Is there any rule of thumb or way to estimate the wind speeds or up and down drafts inside a storm by the size of hail that reaches the ground?


The October 2018 issue of AOPA's "Flight Training" magazine provided the following illustration and comments. enter image description here

  • A 24mph updraft will form ~1/4" hail, while a 98mph updraft will form 4" hail.
  • Down drafts are usually about 1/2 the speed of the updraft (due to a larger outer core area).
  • When a pilot encounters a downdraft, expect an updraft twice the speed to soon follow.
  • The change will be a quick dramatic shear.
  • Only about 10% of thunderstorms produce hail.

Certainly. There are two effects involved here which I will describe briefly as follows:

first of all, a small spherical object will exhibit a well-defined terminal velocity when free-falling in air. This means for a hailstone to have formed at a given diameter, the updraft velocity would necessarily had to have been close to the terminal velocity for an object of that size, for it to have stayed in the storm cloud and grown to that diameter.

Second, if you section a large hailstone you will find that it is layered, like an onion. this means that hailstone contains evidence of not just one freezing event but instead as many as there are layers in its structure.

In each case, then, the velocity of the updraft it experienced at each individual stage of its growth had to be significantly greater than its terminal velocity at that stage in order for it to have been lofted back into the storm cloud where it then got another ice coating before it began falling again.

  • $\begingroup$ Current research debunks the (somewhat commonly held) notion the hail cycles through the updraft multiple times, but rather makes one trip one and then falls to the ground and does all its growing and accretion within that single trip. $\endgroup$
    – casey
    Sep 12 '18 at 2:43

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