Saturday, April 29, 2017

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Weather Lore And Fact Sheets - Hail Speeds

Here are some numbers for those of you who are interested in the how much "punch" a hailstone has. Values here are for freefall only. You'd have to increase/decrease it for wind effects.  This was posted by Tim Marshall on the U.S. chase listserver.

Hail Size (IN) Freefall Velocity 
Freefall Energy (FT-LBS)
1/2 55.8 (34.7) .009
3/4 68.1 (42.3) .044
1 80.1 (49.8) 1.43
1.25 90.0 (55.9) 3.53
1.50 98.8 (61.4) 7.35
1.75 106.5 (66.2) 13.56
2 115.2 (71.6) 23.71

Thus, a 1.25 inch hailstone has about 2.5x the punch of a one inch hailstone and a 2 inch hailstone has about 17x the punch of a one inch hailstone. OUCH! The rapid increase in freefall energy causes a sharp damage threshold for most roofing and siding products as well as vehicle metal. This occurs in the 1 to 1.5 inch diameter range. Keep in mind that hail will fall at different velocities due to its shape, density, spin axis, collisions with other stones, etc. Small hail tends to "float" due to air resistance.

Rainfall Information

Every wonder how we get the terms regarding rain and rainfall rates.  This chart should explain it all.

Drops Per sq. ft. per sec. Diameter of Drops (mm) Intensity (cm. per hr.) Intensity (in. per hr.)
Cloudburst 113 2.85 10.24 4.00
Excessive Rain 76 2.40 4.01 1.60
Heavy Rain 46 2.05 1.54 .60
Moderate Rain 46 1.60 0.38 .15
Light Rain 26 1.24 0.10 .04
Drizzle 14 .96 0.025 .01
Mist 2,510 .10 0.018 .002
Fog 6,264,000 .01 0.012 .005

Here's a great little trick sent to me by Sandy:

Keep a small mirror in your instrument screen or other sheltered outdoor
space so that it'll be at ambient temperature. Expose it to the sky for
half a minute or so. One mm is the threshold between drizzle and rain. The mirror also helps to distinguish hail, ice pellets, snow grains, and
freezing precipitation.


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