What is a supercell?
A supercell is an often dangerous convective storm with a very organized internal structure including a rotating updraft that allows it to keep going for up to several hours. Supercells are capable of producing severe weather including high winds, large hail, and strong tornadoes. They are most frequently isolated and often develop in the warm air ahead of a squall line. A supercell also usually forms in an environment with strong vertical wind shear that causes the updraft to begin rotating.
Why are there thunderstorms?
Thunderstorms are a great way for the atmosphere to release energy. When warm moist air meets colder drier air, the warm air rises, the water vapor condenses in the air, and forms a cloud. As the water vapor condenses it releases heat, which is a form of energy. A large amount of the thunderstorm's energy comes from the condensation process that forms the thunderstorm clouds. As the thunderstorm progresses, eventually the rain cools the entire process down and the energy is gone.
Thunderstorms also help keep the Earth in electrical balance. The Earth's surface and the atmosphere conduct electricity easily – the Earth is charged negatively and the atmosphere, positively. There is always a steady current of electrons flowing upwards from the entire surface of the earth. Thunderstorms help transfer the negative charges back to earth (lightning is generally negatively charged). Without thunderstorms and lightning, the earth-atmosphere electrical balance would disappear in five minutes! We aren't really sure what would happen if this balance wasn't maintained. But thunderstorms are not the only way the atmosphere conducts electricity – the solar wind and ionospheric wind play a role too.
Where can I find out information about a storm?
Several places will have storm information. The local National
Weather Service office in the area of the storm conducts damage surveys from
severe weather events. The Storm
Prediction Center 
documents
storm reports, but the official designation is left up to the local National
Weather Service Office.
The NOAA
National Climatic Data Center maintains
the official national database for weather.
What direction does the wind come from during a thunderstorm?
There is no one direction the wind comes from when thunderstorms or tornadoes occur. Where warm moist air is forced to rise by hills, mountains, or areas where warm/cold or wet/dry air bump together, thunderstorms can form. The surface direction of wind varies depending on your location (northern/southern hemisphere, and even coastal/plains/mountains), and what weather patterns usually affect the area.
Should I drive through a thunderstorm?
Whether or not you should drive through a storm is a complicated question. It would be important to know what kind of storm you are dealing with – is there a potential for tornadoes, hail, flash flooding, or blinding rain? If so, I would suggest leaving your car and taking shelter in a substantial building. Do not take shelter in your car under an overpass – you increase your risk for being a victim of flash flooding, blocking traffic and causing an accident, and you also increase your risk from being injured by flying debris from strong winds or tornadoes.
Be aware of your surroundings – the weather, routes to safety, sturdy shelter – these are all things that will contribute to your safety. If you know there is potential for thunderstorms, you can listen to the radio to find out where they are, how quickly they are moving, and what risks are involved. Based on this information, you can take appropriate precautions to protect yourself. NOAA Weather Radio is a valuable tool that provides constant weather information and would be helpful in your decision making.
Is there a specific thunderstorm season?
One of our scientists has compiled statistics on severe
weather hazards for the U.S. You can click on your area and a graph will
come up showing what days of the year have the most thunderstorms. You might
consider contacting your local National
Weather Service Forecast Office because
they will be more in tune with local climatology.
In addition to actually seeing them, how are they detected?
We can see thunderstorms with a variety of tools. Radars let us see where rain and hail are located in the storm. Doppler radars also let us see how the wind is blowing within and near the storm. Some features of thunderstorms, such as the anvil that spreads out at the top of the storm, can be seen from satellites.
Why are severe thunderstorms so dangerous?
Many hazardous weather events are associated with thunderstorms. Fortunately, the area affected by any one of them is fairly small and, most of the time, the damage is fairly light. Lightning is responsible for many fires around the world each year, as well as causing deaths when people are struck. Under the right conditions, rainfall from thunderstorms causes flash flooding, which can change small creeks into dangerous raging water in a matter of minutes, washing away large boulders and most man-made structures. Hail up to the size of softballs damages cars and windows, and kills wildlife caught out in the open. Strong (up to more than 120 mph) straight-line winds associated with thunderstorms knock down trees and power lines. In one storm in Canada in 1991, an area of forest approximately 10 miles wide and 50 miles long was blown down. Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.
Why does it rain on one side of the street and not the other?
The answer to this question is that most things have a beginning and an end. The rain has to start somewhere and end somewhere. Sometimes it happens to start on one side of the street, and moves away, never getting the other side wet.
Why doesn't the west coast of the U.S. experience more thunderstorms?
There is less convection on the west coast in part because of the cool water temperatures off shore in the Pacific Ocean. That impacts the temperature of air that moves inland, making it generally more stable and less convective; convection = thunderstorms). To get thunder you need strong updrafts (convection), which means rapidly falling temperatures with height (vertically) in the atmosphere.
Are there winter thunderstorms?
Winter thunderstorms are rare because the air is more stable – strong updrafts cannot form because the surface temperatures during the winter are colder.
Why are some clouds darker than others?
The thicker the cloud, the less sunlight can come through. If there is a cirrus cloud above another cloud, it will shade the lower cloud making it a little darker. Cloud drop size also has an effect on how much sunlight gets through has well, with smaller drop sizes blocking more sunlight.
Why do some thunderstorm have a greenish hue?
No one really knows what causes thunderstorm clouds to look green at times. It could have something to do with sunlight refracting through the storm.
Why does the sky sometimes turn orange after a thunderstorm?
Most thunderstorms occur in the late afternoon. By this time of day, the sun is setting. The orange hue is caused by the same process that causes the vivid colors at sunsets. Shorter wavelengths of light (blue) are scattered quickly, leaving only the yellow-orange-red end of the spectrum.
What is a derecho?
A derecho is a fast-moving windstorm that is made up of thunderstorms that repeatedly develop along the leading edge. These lines of storms can move very quickly and produce widespread straight-line winds over long periods of time. Derechos can move anywhere from 35-70 mph and last 8 hours or more. Most derechos that produce severe weather move at speeds greater than 50 mph. Warm season events probably move a little slower than cold season events.
Do all thunderstorms have hail?
Most thunderstorms have hail, but not all thunderstorms produce hail at the ground. Temperatures at the upper levels of a thunderstorm are well below freezing, allowing for the development of hail, but sometimes it melts before reaching the surface of the earth.
What is SKYWARN?
SKYWARN is a National Weather program that is run by and coordinated by the NWS. It was a concept developed in the early 1970's that was intended to promote a cooperative effort between the National Weather Service and its communities. The emphasis of the effort is often focused on the storm spotter, a volunteer who takes a position near their community and reports wind gusts, hail size, rainfall, and cloud formations that could signal a developing tornado. Another part of SKYWARN is the receipt and effective distribution of NWS information.
next -- RELATED LINKS