Hail and Thunderstorm Updraft Strength

This blog was written just before departing for the GLOBE Learning Expedition meeting in South Africa. I’ll be posting some additional blogs about the meeting in the coming weeks. In the meantime, after you read this blog, check out the GLOBE home page for student blogs and photos!

The weather report always tells you the wind direction and speed reported by a weather station near you. Sometimes you hear about the strong winds in the “jet stream” that exists several kilometers above the ground.

Did you ever wonder how strong the winds are in a thunderstorm? The up and down winds, I mean. You can make a rough guess on how strong the updraft in a thunderstorm is, if you have hail.

On the night of 4 June 2008, we had hail, so I decided to see how big it was. There are two ways to do this. You can go out and collect the hail, and measure it before it melts (which I have done), or you can take a picture of the hail – with a ruler or something to compare the hail to, and measure the size of the hailstones from a photograph.

Figure 1. Picture of hail on our back porch, 1830 Local Daylight Time, 4 June 2008. Typical size is one centimeter in diameter. Since the slate surface was warm some of the hail that fell earlier may have melted some. Location: north part of Boulder, Colorado, USA.

Figure 2. As in Figure 1, but hail on the grass. Typical size is 1 centimeter in diameter. The grass was cool enough so that the hail wasn’t melting as much as in the first picture.

In both pictures, the larger hailstones are typically about a centimeter in diameter, with a few that even larger. I don’t think there was much melting after the hailstones hit the ground, because I was taking the pictures as the hail was falling.

How can hail size tell you how strong the updraft is? The updraft has to be strong enough to hold the hail while it is growing. In other words, the hail continues to grow until its downward speed (which goes up with size and weight) is greater than the upward speed of the air.

Hail fall speed is determined by a balance between two forces: the downward pull of gravity and the drag force (air resistance) on the hailstone created by the air. As the hailstone falls faster, the air resistance gets bigger. Gravity of course stays the same. When the drag force is equal to the force of gravity, the hailstone reaches a constant downward speed, called its terminal velocity or terminal fall speed. The updraft has to be this strong to keep the hail from falling.

So we use the terminal fall speed to estimate the updraft speed. The hail will fall to the ground when the updraft weakens slightly, or when the hailstorm travels out of the updraft horizontally.

People have estimated the terminal fall speed of hail using equations, and they have measured it. I actually saw scientists measuring the fall speed of artificial hailstones (same shape and density as hailstones, but not ice) by dropping them down a stairwell that extended vertically about seven stories. Assuming a story is about 3.7 meters, that’s about 26 meters. Sometimes scientists measure the fall speeds of hail in nature. They can photograph them falling with a high-speed camera using strobe lights that flash on at regular intervals. Or they can measure hail vertical speed with a Doppler radar pointing straight up. It is more likely that the “natural” hailstones reached their terminal fall speeds than those in the stairwell.

Knight and Knight (2001) argue that the terminal fall speed is related to:

1. Air density (hail falls faster through thinner air)
2. Hailstone density (less dense hailstones fall more slowly)
3. Drag coefficient (the effectiveness of the air in slowing down the hailstones)

The shape of the hailstone is also important, but Knight and Knight assume the hailstones are spherical to keep the problem simple.

The graph shows how hail terminal velocity (or fall speed) is related to hail diameter.

Figure 3. Hail fall speed (and hence updraft needed) as a function of hail diameter. Red curves are from Knight and Knight (2001); Black points read off figure in http://www.jdkoontz.com/articles/hail.pdf.

For our one-centimeter hailstone, the graph shows a range of values, based on assumptions on air density at the height the hail is forming (taken by Knight and Knight as somewhere around 5.5 kilometer above sea level, where the air pressure is about 500 millibars or hectoPascals, temperature 253.16 K), drag coefficient, and the ice density in the hailstones. I picked up the hailstones, and they appeared to be solid ice rather than soft, so the ice density was probably about 0.9 grams per cubic centimeter. This suggests the updraft speed was between 13 and 18 meters per second, or between 29 miles per hour and 40 miles per hour.

According to the U.S. National Severe Storms Laboratory website, a one-centimeter hailstone falls at about nine meters per second – meaning that the updraft has to be that strong. This means the air had to be moving upward at 32 kilometers per hour or 20 miles per hour. This is more consistent with the less-dense hail.

So – to be safe, I would say the updraft overhead was between 9 meters per second and 18 meters per second. There are too many factors that we don’t really know to get much more accurate than that. This is between 32 and 65 kilometers an hour, or between 20 and 40 miles per hour.

The Encyclopedia of Climate and Weather (New York, Oxford University Press, Stephen Schneider) quotes a 47 meter per second fall speed (or necessary updraft) for a 14.4 centimeter hailstone, translates to a little over 100 miles per hour!

So – next time you have a hailstorm, measure the diameter of some hailstones to find out roughly how strong the updraft was! But if the hail is large, either photograph it from a safe place or wait until the large hail has stopped. If you don’t have a camera, collect some hail stones, put them in a plastic bag, and put them in a freezer until you have time to measure them.

Related blog: “More about Hail,” (No 19, 1 November 2006).

Reference:

Knight, Charles, and Nancy Knight, 2001: Hailstorms. In Severe Convective Storms, C. A. Doswell III, Ed., Meteorological Monographs, volume 28, No. 50. Published by the American Meteorological Society