Did you know...(how rain starts)?

Did you know that every rain drop that falls from a thunderstorm over Wisconsin in summer started off as an ice crystal/snowflake?

Yes, it may seem incredible or even weird that all thunderstorm rain drops start off as ice crystals/snowflakes.  Think about that thunderstorm in summer that dumps tons of rain at your location.  Wow!  That’s a lot of ice crystals/snowflakes.  How can that happen - what’s going on here?  If you’re wondering how this can be, read on.

Here’s how it happens, for those summertime thunderstorms...

1. First, some basics:  water comes in three forms - a liquid (rain), a solid (ice crystals, snow, ice), and a gas (invisible water vapor).  We’ll come back to this idea later.

2. The sun warms the earth’s surface.  The air in contact with the ground absorbs heat that is radiated from the ground. This warmed air begins to rise, since warm air is lighter than the surrounding cooler air.

3. From this point on, we’ll refer to the warmed air that is rising as a "bubble of air."

4. The bubble of air continues to rise into the sky and cools at the rate of about 5.5 degrees F for every 1000 foot increase in elevation (9.8 degrees C per kilometer).  It continues to rise as long as it remains warmer than the surrounding air.

5. Eventually, the temperature of our bubble of air cools enough to allow the relative humidity inside the bubble to approach 100%.

6. As the relative humidity approaches 100%, the original, invisible, water vapor inside the bubble condenses as tiny cloud water droplets onto microscopic dust/dirt/clay particles that were embedded within the bubble.  These tiny cloud water droplets are the "white" cloud that your eyes observe.

7. The cloud grows as our bubble of air rises.  Eventually, the temperature of our bubble of air cools down to below 32 degrees F (0 degrees C), the freezing mark.  However, tiny cloud water droplets do not instantly freeze once they have cooled to 32 degrees F (0 degrees C).   Instead, they remain in the liquid state - as tiny cloud water droplets.   As the bubble of air continues to rise and cool below 32 degrees F (0 degrees C), the tiny cloud water droplets become what meteorologists refer to as "super-cooled" water droplets.  Refer to the image below - notice that the top portion of our cloud (made up of numerous bubbles of air that have become saturated with relative humidity at 100%) has grown above the freezing level which is represented by a white dashed line.

Growing Thunderstorm

8. Assuming our bubble of air continues to rise higher into the sky, eventually it will cool to the point where some of the tiny cloud water droplets do freeze into tiny ice crystals (they have no choice).  This usually occurs when the temperature of our bubble of air cools to about 14 degrees F (-10 degrees C).  When the top of a cloud has cooled to 14 degrees, research indicates that there is a 60% chance that the top of the cloud contains ice crystals.

9. Now that we have ice crystals developing in our bubble of air (at the top of the rising cloud), the ice crystals start to grow by either colliding with other ice crystals, or by "sweeping up" nearby super-cooled cloud water droplets.  In a sense, the ice crystal acts like a magnet, or vacuum sweeper.  Eventually, the ice crystal grows into a snowflake that begins to fall toward the earth.

10. On the way down toward the ground, but still within the cloud, our snowflake eventually falls below the freezing level (the level above the ground where the temperature equals 32 degees F (0 degrees C) for the first time, it begins to melt into a rain drop that reaches the ground.

There you have it....10 simple steps!  We started off with invisible water vapor (a gas), took it into the sky, converted it to tiny cloud water droplets (a liquid), converted it to ice crystals/snowflakes (a solid), and then melted it to rain drops (a liquid) that fell on your head. 

Refer to the image below of a mature thunderstorm.  The freezing level is a white, dashed line.  The orange colored arrows  represent warm, moist air rising upwards in the updraft at speeds of 40 mph to over 100 mph!  The dark green/black arrows represent ice crystals/snowflakes and rain descending in the downdraft.

Typically, summertime thunderstorm cloud bases on those hot, sticky days, are about 3,000 to 5,000 feet (1 to 1.5 km) above the ground, and the freezing level is usually about 13,000 to 15,000 feet (4 to 4.5 km) above the ground.  However, for a thunderstorm that grows to 45,000 feet (14 km) above the ground, there is roughly 30,000 feet (9 km) of cloud material found above the freezing level.  So, there’s an incredibly large amount of cloud material that can be converted to ice crystals/snowflakes!  Just think about it...most of the thunderstorm engine is found above the freezing level - even in summer!  Wow!  In some cases, severe thunderstorm tops have grown to 60,000 to 70,000 feet above the ground (5 to 6 miles)!

When the growing cloud top has cooled to about 5 degrees F (-15 degrees C), the growth rate of our ice crystal is maximized and very proficient.  At these temperatures the ice crystal "magnet" is on "overdrive."   This usually occurs at about 18,000 to 23,000 feet (5.5 to 7 km) above the ground.

An interesting fact: at temperatures of -40 degrees F (-40 degrees C), it is nearly impossible for the cloud  water droplets to continue to exist in the super-cooled liquid state.  Therefore, at these frigid temperatures, nearly all of the cloud water droplets have frozen into tiny ice crystals.  The tops of thunderstorms can easily cool to -40 degrees F (-40 degrees C), therefore the tops of thunderstorms consist mostly of ice crystals.  When a cloud grows into a thunderstorm, the conversion process to "all-ice-crystals" is referred to as "glaciation."  At this time, the top of the thunderstorm resembles a blacksmith’s anvil, and the top of the storm becomes smooth and wispy as the jet stream winds shred the ice crystals downwind.  Refer to the image below to see what the anvil looks like - the jet stream winds are blowing the anvil downstream from right to left.  At the base of the storm, you can see the rain descending to the earth’s surface.   The entire storm is moving right to left. 

Thunderstorm Side View

Another interesting fact...One inch of rain over one square mile equals 17.4 million gallons of water weighing 143 million pounds (about 72,000 tons), or the weight of a train with 40 boxcars.  Now, that’s a lot of ice-crystals/snowflakes!!!

Did you ever wonder why some clouds produce rain and others don’t?  Well, let’s go back to steps 8 and 9 above.  If our bubble of air doesn’t rise high enough into the sky to cool down to 14 degrees F (-10 degrees C), it doesn’t have enough ice crystals developing to kick-start the precipitation process.  There are several factors that determine how high in the sky our bubble of air will ascend, such as the temperature and amount of moisture our bubble of air initially possessed when it started to rise above the ground level, and what the temperature and moisture profile of the free atmosphere is outside of the bubble of air.

Here’s a couple links to check out...

http://www.srh.weather.gov/jetstream/index.htm   (Go to #7 - Thunderstorms)

http://www.spc.noaa.gov/   (Go to Education & Outreach section...."About Tornadoes and "About Thunderstorms)



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