cloud can be constantly replenished and replenished, so that the surface of the cloud droplets is often in a state of supersaturation, then this condensation process will continue, making the cloud droplets grow and become raindrops. However, sometimes the water vapor content in the cloud is limited, and in the same cloud, the water vapor is often in short supply, so it is impossible to make every cloud droplet grow into a larger raindrop, and some smaller cloud droplets have to be merged into a larger cloud droplet. If there are water droplets and ice crystals in the cloud, the process of condensation and sublimation will be greatly accelerated. When the cloud drops increase to a certain extent, due to the increasing volume and weight of large cloud drops, they can not only catch up with the slower small cloud drops, but also "swallow" more small cloud drops, making themselves stronger. When the big cloud drops get bigger and bigger until the air can no longer hold them, they fall straight from the clouds to the ground and become our common rain.

Dew: Water vapor in the air liquefies on objects covered on the ground in the form of water droplets. When the temperature drops at night, the closer to the ground, the faster the temperature drops, forming a temperature distribution opposite to that during the day. When the ground temperature is cooled to saturation, dew can be observed on the ground objects. If the temperature continues to drop below 0℃, dew freezes into ice beads, which is called frozen dew. After sunrise, the distribution of temperature and humidity on the ground becomes completely opposite to that at night, and the warming of air near the ground also makes the water vapor content in the air layer unsaturated. All kinds of conditions will be conducive to the evaporation of ground water, and dew will gradually disappear.