Most city-dwellers can get away with not knowing or caring about the weather like that. If you're a pilot, you probably (hopefully!) know how to recognize a thunderstorm. If you want to be a pilot, you'll have to learn.
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What causes thunderstorms to form?
There are three ingredients to a thunderstorm: sufficient moisture, unstable air and uplift. If all three of these are present, a thunderstorm will form. Thunderstorms can occur anywhere in the world, though with widely varying probability.
For example, in tropical regions, they happen just about every afternoon. Florida acts like it's tropical most of the year even though it's higher latitude than the tropics, because of the influence of the warm water of the Gulf of Mexico.
Here in California with our diverse topography, it depends where you are. Any of our many mountain ranges, especially the Sierra Nevada, can provide the uplift ingredient for thunderstorm formation. All you need is a day with moisture and unstable air. So it can and does get in the way of mountain flying.
What does a thunderstorm cloud look like?
In California, usually pilots are among the few who can answer this question. Make sure that's true for you.
Thunderstorms always form from cumulus clouds. You can recognize cumulus clouds for their cotton-like puffy shape.If the clouds tops are jagged, not curved, then the cloud is not growing and not currently a danger of becoming a thunderstorm. However, they can change to growing. So you still have to keep an eye on them when you're flying or planning to fly.
Cumulus clouds evoke opposite reactions from glider and powered plane pilots. Glider pilots see cumulus and think "lift". Powered plane pilots see them and think "turbulence". But both agree thunderstorms mean trouble.
When cumulus clouds have curved tops, they're growing. If there's enough moisture to sustain them, they'll turn into a "towering cumulus" or a "thunderhead" on the way to becoming a full-fledged thunderstorm. Once there are wispy icy cirrus clouds in the top of the storm, it's a thunderstorm. It will spread out and flatten at the top, giving it an "anvil" shape which makes most thunderstorms easy to recognize. About the same time, it will begin to rain under the cloud. You don't have to be under it to know that.
Once it starts precipitating, the cumulus cloud becomes cumulonimbus. The nimbus suffix simply means a rain cloud.On a dry day here in the west, precipitation from a thunderstorm doesn't always reach the ground. If it evaporates before falling to the ground, we call it virga.
When does California get thunderstorms?
Not on one of our trademark "California severe clear" visibility-unlimited days.
But any day with some moisture in the air, for example when you see clouds around the mountains, there's something to watch. If there's instability in the air, the bigger mountains will be the first to start growth of thunderstorms.
Thunderstorms can occur ahead of and embedded in fronts. Immediately following a front, glider pilots often enjoy flying in the lift provided by unstable "post frontal" weather. That instability can also cause isolated thunderstorms.
On warm days, thermals from the sun-heated ground can start growth of cumulus clouds. We don't often have the mix of "muggy" warm and moist air in Northern California. When we do, it's more common in August to October during the Eastern Pacific hurricane season. The National Hurricane Center's Eastern Pacific page tells whether there's any activity which could later affect us on the West Coast. Storms usually head to the west and angle slightly more to northern latitudes until they get swept into the low point of a wave in the jet stream. Those waves in the jet stream correspond to the storms and troughs we have at our latitudes.
If a trough at our latitude lines up to catch some or all of the remnants of a hurricane or tropical storm and bring it through California, that's what it takes to make significant and widespread thunderstorms here. If you watch the weather, you can almost always see them coming before they happen. (Then you can quietly snicker at the people who tell you that it "never" happens here. You know better.)
I have an example which was literally close to home... see the photos of burn marks on a tree in my backyard, which was struck by lightning just after 6AM on Sept 9, 1999. It was during a thunderstorm that occurred in San Jose when the remnants of Hurricane Greg, which had broken up off the coast of Mexico, passed over Northern California. My yard wasn't alone - over a thousand lightning strikes were recorded in the Bay Area on the night of the 8th and morning of the 9th as the line of very energetic storms crossed the region.
Those conditions were exceptional. But they do happen once every few years.
What should you do about thunderstorms?
The winds inside a thunderstorm are so violent, they have literally ripped airplanes apart. Few people live to tell about it. So stay out of them.
When you're going flying, the first resource is your weather briefing. If it mentions convective activity, take it seriously. That generally means thunderstorms. And if you're flying over mountains, thunderstorms will be more prevalent, probably meaning fewer gaps, than over the valleys. They tend to increase in the afternoon, which may affect your return flight home. Plan for it.
The influence of a thunderstorm extends outside the clouds. Gusty winds and downdrafts can be expected around them. Hail can fall under the icy anvil cloud. The FAA and AOPA Air Safety Foundation (ASF) both recommend that light aircraft should stay 10 miles from any thunderstorm, and 20 miles from a severe thunderstorm. What are signs of a severe thunderstorm? If you can see frequent lightning or if the cloud is very large laterally, it's probably better to treat it as if it's severe, just in case. If you see a funnel or a tornado, definitely treat it as severe.
So that means if you're looking at the cloud from the air, you'll have to go around it. If you can't go around it, divert to an airport far enough away from any storms that you'll have time to secure the aircraft before a storm arrives. Remember that safe airports may be behind you.
If you're at the airport and you see a thunderstorm within 10 miles, just keep the aircraft secured until the storm passes. For example, one day I was among many pilots choosing to stay on the ground at Reid-Hillview Airport in San Jose while watching a bunch of nearby thunderstorms. Before the storms arrived, it had been hot and muggy - so the ingredients for thunderstorms were apparent. But some people were flying around in the pattern even as the thunderstorm clouds were overhead and squirrelly winds were blowing. Confirming my thought that it wasn't a good time to be flying, I photographed a funnel cloud from one of the thunderstorms southwest of the airport. That storm was getting better organized than it appeared.For VFR pilots, the advice is clear - make sure to stay out of the clouds. For IFR pilots, it's trickier because you're able to fly in the weather. And it can hide thunderstorms. The AOPA ASF's Safety Advisor #26 publication is called Thunderstorms and ATC. It has info how you can use ATC as a resource. ASF has more resources about thunderstorms at www.asf.org/thunderstorms including a quick reference card and an online training course which is good for FAA Wings recurrent training credit and incentives with some insurance carriers.
Useful but more general information about thunderstorms can be found at the Thunderstorms section of the National Weather Service's "JetStream online weather school" site.
As pilot in command, it's your responsibility to be aware of the location of thunderstorms and stay out of them. Sometimes it may be a good idea to land to figure out the weather situation from the ground. There's a saying... It's better to be on the ground wishing you were in the air, than in the air wishing you were on the ground. Use that as a factor in making sure you exercise good judgment.
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