How a Thunderstorm Builds a Hailstone in Layers

Hail is ice that grows inside a thunderstorm, not ice that forms while falling. A strong updraft carries a small ice pellet up into a supercooled cloud layer, where it collects additional coats of ice on repeated trips up and down, before finally falling once it is too heavy for the storm to hold aloft. Southern California sees hail most summers, mostly as small, pea-to-marble-size stones from mountain and desert thunderstorms, with the Los Angeles Basin itself getting an isolated hail-producing cell only occasionally. Large, damaging hail is rare here because the region typically lacks the deep instability that supports the biggest storms.

What Is Hail?

Hail is a ball or irregular lump of ice that forms entirely inside a thunderstorm cloud, built in layers as it is carried through pockets of supercooled water before falling to the ground. That single fact, formation inside the storm versus formation on the way down, is what separates true hail from every other kind of frozen precipitation. Sleet and freezing rain form when snow or rain passes through a shallow cold layer near the surface and refreezes or freezes on contact. Graupel forms when a falling snow crystal picks up a light coating of frozen droplets. Hail alone requires a thunderstorm updraft strong enough to suspend and cycle an ice particle through the coldest, wettest part of the cloud, sometimes more than once, before gravity finally wins.

Because it requires an active thunderstorm, hail is also, by definition, tied to convection rather than to a layered winter storm system. A reader who wants the full three-way breakdown of how hail compares with graupel and true sleet, including hardness, bounce, and typical Southern California setting, should see WeatherEscape's sleet vs. hail vs. graupel guide, which covers that comparison in detail. This article goes further into the mechanics of hail itself: how a stone actually grows inside a storm, what determines its final size, and when and where Southern California is most likely to see it.

How Does a Hailstone Grow?

A hailstone grows by cycling through a thunderstorm's supercooled layer, adding a new coat of ice on each pass until it becomes too heavy for the updraft to support. The process starts with an ice embryo, often a frozen raindrop or a small graupel particle, that gets caught in the storm's updraft, the column of rapidly rising air that powers a thunderstorm. That updraft carries the embryo up into a region of the cloud, generally well above the freezing level, where liquid water droplets remain unfrozen even though the air temperature is well below 32°F. Meteorologists call this supercooled water, and it exists because the droplets lack a surface to freeze onto until they collide with something solid, like the embryo itself.

When the embryo collides with supercooled droplets, the droplets freeze onto its surface almost instantly, a process called accretion. Depending on how quickly the droplets freeze, the new layer comes out either clear (wet growth, when freezing is slow enough for the water to spread before hardening) or opaque white (dry growth, when freezing is nearly instant and traps air bubbles). A stone that cycles through both zones multiple times builds alternating clear and opaque layers, which is why a hailstone cut in half often shows rings similar to a tree trunk or an onion. Each additional pass through the updraft adds mass. Eventually the stone's weight exceeds what the updraft can hold aloft, or it gets ejected sideways out of the strongest part of the updraft, and it falls.

Updraft strength is the single biggest control on final hailstone size. A weak or short-lived updraft releases the embryo after only one or two growth cycles, producing small hail. A powerful, sustained updraft, the kind found in a supercell thunderstorm, can hold a stone aloft through many growth cycles, producing the large hail associated with the most severe storms in the central United States. Southern California's thunderstorms are usually shorter-lived and less intense than the supercells common on the Great Plains, which is a major reason local hail tends to stay on the smaller end of the scale.

When Does Southern California Get Hail?

Southern California sees hail in two distinct seasons that work through different storm types. The first is the summer monsoon window, roughly July through September, when moisture pulled north from Mexico fuels afternoon and evening thunderstorm cells over the mountains and deserts. These monsoon cells can be locally intense even though they are usually brief, and small hail is a common byproduct when one of them develops a strong enough updraft. The second window is the cool season, generally November through April, when unstable Pacific storm systems, particularly cold upper-level lows, can trigger scattered thunderstorm cells embedded in broader rain. Cool-season hail tends to be small and short-lived but can occur closer to sea level than monsoon hail does, since the whole atmosphere is colder at the time.

Elevation and terrain matter as much as the calendar. Mountain and high-desert locations see more thunderstorm activity overall because rising terrain helps trigger the convection that produces hail in the first place. Communities near Big Bear Lake and Wrightwood, both in the San Bernardino and San Gabriel high country, see thunderstorm cells, and the hail some of them produce, more often than the coastal basin does. That does not mean the Los Angeles Basin is immune. An isolated, sufficiently strong cell can track off the foothills and drop hail over lower-elevation communities, including areas near Pasadena, though basin hail events are typically brief and confined to a narrow path under a single storm cell.

