National Weather Service United States Department of Commerce

Learning Lesson: Sizing Up Hail


SKYWARN® is a concept developed in the early 1970s that was intended to promote a cooperative effort between the National Weather Service and communities. The emphasis of the effort is often focused on the storm spotter, an individual who takes a position near their community and reports wind gusts, hail size, rainfall, and cloud formations that could signal a developing tornado.

Focusing on hail, based upon samples picked at random the student learns to estimate the size of hail.

TOTAL TIME 20 minutes
SUPPLIES Sealed box with a 4½" opening; several different size balls (see below) made of wood and/or styrofoam.

Acquire the balls at a local fabric or hardware stores. Randomly number each of the balls (1 through 24), with no association in size. Record each number and size on your answer key.

Collect balls of the following quantities and sizes: ½" - 3 balls; ¾" - 3 balls; 1" - 3 balls;1¼" - 3 balls; 1½" - 2 balls; 1¾" - 2 balls; 2" - 2 balls; 2½" - 2 balls; 3" - 1 ball; 3½" - 1 ball; 4" - 1 ball; 4½" - 1 ball. Place the balls in a box.

SAFETY FOCUS Thunderstorm safety


Choose a student to select balls from the box. Once the ball is selected have the student tell the class the number on the ball. The students should write that number on their paper.

Pass the ball, row by row, around the class allowing the students to hold it and estimate the ball's diameter in inches. Once the entire class has written their estimates, place it aside.

Repeat the procedure with the next ball chosen from the box. (you can save time by allowing several balls to be passed through the classroom at the same time.) Once the last estimate has been made, tell the students which number ball represented which size.


Take a poll of the class asking their results of their estimates. For example, hold up a 1" ball and ask...

  • How many students had the correct estimate?
  • How many estimated the ball was greater than 1"? If so, by how much?
  • How many estimated the ball was less than 1"? If so, by how much?

Again, holding up the 1" ball say the National Weather Service defines a severe thunderstorm as one containing hail size of 1" (2.5 cm) or larger (and/or any wind speed 58 mph (93 km/h) or greater).

Building a Weather-Ready Nation

When the National Weather Service issues a severe thunderstorm warning it means a severe thunderstorm is occurring, is imminent, or has a high probability of occurring. The warning will contain...

  • County or counties affected by the severe weather event,
  • Warning expiration time,
  • Location and direction of storm movement,
  • Locations in the path of the storm, and
  • Additional information and/or call-to-action statement(s).

Remember, due to the nature of a thunderstorm's size, there may be a severe thunderstorm warning in effect for your county but you may experience mostly blue skies. Know where the storm is in relation to your location and which direction it is moving.

Just because a thunderstorm may not be severe that does not mean cause damage. A thunderstorm can be deadly due to lightning alone. If you can hear thunder, you are close enough to be struck by lightning. Seek shelter indoors immediately and remain indoors until 30 minutes after the last thunder is heard.

Fast Facts

On June 22, 2003, a hailstone recovered in Aurora, NE, had a diameter of 7" (17.8 cm) and a circumference of 18 3/4" (47.6 cm). This hailstone was larger than the previous record large hailstone that fell in Coffeyville, KS, in 1970 (5.7" (14.5 cm) diameter and 17.5" (44.5 cm) circumference).

However, weight is the most important measurement. An accurate weight could not be determined for the Aurora hailstone; so, the Coffeyville hailstone of 1970 remains the heaviest hailstone weighed and verified in the United States at 1.67 pounds (0.76 kg).

Hail causes $1 billion+ in damages to crops and property each year.

Hailstones can fall at speeds up to 120 mph (193 km/h).

Costliest United States hailstorm: Denver, Co., May 8 2017. Total damage was $2.3 billion.