Another Electrifying Birthday on the Summit!

2015-08-01 18:59:02.000 – Michael Dorfman, Weather Observer/IT Specialist

 

We here at the Observatory have a continuous 8-day-on, 6-day-off schedule, maintained 365 days a year through holidays and birthdays. It is typically the luck of the draw whether you’ll be on the mountain or off the mountain on any given day, so I’ve had a couple of birthdays on the summit. So, it being my birthday, I’m going to discuss one of my favorite things here (as well as a common weather theme): thunderstorms!

Today, we experienced some impressive temperature drops as lines of convective rain showers moved through. To understand these temperature drops one must first understand how thunderstorms form. As the sun beats down on the earth’s surface, the air directly next to the ground begins to warm. As this layer of warm air grows, a buoyant force pushes it upward due to the temperature difference of the cold air directly above it. This layer of warm air then eventually forms bubbles that lift off like a hot air balloon departing the ground on a cool summer morning.

Of course, you can’t yet see any of this since air is clear. As the bubble of warm air rises, it cools due to decreasing pressure with height (temperature and pressure are directly correlated thanks to the Ideal Gas Law) . In unstable conditions, the surrounding atmosphere remains below the temperature or the rising bubble of air, allowing it to continue to rise.

Colder air can’t hold water vapor in gas form as easily as warmer air. Because of this, once the air cools below its dew point a cloud will start to form. Then something happens to this air mass that is not so “Ideal” (in the sense of the Ideal Gas Law). An impressively large amount of energy is stored in gaseous molecules. When the gas molecules condense into liquid cloud droplets, this energy is released in the form of heat. To follow the hot air balloon comparison, this would be like pulling the burner, allowing the balloon to heat up even more and accelerate upward. As it accelerates upward, more air condenses and allows it to accelerate even more.

This air will accelerate until either the surrounding air is warmer than the bubble of air or atmospheric conditions interrupt the upward flow of this air (for example a level of fast moving air would disrupt this convection). Severe thunderstorms can push through the troposphere (the lowest layer of the atmosphere) into the tropopause (a layer of the atmosphere where air doesn’t cool as much with height). Once the cloud rises to a level where there is no buoyant force, it spreads out creating the stereotypical anvil shape that we all recognize as a thunderstorm.

While all this convection is occurring, collector droplets (soon to be rain drops) form within the cloud. The convection in the cloud takes these droplets for a ride, sometimes pushing them so high in the cloud that even summer temperatures dip below freezing and freeze the drop. If this drop makes this trip in and out of the below-freezing zone several times, it will collect more liquid and freeze this liquid in layers. This eventually falls as hail in severe storms.

The hail doesn’t perpetually ride the Ferris Wheel of convection–it may simply get too heavy to be pushed up by the updraft, or it may get forced down by a downdraft. These downdrafts consist of air falling rapidly towards the ground through a storm. This falling air is colder than the surrounding air, and is often the reason why we see significant temperature drops during thunderstorms. These downdrafts can be extreme, regularly surpassing hurricane force (74 mph) on the summit. Combine this with hail and lightning regularly striking the summit and it is very evident that thunderstorms are exciting yet very dangerous from our perspective on the top of the mountain.

So, the next time you’re planning on venturing above tree line during a warm summer day, be sure to check our higher summits forecast to understand what you might encounter along the way! In addition to monitoring the forecast, always be aware of your surroundings. Tall, convective clouds, especially in the morning, can be a sign that the atmosphere is unstable and supportive of thunderstorms. Safe hiking!

 

Michael Dorfman, Weather Observer/IT Specialist

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