Halny
The halny is Poland's foehn: air forced over the Tatra crest from the south, descending into Podhale, warming about a degree for every hundred metres it falls and drying as it does. It takes roofs off in Zakopane, flattens spruce stands in the Tatra National Park, and it has produced 288 km/h on Kasprowy Wierch. The Sudetes have their own weaker version down the Jelenia Góra and Wałbrzych valleys.
Why this site draws the flow at 253 metres
Most forecast sites show you the halny as a wind-gust map, which is the result. The trouble with the result is timing: in the lee of a ridge the 10 m wind often stays almost calm right up to the moment the descending flow breaks through to the surface, and then it is 100 km/h within minutes. A gust map is a report.
The driver is above the surface friction, so that is what is drawn here. COSMO publishes wind on fifty hybrid levels; the centre of layer 45 sits 253 metres above the ground. The field is the component of that wind directed down the local terrain gradient:
halny = −(u, v) · ∇h / |∇h| [m/s]
Positive means the air is descending the slope; a southerly crossing the Tatra crest and falling into Podhale is strongly positive, and the same wind climbing the Slovak side is negative. The terrain gradient is smoothed over about 12 km first, because a foehn is a response to a mountain range and not to one 2.8 km cell of it.
The field is masked to where COSMO's own sub-grid orographic standard deviation exceeds 40 m. Over this domain that selects the Tatras, the Beskids, the Bieszczady, the Sudetes and the Świętokrzyskie, and excludes the entire northern plain — where “downslope flow” over a 30 m moraine is arithmetic rather than meteorology.
See it on the map → · then compare with the gusts a few hours later.
The three signatures, together
A halny is unmistakable in observations because three things happen at once on the lee side:
- Temperature rises, often sharply and often at night — Zakopane can gain 10 K in a couple of hours in January.
- Humidity collapses. The dewpoint falls as the air is compressed. This is the cleanest single indicator.
- Gusts arrive, and the summit station has been blowing hard from the south for hours before they do.
So: watch Kasprowy Wierch for a strong southerly, then watch Zakopane's humidity.
Live, from the IMGW network
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| Station | °C | RH % | Wind km/h | Dir | Reading |
|---|---|---|---|---|---|
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Kasprowy Wierch is at 1991 m and Śnieżka at 1602 m — the two Polish summit observatories. A southerly above about 60 km/h on Kasprowy with the pressure falling to the north is the classic set-up.
What the index will and will not tell you
This is a flow diagnostic, not a foehn detector. It says the model has air being pushed down a slope; it does not know whether the flow will stay attached to the ground, where the hydraulic jump will land, or whether a critical layer aloft will trap the wave and prevent the breakthrough entirely. Those are the questions that decide whether a halny is a warm afternoon in Zakopane or a disaster, and a 2.8 km model resolves them only approximately.
Used the way it is meant to be — as the leading indicator, read beside the gust field and the summit observations — it is worth a good deal. Used alone it will occasionally promise a storm that never reaches the valley floor.