First off, what exactly is a thunderstorm? Sure, we've all seen them but how do they get there? To build a thunderstorm we normally need three things: moisture, an unstable air mass and heat. On any given summer day in the Southeast or Midwest you can watch this take place. You'll see the big puffy clouds start building up as the warm, moist air rises. These are called "towering cumulus". They're not thunderstorms yet, but once they build above 20,000 feet or so it's best to steer clear of them.
Stages of a thunderstorm
At some point the warm air rises until it hits another warm air layer (usually the tropopause) and can't go up anymore. We now have a "mature" thunderstorm. At this point the moisture is condensing and falling back down as rain. This creates a system of updrafts and downdrafts inside the storm cell.
At some point most of the moisture has been dumped on the ground and the cell is considered to be "dissipating". This whole process from start to finish may take as little as an hour.
Frequently there will be more than one storm cell clustered together with some building while others are dissipating. This effectively looks like one giant thunderstorm and can last for several hours.
That's a big one. I don't want any part of this thing!
Then there's the stuff of my nightmares - squall lines. These are frequently caused by a cold front. The cold air mass acts like a giant snow plow - pushing warmer air up into the atmosphere as it moves along. You see these a lot in the springtime. These tend to produce long lines of very powerful storms along the leading edge of the front. Sometimes stretching for hundreds of miles.
That's the quick and dirty explanation of what makes a thunderstorm. A meteorologist could explain it much greater detail. But what makes them so dangerous? It's what's going on inside.
Inside a thunderstorm is an extremely unstable air mass. Some of the hazards associated with them are:
Severe turbulence (if you've never experienced it pray you never do)
Extreme precipitation (jet engines don't like to drink tsunamis)
Hail (jets don't like getting whacked with ball-peen hammers either)
Severe updrafts and downdrafts (bad, trust me)
Rapidly shifting winds (bad)
And of course, lightning, which surprisingly isn't the worst thing (scary though)
Every Air Force flight manual carried an ominous warning:
Warning. There is no peacetime mission that warrants penetrating a thunderstorm.
Hey, you don't have to tell me twice!
The key to dealing with thunderstorms is simple - avoid them at all costs. But how?
We are fortunate to have some tools at our disposal:
Ground based radar
And of course the good old-fashioned Mark One eyeball.
Before going out to fly we always take a good look at the radar picture. The green blobs are rain showers. No big deal. Might get bumped around a little. The yellow areas are heavy rain. I'll avoid those unless there's no other way. The red areas are thunderstorm cells and I will give those a wide berth. Preferably several miles.
Ground based radar image
Now let's look at another radar picture. This one shows the tops of the cells. This tells me if I might be able to get over the top of the system. In the winter it's usually possible. In the summer these things seem to go all the way to low earth orbit.
On this picture, the blue areas are in the 20,000 foot range. No problem topping those. The green is up to 35,000 so I might
go over the top, especially if it's dissipating (and hence getting smaller). Anything yellow or red is higher than I can go with any kind of safety margin. Plus it's probably something I don't want to get that close to.
Finally let's look at the lightning detection. Large pulses of electrical energy are easy enough to detect. This shows us a time lapse of all the lightning strikes over the last hour or so. They go from most recent (white) to oldest (dark red).
Now in a perfect world, I'd have a satellite feed of this data to a screen in my cockpit. Systems like that are readily available for general aviation. It's on my "wish list" but I doubt I'll ever see it.
Fortunately my dispatcher does have this information readily available and can send us a message via ACARS that we need to re-route.
Air Traffic Control also can see precipitation on their radars (somewhat) and can give us a heads up. That's not their primary job, however, and if they're busy they may not have time. If they do, they'll use the terms "Moderate", "Heavy" or "Extreme". Generally I don't like to hear the word "Extreme" with anything aviation related.
Our primary tools for weather avoidance are our plane's weather radar the tried and true "look out the window".
In the daytime we look for "towering cumulus" buildups. They're usually easy enough to spot. At night we look for lightning flashes. There is a rule of thumb that yellow flashes are further than 80 miles and white flashes are closer than 80 miles. My very unscientific observation seems to support this.
Thunderstorms are pretty at night. I prefer to admire them from a distance.
