Precipitation
So apparently we’re meant to get a lot of rain here over the next couple of days. That’s fine with me, I love the rain. Also I got three loads of washing dry today. So bring the rain!!!
Of course, rain is just one type of precipitation. Others include freezing rain, ice pellets, snow, snow pellets, and hail.
Water molecules exist in the atmosphere in all three of the common states of matter - solid, liquid and gas. Water vapour in the atmosphere is what we refer to as humidity. On very humid days there is a lot of water vapour. When the atmosphere is cold enough, this vapour condenses (changes from gas to liquid) to liquid state or deposits (changes from gas to solid without passing through liquid) to solid state. If the liquid and solid forms of water are heavy enough to overcome atmospheric updrafts they can fall to the Earth’s surface. This is what we call precipitation.
Precipitation is any aqueous (watery) deposit, in liquid or solid form, that develops in the atmosphere and falls from clouds to the ground. However, most clouds do not produce precipitation. Water droplets and ice crystals are often too small to overcome natural updrafts, and remain suspended in the atmosphere.
Turbulent atmospheric mixing can result in collision and coalescence of droplets and crystals, causing them to grow larger, and therefore heavier. Eventually they may be large enough and heavy enough to overcome natural updrafts and fall to the Earth.
Let’s take a quick look at a few types of precipitation:
Rain is any liquid that A) falls from the atmosphere to the ground, and B) has a diameter greater than 0.5 mm. The maximum size of a drop of rain is about 5 mm. Once the drops get larger than this the intermolecular cohesive forces are too weak to hold the mass of water together as a single drop and it splits into smaller droplets.
Freezing rain takes place when liquid water droplets land on a surface with a temperature below 0 °C. The rain quickly turns into ice when it hits such a surface.
Ice pellets (a.k.a. sleet) are transparent (you can see through them) or translucent (you can’t see through them very well) spheres of frozen water. They have a diameter smaller than 5 mm. They start out as raindrops in a relatively warm layer of the atmosphere. Then, as they descend into a colder layer of the atmosphere where freezing temperatures occur, they freeze into solid ice pellets.
Snow develops when water vapour deposits itself (becomes solid without first becoming liquid) directly on a deposition nuclei (a particle of dust, smoke, etc.) as solid crystals, at temperatures below 0 °C.
Snow pellets develop when supercooled droplets freeze onto the surface of falling snowflakes. They have a diameter less than 5 mm, but differ from ice pellets in that they are white in colour, and opaque (you can’t see through them).
Hail is frozen precipitation that is more than 5 mm in diameter. Hailstones often have layers of ice alternating between a cloudy white appearance to being clear and colourless. The cloudy white layers contain partially melted snowflakes that freeze onto the surface of the growing hailstone. The clear layers develop when liquid water freezes to the hailstone surface. Hailstones can melt significantly on their way to the ground, becoming much smaller than they were up in the atmosphere. Small hailstones often melt completely before they reach the ground.
So you see? All sorts of stuff falls to the ground, not just rain :)