Rain falls only under certain conditions

April showers are supposed to bring May flowers, so can we count on bountiful blooms next month? So far we’re coming up a bit short on rainfall with a little over 1 inch received so far in a month that averages 1.75 inches. Year-to-date rainfall totals are actually running a little bit above normal, by about a half-inch for a total of 9.54 inches. This pales in comparison to last year’s soggy spring, when by late April we had already gauged more than 15.6 inches of rain.

Just how do we get rain anyway? Most of us as children learned about the water cycle, which revolved around rainfall/evaporation/clouds/and more rainfall. Not all clouds bring us rain, however. Some pass us by with nary a drop, while others can bring torrential downpours.

The process that makes rain from a cloud is not as intuitive as you might think. Clouds are made from tiny water droplets or ice crystals suspended in the air. It would make sense to think that the tiny drops of water would bump into each other, combine to form bigger drops, and eventually fall as rain when they got too heavy to stay suspended. That does happen sometimes, and it is called collision and coalescence. That process is most common, however, in the tropics, where warm clouds are made up of mainly liquid droplets.

At our latitude, most of the clouds are a mixture of “supercooled” liquid droplets (supercooled meaning water that is still liquid below 32 degrees) and ice crystals. Laws of physics dictate, that in a cloud, an ice crystal will “grow” at the expense of the liquid water droplet. Water vapor (the gaseous form of water) in the cloud is more readily deposited on the ice crystals, causing them to grow larger. When these ice crystals get too heavy to stay suspended, they will fall first as a snowflake, then melt into rain (or stay as snow depending on the time of year and the temperature profile of the atmosphere.)

In order for the ice crystals to grow, a supply of water vapor must be constantly transported up to cloud level. We now have to backtrack one step to look at the ways the water vapor makes its way up to the clouds. We can thank area mountains for providing one mechanism. Air cannot pass “through” a mountain, so it is forced to rise up the slope, bringing water vapor up with it.

Advancing cold and warm fronts also “lift” the air ahead of them, causing clouds to form and sometimes precipitation.

A third method of moisture transport is instability. This can be achieved in two ways – by heating the air below or by cooling the air above. Think of instability as being like an inflated beach ball held underwater. As soon as you let go, the ball’s buoyancy makes it rise to the surface without anything else lifting it up.

So to get rain, you obviously need a source of moisture, which can come from evaporation of water from the ground, or the moisture can be carried in from other areas by the horizontal movement of air. This water vapor must then be lifted high enough to cool and condense into clouds.

Finally, the clouds must receive an adequate supply of water vapor before they will precipitate. Even then, precipitating clouds are still not a guarantee of rainfall on the ground. If the air below the clouds is too dry, it is possible for the rain or snow to evaporate before reaching the ground. This is called virga, and can be seen as fall streaks or rain shafts that seem to disappear before touching the ground.