Frequently Asked Weather Questions

Weather Team Thunder, lightning and waterspouts are just a few of the weather phenomena our Bay News 9 Weather Experts explain in the following question-and-answer session.

E-mail them your weather questions.

What is a severe thunderstorm or tornado watch?

Storm A severe thunderstorm watch outlines an area with hail 3/4-inch diameter or larger and damaging thunderstorm winds are expected to occur during a three- to six-hour period.

A tornado watch includes the large hail and damaging wind threats, as well as the possibility of multiple tornadoes.

What's the difference between a severe weather watch and a warning?

A watch means severe weather is possible during the next few hours, while a warning means that severe weather has been observed, or is expected soon.

We often talk of severe weather. What is severe weather?

Flood There are many forms of hazardous, or severe weather. Nationally, warnings for heavy snow, freezing rain, high winds, flash flooding, river flooding, thunderstorms, tornadoes, tropical storms, and hurricanes and others can be issued. In a broad sense, these are all defined as severe weather, as any of them can and do pose a threat to life and property.

With respect to thunderstorms, a severe thunderstorm has winds in excess of 58 mph, hail greater than 3/4 inch in diameter and/ or tornadoes.

Although lightning can be deadly it is not a criterion for what is defined as a severe thunderstorm, since any ordinary thunderstorm can produce a lot of lightning. Also, excessive rainfall may lead to flash flooding, but heavy rain is not a criterion for a severe thunderstorm.

What causes lightning?

Lightning originates around 15,000 to 25,000 feet above sea level when raindrops are carried upward until some of them convert to ice.

For reasons that are not widely agreed upon, a cloud-to-ground lightning flash originates in this mixed water and ice region. The charge then moves downward in 50-yard sections called step leaders. It keeps moving toward the ground in these steps and produces a channel along which charge is deposited. Eventually, it encounters something on the ground that is a good connection.

The circuit is complete at that time, and the charge is lowered from cloud to ground. The return stroke is a flow of charge (current), which produces a luminosity much brighter than the part that came down. This entire event usually takes less than half a second.

What causes thunder?

Thunder is caused by lightning. The bright light of the lightning flash caused by the return stroke mentioned above represents a great deal of energy. This energy heats the air in the channel to above 50,000 degrees Fahrenheit in only a few millionths of a second!

The air that is now heated to such a high temperature had no time to expand, so it is now at a very high pressure. The high pressure air then expands outward into the surrounding air, compressing it and causing a disturbance that propagates in all directions away from the stroke.

The disturbance is a shock wave for the first 10 yards, after which it becomes an ordinary sound wave, or thunder. Thunder can seem like it goes on and on because each point along the channel produces a shock wave and sound wave.

How far away from a storm can lightning strike?

It's not clear what the maximum possible distance might be. Lightning has been known to strike more than 10 miles from the storm in an area of clear sky above. This is why it is important to seek shelter whenever you can hear thunder. Most lightning-related injuries and deaths are outside the actual thunderstorm.

How do thunderstorms form?

Three basic ingredients are needed to make a thunderstorm. The basic fuel is moisture (water vapor) in the lowest levels of the atmosphere. The air above the lowest levels has to cool off rapidly with height, so that 2-3 miles above the ground, it is very cold.

Finally, something in the atmosphere is needed to push that moist air from near the ground up to where the air around it is cold. This "something" could be a cold front or the boundary between where the cold air from one thunderstorm meets the air outside of the storm (called an outflow boundary) or anything else that forces the air at the ground together. When that happens the moist air is pushed up.

What happens to a blob of moist air as it rises? It cools off and after a while, some of the water vapor turns into liquid drops (that we see as clouds). That warms up the rest of the air in the blob so that it doesn't cool off as fast as it would if the air was dry.

When that blob of air gets to the part of the atmosphere where it is very cold, it will be warmer and less dense than the air around it. Since it is less dense, it will start to rise faster without being pushed, just like a balloon filled with helium does. Then more water vapor turns into liquid in the blob and the blob warms up more and rises even faster until all of water vapor is gone and the blob eventually reaches a part of the atmosphere where it isn't warmer than the environment (typically 5-10 miles).

What is a tornado?

Tornado A tornado is "a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud."

Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base.

What is a waterspout?

A waterspout is a tornado over water, usually meaning non-supercell tornadoes over water.

Waterspouts are common along the southeast U.S. coast -- especially off the Florida coast -- and can happen over seas, bays and lakes worldwide.

Although waterspouts are always tornadoes by definition, they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn small boats, damage ships, do significant damage when hitting land, and kill people.

What's the nature of tornadoes in hurricanes and tropical storms?

Storm damage Hurricane-spawned tornadoes tend to occur in small, low-topped supercells within the outer bands, NNW through ESE of the center -- mainly the northeast quadrant. There, the orientation and speed of the winds create vertical shear profiles somewhat resembling those around classic Great Plains supercells -- the shear being in a shallower layer but often stronger.

Occasionally a tornado will happen in the inner bands as well, but the large majority still form outside the hurricane-force wind zone. Because tornado-producing circulations in hurricane supercells tend to be smaller and shorter-lived than their Midwest counterparts, they are harder to detect on Doppler radar, and more difficult to warn for.

But hurricane-spawned tornadoes can still be quite deadly and destructive, as shown by the F3 tornado from Hurricane Andrew at La Place, La., (1992, two killed) and an F4 tornado at Galveston, Texas, from Hurricane Carla (1961, eight killed).

What are the different tropical cyclones?

Flood A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.

Tropical cyclones with maximum sustained surface winds of less than 39 mph are called tropical depressions. Once the tropical cyclone reaches winds of at least 39 mph they are typically called a tropical storm and assigned a name. If winds reach 74 mph, then they are called hurricanes.

How do tropical cyclones form?

Several ingredients are needed for the formation and eventual strengthening of a tropical low, including warm ocean water throughout a sufficient depth.

Warm waters are necessary to fuel the heat engine of the tropical cyclone. The system must also be warm core, meaning it is the opposite of cold core systems, which are common over land, especially during the non-summer months.

The wind aloft must also be light. This is referred to as wind shear and needs to be light or non-existent in order to maintain a tropical cyclones structure.

During a hurricane are you supposed to have the windows and doors on the storm side closed and the windows and doors on the lee side open?

Windows No! All of the doors and windows should be closed (and shuttered) throughout the duration of the hurricane. The pressure differences between inside your house and outside in the storm do not build up enough to cause any damaging explosions, as no house is airtight.

The winds in a hurricane are highly turbulent and an open window or door -- even if in the lee side of the house -- can be an open target to flying debris. All exterior windows should be boarded up with either wooden or metal shutters.

How about tape for my windows?

Taping windows is a waste of time and effort and, of course, tape. It offers almost no protection to the threat of flying debris. Plus, after the storm you will likely spend hours trying to scrape caked-on tape off your windows.

Why do winds rotate counter-clockwise in the northern hemisphere around low pressure?

The reason is that the earth's rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the northern hemisphere (and to the left in the southern hemisphere).

So, when a low pressure starts to form north of the equator, the surface winds will flow inward, trying to fill in the low, and will be deflected to the right and a counter-clockwise rotation will be initiated. The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator.

Does that force affect the flow in a toilet?

This force is too tiny to affect rotation in, for example, water that is going down the drains of sinks and toilets. The rotation in those will be determined by the geometry of the container and the original motion of the water.

Thus, one can find both clockwise and counter-clockwise flowing drains no matter what hemisphere you are located in. If you don't believe this, test it out for yourself.