Either way, if we think there's even a chance of thunderstorms along our route of flight we turn the radar on. The weather radar sweeps 90 degrees either side of the nose and has a range of maybe 320 miles. Because the beam loses strength over distance, I don't think you get "truth in advertising" until 160 miles or so.
What the radar is actually picking up is the precipitation inside the thunderstorm. The newer radars can also see the movement of the precipitation and predict turbulence (but only if its associated with precip). The newer radars have color displays similar to the ground-based imagery. Green = not so bad, Yellow = bad, Red = really bad, Magenta = turbulence.
There's a bit of an art to getting the most out of the weather radar and I don't claim to be an expert. Setting the tilt angle of the beam is very important. Point it too low and you end up "painting" the ground. Point it too high and it may not pick up that storm that's right under your nose.
This picture shows a storm cell on airborne radar. We can change the scale of the display. In this case each ring is 20 miles so the storm is 60 miles away. The green line represents the aircraft's flight path. It should pass roughly 10 miles to the left of the cell.
Airborne Radar depicting storm cell
I find that it's best to keep one radar display looking close in and one looking at the "big picture". If I'm picking my way between storms, I don't want to go down a blind alley and find myself hemmed in. The picture on the left show's the plane clearing the storms at 35 miles. However, the big picture shows another large cell at the 60 mile ring. Probably not where you'd want to go.
This is called a "blind alley". You don't want to let yourself get boxed in.
Here's another potential "gotcha" with the airborne radar. The beam may not penetrate a strong storm due to the intense precipitation. This creates a "shadow" behind the image of the cell. Take a look at the two dark areas on this radar display. We really don't know what's behind the storms at ten and two o'clock because they're blocking our radar beam. Sometimes increasing the gain (power) of the beam helps, but as a general rule if I can't see it I'm going to assume it's something bad.
This is caused by attenuation of the radar beam.
Here's an example of doing and end-run around a line of storms. This was a pretty strong line of storms all the way from Michigan through northeast Ohio. The really big cells (red and magenta) were around Akron. All we had to do was take a slight jink to the right to avoid the whole mess. It maybe added a few minutes to our flight time but it kept the chickens (and me) happy. We always add a little extra gas if we there we'll have to dodge storms.
End run around a line of storms
Sometimes you have no choice but to pick your way between the storms. You'll avoid the really bad stuff but you're still going to get bounced around a bit. These guys were going from Newark (KEWR) to Stewart/Newburgh (KSWF) and had to dodge what looks like heavy rain showers near Stewart. That's why they took such a roundabout route getting there.
Picking your way through the cells. Not my favorite thing to do.
I don't like having to work my way between cells but sometimes you have to. That's usually when I start wondering if I made a bad career choice somewhere along the way.
What if you're trying to depart an airport with thunderstorms? It's normally best to wait it out. Thunderstorms are relatively short lived. If you wait an hour, or sometimes even a few minutes, they'll probably move.
Departing Denver one afternoon I see one lonely thunderstorm. It's not even very wide, just a few miles across, but I can see it's very dark and spitting out lightning. No big deal except we're heading right for it.
Me: "Hey boss, I think we might want to go around this thing."
Capt: "Denver, Boxhauler 605 request deviation left of course."
Friendly helpful Denver controller: "I'm dealing with a lot of airplanes here!"
Capt: "I'M NOT FLYING THROUGH THAT CELL!!!!"
Denver: "Roger. Boxhauler 605 clear to deviate."
What about thunderstorms at your destination? You pretty much have to wait it out in a holding pattern. At some point the storms will either clear out or you'll go to your alternate airport when you don't have fuel to wait any longer. We always put extra gas on if storms are forecast at our destination.
This picture shows a line of strong storms just north of Kansas City (KMCI). Kansas City seems to be a magnet for thunderstorms. Flights are getting into the airport from the south, but if the storms move closer they may shut the airport down for a time.
Line of storms just north of Kansas City (KMCI)
You're probably wondering if I've ever been hit by lightning. No, but I'm told it will scare the you-know-what out of you. It's been compared to "a shotgun going off in the cockpit". The good news is that it's not all that dangerous. It passes through the skin of the aircraft, in one end and out the other. Sometimes it puts a quarter-sized hole in the radome.
I'd just as soon not find out first hand. It's very undignified when I start shrieking like a little girl.
I'm told that due to global warming I can expect larger and more frequent storms. So I've got that to look forward to.
Stay safe out there.
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