How does water vapour turn back into liquid water?

Answer 1

Naturally occurring clouds are composed mostly of water in its liquid or solid state. Firstly we need a sufficient quantity of water vapour to transform into the liquid or solid states. The water vapour content of the atmosphere varies from almost 0% to about 4%, depending on the moisture on the surface beneath and the temperature of the air. Second we need a small amount of fine dust. This is needed for condensation nuclei, upon which water vapour is able to condense or deposit as a liquid or a solid. Certain Particles, such as sea salts and clay, make very good condensation nuclei. When there is sufficient water vapour in the air along with the required dust particles it must be cooled to a temp where droplets or ice can form on the particles. At this point clouds are formed Air moving vertically (up or down) is Very important in the weather processes, especially where clouds and rain are concerned. Ascending air currents take us up into the areas of the atmosphere where Precipitation is more likely to be initiated. Descending currents reverse the process until only water vapour and dust makes up the air mass. Whatever the cause, when air rises it's pressure has to change to match surrounding air, Air pressure goes down as it goes up, which causes it to expands. And as it expands, it cools. The higher it rises the cooler it becomes. As air cools, its relative humidity increases, (Humidification) .the water vapour content in the air has remained the same, however the ability of the air to hold water vapour (The saturation threshold) has decreased at it was cooled. As a result of decreasing the saturation threshold, relative humidity increases. Cooling is the effective method for increasing relative humidity but it is not the only one. Adding water vapour through evaporation or mixing with a more humid air mass is another. To form clouds humidification may eventually bring the air in a given area to saturation. (i.e.) relative humidity reaches 100 percent. Usually a little more humidification is required taking the relative humidity to over 100 percent, this is known as the state of supersaturation needed to form cloud. When air becomes supersaturated, the water vapour in it looks for ways to condense out. If the quantity and composition of the dust content is ideal, condensation may begin at a relative humidity of below 100 percent. If the air is very clean, it may take high levels of supersaturation to produce cloud droplets. But typically condensation begins at relative humidity a few tenths of a percent above saturation. Condensation of water onto nuclei (or deposition of water vapour as ice on freezing nuclei) begins at an altitude known as the cloud base (or lifting condensation level). Water molecules attach dust particles to form cloud droplets which have are about 20 micrometres (0.02 mm) or less in size. The droplet volume is usually a million times greater than the typical condensation nuclei. Clouds are made up of large numbers of cloud droplets, ice crystals, or both. As a result of their small size and relatively high air resistance, they are able to remain suspended in the air for long periods of time, especially if they stay in rising air currents. The average cloud droplet has a terminal fall velocity of 1.3 cm per second in still air. When rain falls, drops of rain are more than one millimetre in size. A raindrop with a diameter 2 mm has the equivalent of a million cloud droplets in of (0.02 mm diameter). Therefore to get rain from a cloud more has to happen within it to that form droplets heavy enough and big enough to fall to the ground. Increased cloud water content is required before we can expect any rain This means the cloud has to continue to rise. Water vapour has a property of releasing heat when changing to liquid and also to solid states, (latent heats of condensation and of deposition), respectively. When this heat is released the surrounding air is warmed, causing the surrounding air to rise. We can see this in the formation of cumulus clouds growing vertically. At this point condensation continues however the coalescence of the cloud droplets causing them to increase in size until they become viable raindrops is a much faster process. This process is further aided by strong updrafts and turbulence forcing droplets to collide and grow more rapidly. The best conditions for droplet growth are when ice crystals are present in a cloud. Under prime conditions a droplet may reach -40 oC (or oF) before it freezes. So there are areas in a cloud were ice crystals and water droplets co-exist. If ice crystals and supercooled droplets are near each other, a movement of water molecules from the droplet to the crystals occurs. As a result the ice crystal gets bigger and the water droplet reduces in size. As crystal grows at temperatures of around -10 oC (14 oF), it begins to develop arms and branches, resulting in the formation of snow. These crystals besides being very efficient at growing at the expense of water droplets also stick to each other forming snowflakes. At last water ice or snow has built up in sufficiently large quantities to be able to fall back to earth.

Answer 2

the water particles in the air bond and become a liquid due to condensation.

Answer 3

100 degrees