It can rain all day, and it can snow for hours on end, but hail always seems to fall for short periods only. Why don't you get prolonged periods of hail?

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    $\begingroup$ The all-day or multi-day rain and snow storms are created by different processes than are the thunderstorms that sometimes create hail. It's why you seldom get thunder & lightning in these sorts of storms, either. $\endgroup$ – jamesqf Mar 2 '17 at 20:28
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    $\begingroup$ Because I've yet to experience, or learn of, hail that is not associated with the strong convective updrafts of thunderstorms. I'm certainly not claiming that there isn't such a thing, just that I'd like an example. $\endgroup$ – jamesqf Mar 3 '17 at 3:21

The first reason is that hail is intimately connected to extreme updrafts in violent, high-energy, high-wind storms, mostly supercells. High-wind storms by their very nature tend to move quickly, so they go as fast as they came. And for obvious reasons an updraft must be a local phenomenon (or the air in a region would go away). One can have multiple hail storms on any given day when the conditions are right, but each one is necessarily a local phenomenon.

So we need to be more precise: Hail storms are short at any given location, but the storm can be longer lived and create more hail elsewhere, together with tornadoes, hard rain and other short-lived whether phenomena. Sometimes we can see a cell move over land from a distance. We can see that rain or hail continue to fall out of it, but the cell is constantly changing location.

The second reason, concerning a somewhat longer timescale, is that the condensation depletes the storm of energy which must be replenished, or the storm dies eventually, as can be seen with hurricanes: They are long-lived only over warm bodies of water which serve as the energy source. Over land they abate quickly.

This would make it possible that there are longer-lived hail storms over the sea, but I have no data about that. A remark on theweatherprediction.com says that hail in hurricanes is rare because the air is so moist and warm (i.e. energy laden) that it freezes too late and too far up.

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    $\begingroup$ I like this answer. In the reference frame of an observer the hail is over quick but in the reference frame of the storm motion and near the updraft the hail may last a long time. Might have to go find some imagery of hail swaths left in the wake of supercells. $\endgroup$ – casey Mar 3 '17 at 2:35
  • $\begingroup$ This has my upvote. Any answer which does not mention that the clouds which generate hail and heavy downpours are spatially local and they only rain on you for a short period of time because they travel overhead, has at best a tiny sliver of the answer. To flesh it out you might mention that there are also nimbostratus clouds that instead generate slow precipitation over long periods of time: these are formed by more gentle processes and are spatially diffuse and even if they move quickly (which they can!) can remain overhead for much longer times as a result. $\endgroup$ – CR Drost Mar 3 '17 at 19:40
  • $\begingroup$ I have chosen to accept this answer over the more upvoted answer by Gary Kindel because, in my view, it's more focused on answering the question I asked. Gary's answer gives a great explanation of hail formation but glosses over why this leads to short storms while this answer tackles it head on. $\endgroup$ – Jack Aidley Mar 7 '17 at 10:30

Hail is created within a thunderstorm by updrafts that carry water droplets to an altitude where temperatures are lower than the freezing point for water (32°F or 0°C). This freezing process forms a hailstone. This process can repeat itself each time increasing the size of the hailstone until the hailstones become too heavy to be picked up again.

The creation of hail requires high wind speeds that may only be sustained in the storm for a brief period of time. Longer periods of hail are possible but typically require large strong storms such as tornadic supercell thunderstorms.

