Probability of hail after a few minutes of rain?

[KLM212]

Hi everybody, I'm just curious about this.

Why is that hail almost always comes at the beginning of a storm? Is that after that the rain colds the terrain/air?

Anyway, I guess there may still be chances, how rare is to see hail dropping after rain was falling for a while?

[lookingnorth]

Well, hail is heavier than rain and encounters less air resistance, so it falls faster. That may be part of the reason why.

[Moderately Unstable]

Well, the short answer is, it doesn't always. For small hail, you can get rain-hail-rain. As you know, hail is produced when we have an updraft strong enough to lift rain droplets above the freezing level, which then condense into a hail embryo, which grows with time. If you picture a storm traveling along, the updraft is ahead of the storm, pulling in warm air. That's also the part of the storm that keeps hailstones aloft. Thus the densest objects, aka heaviest relative to surface area, will fall faster, after growing to sufficient size high up in the upper levels of the storm clouds. Smaller hail will fall next, relative to the direction of motion, as it gets more tied up in the overall wind structure of the storm. Rain follows last. Large hailstones are falling from beneath the tallest parts of the storm cloud--areas that have the highest updraft speed, right after the rain free base area. As they fall back into the lower levels of the storm, they're too heavy to be "mixed up" by the turbulence in the mid-level flow. The main cold downdraft region is behind this segment of the storm. Hence why you will often see large hail-small hail-rain.

 

Now, this rule applies for supercells and other storms with very strong updrafts. But not for marginal hail cases. In other words, if you have, say, a storm producing pea sized hail, the dynamics will often have that small hail corridor within a surrounding area of heavy rain. You can see this easily using dual pol radar products. In this case, the storm is producing through one of a few mechanisms, a region of marginal hail, and that correlates with the greatest intensity of precipitation. E.g, maximum downdraft intensity = hail zone, for small hail. Having a relatively cold air mass with low freezing height coupled with a nice strong cold front is a great way to produce this type of hailstorm.