Friday, January 4, 2013

And then the rains came!

My rain gauge measured 6.56 inches of rainfall for December... so welcome in this dry land! The story of how rain is formed reads like poetry or science fiction.  It demonstrates the interconnectedness of many elements of the natural world... Stick with me - this week I want to share with you a wonderful story...

Its hard to visualize what happens to invisible air - so think about a large bag of gas right on the earth's surface.  When the sun shines down, the soil takes in the heat and the air becomes warm by contact with the soil. When air becomes warmer its particles start to move faster and that causes the air mass to  expand.
Cumulus clouds formed by uplift of warm surface air
Suppose that a balloon full of air is held down at the bottom of a swimming pool.  The weight of the water overhead would push in on the air in the balloon and compress it.  As the balloon is allowed to rise closer to the surface of the pool the compression forces become less and the balloon would expand.
The air below a cloud  also has water - but it is all dissolved into the air

Our bag of gas at sea level is like the balloon at the bottom of the swimming pool.  The weight of the gases of the atmosphere has the effect of pushing in on the bag of gas just like the weight of the water in the pool compresses the gas.  Next time you go out doors look up and imagine the great weight of all that gas above you pushing on every square inch of your body (14.7 pounds per square inch)!
The colder air becomes - the less able it it to hold moisture - the more cloud particles are formed

This bag of less dense gas is now forced to rise through the more dense air that pushed in on it.  When  air rises it expands because there is less atmosphere above it to compress it.   The expanding air still has to push the surrounding air aside and this requires spending some of the stored heat energy of the enclosed airmass.   This rising air cools at the rate of -5.5 degrees F for every 1000 ft. it rises.

As the air cools it becomes more dense and if no other factors are present it would tend to sink back to its original level with no net change. 

When the clouds are thick enough, sunlight is absorbed, and from underneath they look quite dark

This normally does not occur because of another effect.  As air cools the ability of that air to hold moisture drops.  Every air mass has a temperature at which the air becomes saturated with moisture.  If the air is cooled still more, water dissolved in the air will turn from gas to liquid droplets – cloud particles are formed.  This temperature at which air is saturated is called the dew point.

The region of falling rain is visible from this cloud
Converting water-as-a-gas into water-as-a-liquid releases heat into the air mass. Molecules of water in the air combine into groups of water molecules when cloud particles are formed. This new bonding arrangement requires less energy than free molecules alone.

Some clouds can appear quite dense and produce no rain - it all depends on a portion of the cloud extending into the zone of freezing

The released energy has a warming effect on the air mass.  For each 1000 ft. that air is forced to rise above its dew point the air is heated by +3 degrees F.  But since it is still cooled by 5 degrees/1000 ft., the over all change for raising 1000 ft. above the dew point is -5+3= -2 degrees per thousand.

"Flat bottomed" clouds indicate the elevation at which the dew point of the air is reached
When air is forced to rise for any reason, and it rises past its dew point, clouds start to form at that point.  When we see flat-bottomed clouds – that represents a column of rising air that passes its dew point at the level of the flat cloud.  The cloud below that level can hold all of its moisture in a dissolved state – when it is pushed above that line some of the moisture begins to convert into cloud droplets, and also heats the air enough that it is prevented from contracting and sinking.

The cloud must extend into the region of freezing temperatures in order for rain drops to form
The stability of the air depends on the final temperature of the air, which results from both the cooling-due-to-rising/expansion and the warming-due-to-changing-gaseous water-into liquid cloud particles.

Clouds can be forced to rise by various causes – surface heating, collision with a dense mass of cold air, or being forced to rise over a mountain...

In addition to surface heating, air can be forced to rise by encountering mountains or cold air fronts
Here clouds have formed as air is pushed over the hills

For the clouds to produce rain the cloud must be forced to raise high enough that the top of the cloud is cooled to below freezing – with warm air currents rising up, and frozen air sinking there has to be vertical mixing of frozen and unfrozen cloud particles to create a sticky situation that causes the cloud particles to combine and form rain drops... When they become so heavy that the air currents cannot longer hold them, then they fall as rain (or snow if freezing temperatures extend to the surface,)

If air is cooled below its dew point on the surface - fog forms...
The water is warmer than the cold air above - Evaporating water can not remain evaporated in the colder air  - so it forms liquid cloud particles

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