Reading the weather

A radar does not measure rain at the ground — it measures precipitation reflectivity in the atmosphere (in dBZ). The higher the reflectivity, the heavier the precipitation: blue and green mean light rain or showers, yellow and orange steady rain, red a downpour and purple usually hail or a storm core.

As a rule of thumb: ~20 dBZ is drizzle, ~35 dBZ rain around 5 mm/h, ~45 dBZ a downpour above 25 mm/h, and above 55 dBZ it is almost certainly hail. The exact scale is in the legend on the map.

You can read a storm’s direction and speed from the animation: play the timeline and watch where the cores move. Slejvak also computes a prediction for the next hour — future frames are marked on the timeline and fade with lead time, because extrapolation certainty drops.

Mind the limits: radar can overshoot with high clouds (virga — rain that evaporates before reaching the ground), mountain valleys can shadow the signal, and a very local shower can move between two 5-minute frames.

A thunderstorm needs three things at once. Moist air near the ground, because the rain has to be made of something. Instability — an atmosphere in which a rising bubble of air stays warmer than its surroundings and therefore keeps climbing on its own. And something to give it the initial shove: a sun-warmed slope, a front, converging winds. Take any one away and the day ends in fair-weather cloud.

That rising motion is convection. Air expands and cools as it climbs, and once it cools to its dew point the water vapour in it condenses into cloud. Condensation releases latent heat, which warms the bubble again — and that is the engine of the whole thing. A flat cumulus grows into a towering cumulus congestus, and once the tower reaches high enough for its top to glaciate into the familiar anvil, it is a cumulonimbus: a thunderstorm cloud.

Every storm cell then passes through three stages. In the cumulus stage the updraft dominates and no rain has fallen yet. In the mature stage both currents run at once: the updraft feeds moisture upward while falling rain drags air down with it. The shear between them is what makes a storm a storm — turbulence, gusts, lightning. In the dissipating stage the updraft stops, the moisture is spent, and only weakening downdrafts remain.

And here is the crux: in the mature stage the downdraft undercuts the updraft and cuts off its supply of warm moist air. An ordinary storm, in other words, smothers itself with its own rain. That is why a single cell typically lasts twenty to thirty minutes. Storms that run for hours get around it — either cells regenerate one after another along a line, or it is a supercell, whose tilted updraft lets the rain fall beside it rather than through it, so the cycle never breaks.

All of this is visible on radar. A new cell appears as a small blob that strengthens through yellow into red within a frame or two, holds briefly, then fades. Play the timeline and you also get the direction it is travelling — which a single still frame can never tell you.

An ordinary storm smothers itself with its own rain: the downdraft undercuts the updraft, cuts off its supply of warm moist air, and that is the end. Which is why a single ordinary cell lasts tens of minutes. A supercell escapes that fate, and it has exactly one trick for doing so.

The trick is a mesocyclone — a rotating vortex bound to the updraft. Its presence is what makes a supercell a supercell; without one you have some other storm, however violent. It is roughly three to eight kilometres across and lasts at most a few hours.

It is born of wind shear — the change of wind speed and direction with height. Shear sets the air spinning about a horizontal axis: picture a rolling cylinder lying over the landscape. When a strong updraft develops beneath it, that cylinder is lifted and stood on end. Horizontal rotation becomes vertical, and that is a mesocyclone.

The rotation does the decisive thing: it tilts the storm so the rain falls beside the updraft rather than through it. The supply of warm moist air is never cut, and the cycle that ends an ordinary storm simply never happens. Instead of tens of minutes it runs for two to four hours, exceptionally more than ten.

Its behaviour follows from that longevity. A strong, sustained updraft can hold far heavier stones aloft, which is why the largest hail comes from supercells. So do extreme wind gusts, torrential rain, and the minority of cases in which a mesocyclone spins up a tornado.

Recognising a supercell from an ordinary radar image is hard — you want Doppler data, which shows the rotation. What you can read without it is behaviour over time: play the animation and a supercell holds as a single compact core that refuses to weaken, often curving to the right of the other cells. A storm that still looks just as strong an hour later is not an ordinary one.

Hail needs two things an ordinary shower does not have: a storm cloud with real vertical depth, and above all a strong enough updraft. That updraft is the only thing capable of holding a stone up long enough for it to grow at all.

High in the cloud sits a great deal of supercooled water — droplets below freezing that have not yet frozen because they have nothing to freeze onto. The moment they meet a small ice embryo they freeze onto it instantly. A hailstone therefore does not grow by rolling up snow; it grows by collecting droplet after droplet, each one freezing on contact.

How fast that freezing happens is what gives a stone its layers. Where water freezes instantly, air is trapped in the ice and the layer comes out opaque and milky — dry growth. Where it freezes more slowly, it spreads into a smooth film and the layer is clear — wet growth. The rings in a sliced hailstone are thus a record of the conditions it passed through, not a tally of trips up and down, as the story usually goes.

It starts falling the moment it is heavy enough that the updraft can no longer hold it. Hence the rule that makes hail such a good gauge of a storm: the stronger the updraft, the larger the stone it can keep aloft. A big hailstone is direct evidence of an exceptionally strong updraft, not a fluke.

It may not survive the trip down intact. Below the freezing level, where temperatures are above zero, the stone begins to melt, and smaller ones vanish entirely before reaching the ground. That is why only the biggest make it down on a warm day — and why the same storm can drop ice on a hilltop and nothing but water a few kilometres lower.

Hailstones are commonly between 5 mm and 5 cm across; anything smaller than 5 mm counts as graupel rather than hail. The record stone fell in South Dakota in 2018 and measured 23.6 centimetres.

On radar, a core capable of hail shows as very high reflectivity — the violet and white end of the scale. It is not proof, because radar measures reflectivity rather than ice itself, but the higher the value, the likelier it is ice rather than rain alone.

Normally temperature falls with height — the higher you climb, the colder it gets. In a temperature inversion the opposite happens: through some layer of the lower atmosphere, temperature rises with height instead. Cold air stays at the bottom, warmer air lies on top of it, and because cold air is denser it has no reason to go anywhere. The layers simply stop mixing.

The most common version here forms by radiation. On a clear night the ground radiates its heat away to space and cools the air in contact with it. If there is almost no wind, nothing stirs that chilled surface layer into the air above. Colder air is also heavier, so it drains down the slopes and collects in basins and valleys — a literal lake of cold air.

Fog is just the visible surface of that lake. Cool the air far enough and it reaches its dew point, the vapour condenses, and you get a cloud whose base is the ground. Meteorologically it is stratus; from a summit you look down on it as a sea of cloud. That is exactly why fog sits in valleys and beside water while a few hundred metres higher the air is clear.

The crucial difference from a summer morning mist is how easily it clears. In summer the sun warms the ground, convection stirs the layers, and the fog is gone within a couple of hours of sunrise. In autumn and winter the sunlight is weak, the day is short and the inversion layer is deep — the sun may not break it at all. An inversion can therefore last several days, exceptionally weeks, and what usually ends it is the wind of an incoming front.

In Czechia inversions are most frequent where the terrain is most enclosed: the Ostrava basin, the foothills below the Ore Mountains, and the Prague basin. That has an unpleasant corollary — whatever is emitted into the air collects under the inversion just as the fog does, so inversion episodes tend to be poor air-quality episodes too.

The practical rule that follows is simple. During an inversion there is no point hunting for sun in the lowlands, because it is above them. The gap between a summit station and a lowland one tells you whether the drive is worth it, and a mountain webcam confirms it in pictures.