National Weather Service United States Department of Commerce

Learning Lesson: Updrafts in Action


Rain and hail will be suspended by the updraft inside a thunderstorm until the weight of the hail and water can no longer be supported. Usually, the stronger the updraft in a thunderstorm, the more intense the storm and the larger the size of hail that can be produced. Suspending a ping pong ball in the stream of air supplied by a hair dryer will demonstrates how hail is supported in thunderstorms.

TOTAL TIME 10 minutes
SUPPLIES Hand held hair dryer; at least two ping pong balls
It is better to have several ping pong balls on hand as occasionally they will fall out of the air stream.
SAFETY FOCUS Thunderstorm safety


Point the nozzle of the hair drier up and turn the power on HI. Place the ping pong ball in the stream of air. The ping pong ball will be suspended by the air. Slowly tilt the hair dryer (to the left or right) until the ball falls.

Repeat the demonstration but add a second ping pong ball. Depending upon power of the hair dryer, both ping pong balls will be suspended. Occasionally, the balls will swap their order as they bounce around in the air stream.


The ping pong ball remains in the stream of air due to lower pressure created around the surface of the ball. The effect is called the Bernoulli Principle named after Daniel Bernoulli, an eighteenth-century Swiss scientist, who discovered that as the velocity of a fluid increases, its pressure decreases.

Bernoulli's principle can be seen most easily through the use of a venturi tube (see figure left). A venturi tube is simply a tube which is narrower in the middle than it is at the ends.

When the fluid passing through the tube reaches the narrow part, it speeds up. According to Bernoulli's principle, it then should exert less pressure.

This low-pressure effect also can be seen around the ping pong ball albeit in a different way. Instead of a narrowing in the center as in the venturi tube, the narrowing takes place around the perimeter of the ping pong ball (see figure right). In effect, there is an area of low pressure immediately adjacent to the ball.

The pressure is higher in the air outside of the stream created by the hair drier. The result is the ping pong ball bouncing from side-to-side as it reaches the edge of the flowing air and is pushed back into the region of low pressure.

You can now repeat the experiment and this time have the students notice the back-and-fourth oscillation of the ball as it tries to fall out of the stream but is push inward. Another way of seeing this is inward push.

Updrafts are responsible for the thunderstorms we experience. Generally, the stronger the updraft, the stronger the thunderstorm.

While we cannot predict if you will experience a thunderstorm on any particular day, we can know the area where thunderstorms are possible. If atmospheric conditions are such that the thunderstorms may become severe, the National Weather Service will issue a SEVERE THUNDERSTORM or TORNADO WATCH.

A WATCH, issued by the Storm Prediction Center in Norman, OK, is used when the risk of a severe thunderstorms and/or tornadoes has increased significantly, but its occurrence, location, and/or timing is still uncertain.

It is intended to provide enough lead time so that those who need to set their plans in motion can do so. These watches are issued by county.

The National Weather Service defines a severe thunderstorm as one having wind speed 58 mph (93 km/h) or greater, and/or hail size of 1" (2.5 cm) or larger.

Each watch is numbered sequentially, beginning with number 1 for the first issuance of each calendar year and contains...

  • hail size (in inches),
  • turbulence (for aviation community),
  • surface wind speed in knots,
  • maximum height of thunderstorm tops (in hundreds of feet),
  • estimated direction and speed of thunderstorm movement, and
  • a discussion of the meteorological reasoning that support the watch issuance and forecast for severe weather.

Building a Weather-Ready Nation

The most important safety rule is to known what is happening weather-wise so you will not be caught unaware in a hazardous situation. At the beginning of each day...

  1. Learn if you need to be aware of hazardous weather that might threaten you. You can do this by listening to the NOAA Weather Radio or check out the day's forecast at
  2. Check the Convective Outlooks to discover where thunderstorms are most likely to occur.
  3. Periodically during the day, recheck the forecast to learn of any updates or advisories.

If a WATCH is issued for your area, listen carefully to the message. This message will tell you the type of threat you can expected from severe thunderstorms. If hazardous weather approaches your location, seek sturdy shelter.

Fast Facts

One inch of rain over one square mile equals to 17.4 million gallons (66 million liters) of water. The weight of one inch of water over one square mile equals over 145 million pounds (66 million kg). 145 million pounds of water is almost 73,000 tons or the weight of 241 locomotives. This is a lot of water held up by wind.