The broader atmospheric river storms that deliver most of Southern California's cool-season rainfall are usually not the source of local hail; they tend to be steadier, less convective systems. Readers who want the fuller picture of how those large-scale storms differ from the more localized convective cells that actually produce hail can see what are atmospheric rivers in Southern California.

Why Is Large Hail Uncommon Around Los Angeles?

Large hail is uncommon around Los Angeles because the region typically lacks the deep atmospheric instability and sustained, powerful updrafts that build the biggest stones. Instability is the difference in temperature and moisture between the surface and the upper atmosphere; the greater that difference, the more explosively a thunderstorm can grow and the stronger its updraft can become. The central and southern Great Plains regularly combine intense surface heating, deep low-level moisture, and strong upper-level winds, the ingredients for supercell thunderstorms capable of holding a hailstone aloft through many growth cycles. Southern California's marine-moderated basin climate and generally drier upper atmosphere make that combination far less common.

That said, "uncommon" is not "impossible," and the region has documented severe thunderstorm cells that produced hail well beyond pea or marble size, particularly over the Antelope Valley, the Inland Empire, and the deserts, where surface heating is stronger and moisture from the monsoon can be deeper than along the coast. Any thunderstorm cell that reaches severe criteria, one-inch hail or larger under National Weather Service standards, can trigger a Severe Thunderstorm Warning from the relevant NWS office. Most Southern California hail, however, stays in the pea-to-marble range described in the table above, briefly whitening lawns, sidewalks, and cars before melting.

How Is Hail Different From Graupel or Sleet?

Hail is distinguished from graupel and sleet by where and how it forms, not simply by size. Hail is built exclusively inside a thunderstorm updraft, through repeated cycles of accretion in a supercooled cloud layer, and it is hard enough that a stone of any real size can dent soft surfaces on impact. Graupel forms through a single, gentler pass: a falling snow crystal collects a light rime coating of frozen droplets and comes out soft, opaque, and crumbly, no thunderstorm required. Sleet is different again: it starts as snow that melts in a warm layer aloft and refreezes into a hard, clear ice pellet as it drops through a cold layer near the surface, a process that has nothing to do with a thunderstorm updraft at all.

In practice, the fastest field clue is context. If thunder, lightning, or a dark, towering convective cell is nearby, small ice pellets falling from it are almost certainly hail. If small, soft, crumbly white pellets fall from an ordinary shower with no thunder, especially in the San Gabriel or San Bernardino Mountains during cold weather, graupel is the far more likely explanation. WeatherEscape's sleet vs. hail vs. graupel comparison walks through the full identification table, including hardness, layering, and typical storm environment for all three.

What Should People Do During a Hailstorm?

The core guidance is simple: get under solid shelter and stay away from windows until the cell passes. Hail always falls alongside an active thunderstorm, so anyone caught outside when hail starts should treat it as a lightning risk as well as an impact risk and move indoors or into a hard-topped vehicle immediately. Once inside a vehicle, pulling under an overpass or garage, when it is safe to do so, reduces the chance of windshield or body damage from larger stones; parking in the open during a hail-producing cell leaves a vehicle exposed to whatever size hail that storm can produce.

Drivers caught on the road during a hail-producing cell should slow down, since hail on pavement reduces traction similarly to loose gravel, and should avoid stopping directly under highway overpasses in a way that blocks travel lanes for others. After the storm passes, hail that has accumulated can still make roads slick until it melts, particularly in shaded canyon sections or at higher elevations near Big Bear Lake and Wrightwood.

Because storm cells and road conditions change quickly and locally, this page intentionally does not attempt to forecast current conditions; check the latest Severe Thunderstorm Warnings and point forecast from the NWS Los Angeles/Oxnard office or the NWS San Diego office depending on your location. Broader guidance on staying safe when thunderstorms build over Southern California, including lightning precautions that apply during the same storms that produce hail, is covered in WeatherEscape's lightning safety guide.

The short version: hail is a thunderstorm product, built in layers inside the storm itself, and Southern California sees it most summers in small, mostly harmless sizes over the mountains and deserts, with the occasional cell reaching the basin. It is not the same process as graupel or sleet, and it is not something to predict from this page. Before heading toward the mountains or the desert during unsettled summer or winter weather, check radar, the freezing level, and any active Severe Thunderstorm Warning on WeatherEscape's Los Angeles forecast pages so you know what a building cell near your destination is actually capable of.