References: How hail is created. Size of Hail vs wind speed

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  • $\begingroup$ You say: "[t]he creation of hail requires high wind speeds that may only be sustained in the storm for a brief period of time" and this seems like the key part of the answer. But why are they only sustained in the storm for a brief period of time? $\endgroup$ – Jack Aidley Mar 2 '17 at 15:03
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    $\begingroup$ The input needed for a thunderstorm are: moisture, wind, and air mass instability. Intense rain and hail quickly consume moisture within the thunderstorm. When moisture output is higher than input, the thunderstorm weakens in intensity, lower wind velocities, lower updraft speeds. Rain events requiring lower wind strengths can last longer. $\endgroup$ – Gary Kindel Mar 2 '17 at 15:19
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    $\begingroup$ @JackAidley Because strong winds cost loads imbalance (in pressure, energy, etc.), or if you wish, high entropy. Obviously, the wind "consumes these resources". $\endgroup$ – yo' Mar 2 '17 at 15:34
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    $\begingroup$ Great updraft speeds actually CAN be sustained for many hours... as long track supercells. But even then, it's only a small area, and assuming the storm is moving at a decent speed, each location will only get the hail for a short period. But there are cases of hailstorms lasting probably an hour or more in slow moving supercells. $\endgroup$ – JeopardyTempest Mar 3 '17 at 2:26
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    $\begingroup$ @JeopardyTempest agreed. Just yesterday we had an example with the storm that produced the Perryville tornado. That storm survived for around 6 hours. $\endgroup$ – casey Mar 3 '17 at 2:41

The other answer is correct: the mechanism of hail formation is the hail repeatedly travels up the cloud, gets heavier, and then when it becomes too heavy, it falls out of the cloud. This process has a number of preconditions such as an updraft and a lot of water, and when those preconditions are disrupted, so is the hailstorm.

But I want to look at it from a different perspective. Here, let's do an exercise. Find a fridge with an icemaker, and extract ice from it as rapidly as possible. If your icemaker is like mine, you can get a huge amount of ice out for a very short time, and then you wait a long time for more ice to be made.

There is no lack of moisture in an icemaker; the plumbing supplies as much as it needs. And there does not need to be an updraft, obviously. But in both the hailing cloud and in your icemaker, there has to be some process which is extracting heat from the water and moving it elsewhere in the environment. And that process takes time and energy. Once all the stored ice is gone, you're going to have to wait, and you're going to have to spend energy to get more.

You have to extract 80 calories of heat from each gram of liquid water to freeze it regardless of the current temperature and pressure of the air. And it takes an extraction of a whopping 540 calories per gram of water vapor to turn it to liquid, again, regardless of the temperature and pressure. That heat has to go somewhere! We think of hail as a massive exportation of coldness from the cloud, but you can't get coldness somewhere without putting that heat somewhere else.

That heat energy that is somehow extracted is going to change the characteristics of the cloud. And there are real physical limits on how much time and energy it takes to transfer that heat somewhere else in the cloud.

Your icemaker has a basically unlimited supply of moisture and perfectly stable conditions for making ice, and once its reserves are gone, it cannot supply a rapid flow of new ice. Even if a cloud had an unlimited supply of moisture and perfect ice making conditions, it also cannot supply a rapid flow of new ice once the reserves are gone. It turns out that it is really expensive in energy terms to make ice out of water vapor, and so on energy grounds alone we would expect hailstorms to be short.

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    $\begingroup$ To give some perspective with severe storms, some of the big thermodynamic environments can have in excess of 5 kJ/kg of air lifted from the surface with updraft velocities well over 100 m/s. Not infinite energy, but it can be quite abundant. $\endgroup$ – casey Mar 3 '17 at 2:31
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    $\begingroup$ Yes, once again, supercells fairly disprove this. Windspeeds in upper regions of strong storms are often on the order of 100-200 mph... plenty fast to exhaust the latent heat released. They can go on many hours making large hail. The updraft circulation well overcomes your issue. But moreso, the energy released in latent heat is massively counteracted by the negative work of the rising air parcel expanding. This is more like an industrial freezer than an ice cube tray! Your thinking would just as much say it shouldn't be able to rain for hours, as it takes lots of energy to condense as well. $\endgroup$ – JeopardyTempest Mar 3 '17 at 2:41
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    $\begingroup$ Convective storms typically weaken because they build TOO MUCH cold air (rain/ice loaded downdrafts choking off the inflow/updraft). A good attempt at an explanation you've given, but one that doesn't hold up in application. $\endgroup$ – JeopardyTempest Mar 3 '17 at 2:41
  • $\begingroup$ Interesting; this was the explanation I was given when I wondered this same thing in my youth. Apparently it does not entirely pan out. Thanks for the comments. $\endgroup$ – Eric Lippert Mar 3 '17 at 3:58

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