Last week we looked at moisture, including the different phases, transformations, and methods of measurement (e.g., dewpoint and relative humidity). Now, what happens when the relative humidity hits 100%?
When the air’s temperature equals the dewpoint, the relative humidity is 100% and we say the air is “saturated“. Air can only “hold” so much water vapor, and when it maxes out, some of the water vapor must condense into liquid water droplets or deposit ice crystals. This means saturation can result in dew, frost, fog, or clouds.
How can we attain saturation? The amount of water vapor the air can hold depends on the temperature. Warm air requires more moisture to reach saturation than cold air, so we can reach saturation by either adding more moisture or lowering the air temperature.
Clouds are composed of liquid water droplets or ice crystals. Water vapor doesn’t necessarily condense into pure liquid water droplets, though, but condenses onto tiny particles of dust, smoke, salt, etc., called cloud condensation nuclei (ice crystals form on ice nuclei).
Also, it is important to note that liquid water droplets may exist below 0 °C…these are called supercooled water droplets, and they can exist in liquid form at temperatures as low as -40 °C/F. Supercooled water droplets are important in many ways that we will see later (e.g., freezing rain, hail formation, thunderstorms, etc.).
Clouds are classified with Latin words based on their height and appearance…for example, high clouds have a “cirro-” prefix, clouds a bit further down in the atmosphere have an “alto-” prefix, clouds that are flattish have “strato-” or “stratus” in their name, clouds that are bubbly have “cumulo-” or “cumulus” in their name, and clouds from which precipitation is falling have “nimbo-” or “-nimbus” in their name.
- High clouds: cirrus, cirrostratus, cirrocumulus…usually reside above 6 km, mostly made up of ice crystals, thin and often very white, responsible for many optical effects such as sundogs and halos
- Middle clouds: altostratus, altocumulus…usually between 2 and 6 km, mostly made up of liquid water droplets, usually more gray in color, sun’s disk may be visible through them, but no halo
- Low clouds: stratus, stratocumulus, nimbostratus…usually below 2 km, mostly made up of liquid water droplets, the sun typically cannot be seen through these clouds
- Deep clouds: cumulus, cumulonimbus…clouds with more vertical development, including thunderstorm clouds (cumulonimbus)
There are also many other clouds names that may be affixed to one or another of the above cloud names, including “congestus” (deep cumulus clouds that haven’t quite made it to the cumulonimbus stage yet), “lenticularis” (lens-shaped clouds that are common above or just over mountain tops), and “mammatus” (pouch-shaped clouds common under thunderstorm anvils).
Come back next Monday as we talk about precipitation, the next element of weather.