Monday, July 25, 2016

Science Guest Blog: Why Does it Hail in the Summertime? by Dr. Joe Castanza

I am fortunate to know some really cool people in my life (people with really cool talents).  My friend, (affectionately known as 'Dr. Joe') was kind enough to answer a question for The Jersey Momma's Boy, who once asked me, 'Why does it hail in the summertime?' Now my typical answer to this would be, 'Let's Google it,' but this time I thought I'd have my friend, Dr. Joe, write us a guest blog about this interesting topic.  We hope you enjoy it!

It gets colder (normally) as we go higher up into the sky.
photo courtesy of the National Weather Service

Cold Ice from the Sky on a Hot, Summer Day...What the Hail???
by Dr. Joe Castanza, Theoretical Physicist, Meteorologist and Former Airline Pilot

How can it be that ICE falls from the sky in the summertime? We call those bits of ice “hail” (which is different from sleet—sleet occurs during the winter when rain from warmer air higher up falls into colder air below).  To understand how hail is formed, we need to understand how thunderstorms form and what goes on inside.

Now, a lot of people may think that the wind only blows across the ground; that’s not true!!! Air also rises and falls in the atmosphere. We call those up-and-down currents “convection”.

Another thing that needs to be understood is that, while it may be 88°F on the ground, it can be -20°F at 15,000 feet above the ground and, even on the hottest days, -65°F or colder at 43,000 feet!!!  So, even on really hot summer days, it’s still really cold way up in the atmosphere!!!

Sometimes, on hot, humid days, the warm air rises and cold air falls very quickly… when we have that kind instability and water vapor (gaseous moisture, which we feel as “relative humidity”) in the air, the conditions are just right for the formation of a strong thunderstorm.  If it’s too windy on the ground and a few thousand feet above the ground, however, convection may be interrupted; we might still get a thunderstorm but, it will not be nearly as severe (that’s NOT to be confused with the strong gusts of winds that we see just BEFORE a thunderstorm; that’s the front of the storm passing by and it’s a sign that it’s time to go inside.)

So.  It’s a hot summer’s day… perhaps, in late July or early August.  The sun is out, the air is warm and humid, and there are large birds gliding around in circles overhead.  These large birds are able to glide without having to flap their wings because the sun is heating the ground, the ground heats the air above it; hot air is lighter than colder air and, therefore, it rises—just like a helium-filled balloon.

We sometimes call these rising currents of warm air “thermals”; these are what those big birds are gliding on; that’s a good sign that there’s convection occurring in the atmosphere.  When it comes to the formation of thunderstorms, we call these currents of rising air “updrafts”.  As these updrafts carry that moist air higher and higher into the atmosphere, the warm, moist air eventually reaches a level at which the temperature is cool enough that the water vapor condenses—it turns from water vapor (a gas) into tiny, lighter-than-air water droplets (liquid).  This is what forms clouds, as well.

BUT, the stronger those updrafts are, the higher up those tiny water droplets are carried.  When they reach the top of an updraft, the air carrying these droplets cools to the point that it becomes heavier than the warmer air around it and we wind up with a downward wind, which we call a downdraft.

Again, this continuous cycle of rising and falling air is called “convection”, and it’s required for the formation not only of thunderstorms but, hail, as well.

CONVECTION AT ITS FINEST!  In this image, updrafts are red, downdrafts are blue.
photo courtesy of the National Weather Service
As the water droplets fall with the downdrafts, they combine together with other water droplets and increase in size. Then, they end up in warmer air below and start to rise again with the updrafts.  As this happens, the speed of the updrafts continues.

Also, something else happens: Whenever water vapor turns into liquid droplets, heat is released (this is called the “latent heat of evaporation” and, it’s why sweating keeps you cool; as the water evaporates from your skin, it takes away some of your body heat and cools you down).  When that heat is released higher up in the atmosphere that causes the updrafts not only to accelerate but, also, go even higher, into much colder air.  If the moisture only goes so high to a point that the air is not cold enough to cause the water droplets to freeze, we’ll wind up with just rain.  The bigger the rain drops, the higher up those updrafts carried that moisture—the droplets had time to join together with other droplets to form really big rain drops!!!

On the other hand, sometimes, those updrafts go so high that they reach air that has temperatures below freezing, and these water droplets freeze.  That’s hail—but, we’re not done yet!!! That little piece of ice will fall into warmer air, its outer parts will melt (with ice still inside) and rise again—and, depending on how high they get carried back up in those updrafts, these little bits of ice will bump into water droplets that will accrete (freeze onto) our original bits of ice, and the hail gets even LARGER!!!  In fact, the largest, single piece of hail ever recorded fell in Vivian, South Dakota on July 23rd of 2010; it was 8” across and nearly 18¾” around; it weight almost two pounds!!!  Here’s a picture of it:

The largest hailstone, EVER!
(photo courtesy of the National Weather Service)
The stronger the updrafts are and the more moisture that’s available in the upper atmosphere for freezing onto our already-frozen hail, the larger the hailstones will be.  Eventually, even the strongest hailstones will become too heavy to be supported by the updrafts and they will fall.  Commonly, they’re small stones, like this one in my hand which is only ¼” across:

A 1/4" hailstone
(photo courtesy of ISI Physics Research Observatory, 2014)

…but, sometimes, they can grow to golf ball- or even softball-sized stones and do some serious damage.  For example, look at this poor car!!   
Ouch!  Someone better call the Insurance Gecko!
(photo courtesy of the National Weather Service)
Hail can cause serious damage to cars, homes, trailer homes, tall buildings… it can even hurt or kill people!!!  Airplanes are directed around hail storms and downdrafts because hail is one of the most hazardous weather effects known to aviation; just look at the window of this Boeing 727 airliner!!!

 Thank God, this aircraft landed safe
(photo courtesy of the FAA)
                    Look at the wing of this poor, single engine, propeller plane!
            (photo courtesy of the FAA)
So, to get thunderstorms, we need heat from the sun, moisture and unstable air (convection).  The higher that convection goes up into the atmosphere, the better the chances are that hail and severe thunderstorms will occur.  Hailstones can grow pretty large in those storms and, when they fall, they can do lots of damage to property, and even hurt or kill people!!!  When it starts to hail, “Get the hail inside!!!”

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