Weather and Atmospheric Water

Key Questions

  • Answer:

    Water and condensation nuclei.


    In order for precipitation to form you first need liquid water or ice. In order to have this in the atmosphere you have to first have 100% relative humidity (or close to it). Relative humidity is the percentage of water vapour in the air as a percentage of the amount of water vapour that that air can hold at it's present temperature. The closer it gets to 100% the higher the chance that some of the water vapour will change back to liquid (or solid) water.

    The problem is that water doesn't like to change state readily. It usually needs a little "kick", that is where the condensation nuclei come into play.

    Condensation nuclei are particles of dust or other things that the vapour can actually condense around (hence the name). Imagine a steam room, the liquid water is more on the walls and the ceiling than it is in the air. That is how condensation nuclei work.

    Once you have both of these then all you need is the liquid drop to become large enough so that gravity on it overcomes the updrafts of the atmosphere. Then it falls, tada, precipitation.

  • Answer:



    This is not as simple as I have made it sound, since the temperature aloft is important too.

    Rain, starts as a liquid or solid higher in the atmosphere. As it falls it enters warmer air and melts. The surface temperature must be above freezing, usually by a few degrees.

    Freezing rain, forms as a solid and then passes through an above freezing level layer in the atmosphere (where it turns liquid). It then falls to a below freezing level (but does not refreeze). The surface temperature has to be below zero (generally speaking)

    Ice pellets, form the same as freezing rain but the surface temperature is much colder so it refreezes.

    Snow, forms as a solid from accretion of ice crystals or by stealing water from liquid water droplets (this is called the Bergeron effect and I will explain it if anyone ever asks about it). It falls as a solid. Surface temperatures are below freezing.

    Sleet is just wet snow, it forms like snow but the surface temperatures are right around freezing.

    Hail is a bit more exciting. It forms as a solid or a liquid in large thunderstorms. The droplets fall and melt but are caught in strong updrafts, taking the precipitation back up below the freezing point. It then falls and melts and gets carried back up. Each time this happens more and more layers of ice and water are formed on the precipitation until it reaches a size that the updrafts are no longer stronger than gravity. Then it falls to the Earth.

    Hail melts the whole way down so the hail that hits the ground is much smaller than the hail in the cloud. I have hear reports of hail that hits the ground the size of a baseball being almost the size of a basketball.

  • Clouds form when air that is warm and moist cools and expands. Clouds are just tiny droplets of water in the atmosphere. As you can see from the image below, the air cools as it rises, but cool air is able to hold less water than warm air. Thus, this excess water in the cool air condenses, and when enough of it does so, a cloud is formed. ![

    Clouds are formed by way of four processes : 1) surface heating, 2) mountains and terrains, 3) air masses that are forced to rise and cool, and 4) cold or warm weather fronts.

    1) The first process is perhaps the simplest: the sun heats the earth and thus the air, this warm air rises, expands, and cools, forming clouds like in the image above.

    2) Clouds are also formed when air encounters mountains or other topography. The air rises and cools and, again, the air cannot hold all of the water it held when warm so clouds form.

    3) The air is also forced to rise when wind in a low pressure system forces the air to rise up. Related to number 2, if air is forced to rise because of topography that slopes upwards, clouds may also form.

    4) Finally, weather fronts cause cloud formation. Specifically, warm fronts create clouds because the warm air rises above the cold air and cold fronts create clouds because the cold air displaces or moves the warm air up.

    To conclude, many factors affect cloud formation, including topography, air temperature, and humidity.

    Here's a very comprehensive link on cloud formation if you're looking for more.

    Additionally, clouds can form in areas where precipitation has been ongoing or particularly heavy. The falling precipitation brings moisture lower down in the atmosphere, and we get a ragged layer (either stratus or stratus fractus) form, usually within 1,500 ft of the ground or lower.

    Finally, since Fog is cloud at the surface of the Earth, in areas where fog has formed, surface winds can lift the fog, once it is aloft it becomes a cloud layer (stratus).