365 Days of Climate Awareness 31 - Water Vapor and Cloud Formation

365 Days of Climate Awareness

31 - Water Vapor and Cloud Formation

Water which has evaporated and exists in the air as a gaseous component is water vapor. To do this the water has drawn a large amount of heat--539 kcal/kg, to be precise--from its surroundings. When water is evaporating from a body of water, it pulls the heat from the rest of the water, leaving the water source colder. (This is a topic we will revisit later concerning ocean currents.)

Water molecules (H₂O) are lighter than molecular oxygen (O₂) and nitrogen (N). Therefore, air with water vapor--"wet" air--is less dense than "dry" air. Furthermore, air's capacity for water vapor increases with temperature. So not only does the air become less dense as it warms (another topic to be covered soon: the ideal gas law), but also as it absorbs water vapor. This has gigantic implications for air circulation and weather (yet another topic to come--it's tantalizing, but I have no time for tangents in these posts. We will address storms and hurricanes down the road). Water vapor, a very important greenhouse gas, is by mass about 0.25% of the atmosphere, but it is not well-mixed, meaning the concentration can vary with time and/or by location.

How Precipitation occurs

When the air is saturated with water vapor, due either to lowered temperature or pressure, the water starts condensing, forming tiny droplets around aerosols like dust, pollen or salt. In this way clouds and fog (which is a ground-level cloud) form, as well as dew (droplets which form on the surface of larger objects). Dew warms the surface of the object on which it forms.

If the initial air parcel loaded with water vapor was warm and humid enough, and enough water droplets condense from it, over time the droplets will join, or coalesce. They become progressively larger, to the point that they begin to fall as rain. Raindrops range in diameter from 0.1 to 9 mm, though larger droplets will break up as they descend. (A typical diameter is 2 mm.) Due to air resistance, larger raindrops fall more quickly--they have a higher terminal velocity--than smaller ones. (This is why mist falls so softly, but the pelting raindrops of a thunderstorm, where a huge amount of water is being released, can hurt.)

Snow begins in a similar manner. Vapor condenses into water droplets around tiny aerosols, but the water freezes. Additional water vapor then deposits in crystalline form directly onto the frozen droplet, or nucleus. When the crystal is heavy enough it begins to fall. But not all snow reaches the ground. It can melt back into water, or sublimate into vapor again before reaching the earth.

Tomorrow: the structure of the atmosphere.

Be well!