K-Index (KI)

KI = (T850 − T500) + Td850 − (T700 − Td700)

Where T is temperature (°C) and Td is dew point (°C) at the given pressure level.

The first term (T850 − T500) captures the lapse rate — how steeply temperature falls with height. The second term (Td850) adds low-level moisture. The third term subtracts the mid-level dryness at 700 hPa, penalising environments with a dry intrusion that would erode convection.

Thresholds:

• < 20 — thunderstorms unlikely.
• 20–25 — isolated storms possible (~20%).
• 26–30 — widely scattered storms (~40–60%).
• 31–35 — scattered to numerous storms (~60–80%).
• > 35 — numerous storms; organised convection likely (>80%).

Why K-Index often outperforms CAPE for UK forecasting: UK maritime air masses typically keep CAPE modest (200–800 J/kg) even on active storm days. CAPE depends on parcel buoyancy relative to the entire column; if the atmosphere is cold throughout, CAPE stays low despite real instability. K-Index captures the moisture depth at 850/700 hPa independently — a moist, steeply lapsing column with no dry mid-level intrusion will score highly even when classical CAPE is underwhelming. It was developed for the European/North Atlantic regime, not the southern Plains.

The radar map fetches K-Index on a 1° grid (~12 km effective resolution) — coarser than the 0.5° CAPE grid but appropriate for this synoptic-scale parameter.