Total Totals Index (Miller)

Instability plus moisture. The Total Totals index is worked out from three readings of a weather-balloon sounding to gauge the chance of thunderstorms and how severe they may be. It adds two parts. The vertical totals, $VT = T_{850} - T_{500}$, are the cooling between 850 and 500 hPa and measure how unstable the air is. The cross totals, $CT = T_{d,850} - T_{500}$, are the 850 hPa dew point less the 500 hPa temperature and add the moisture low down. Together, $TT = VT + CT = T_{850} + T_{d,850} - 2\,T_{500}$.

Reading the answer. A vertical total of 26 or more is enough instability for thunderstorms, and 40 is the dry adiabat. A cross total above 18 is often needed for convection to start. The index itself is read in degrees: the National Weather Service reads below 50 as weak and above 55 as a strong signal of severe storms. A high value does nothing if a strong capping inversion holds the air down.

A worked case. The marks of 26 and 18 make a natural test: 16 °C at 850 hPa, a dew point of 8 °C and minus 10 °C at 500 hPa give VT 26, CT 18 and TT = 44. A warmer, moister low level under colder air aloft, 18, 12 and minus 16 °C, gives VT 34, CT 28 and TT = 62. Because the three readings enter as differences, the same 44 case in kelvins (289.15, 281.15 and 263.15 K) gives the same 44. The page also solves for $T_{500}$, $T_{850}$ or $T_{d,850}$ when the index is known.

Where it comes from. Miller's 1972 notes on severe-storm forecasting procedures (AWS Technical Report 200) and the National Weather Service glossary and Louisville pages define it. The K index is the companion, which adds the 700 hPa dew-point depression for the depth of the moist layer. The vertical totals are the lapse rate between 850 and 500 hPa times the depth of the layer, and the dew-point page gives a dew point from a temperature and a humidity when a station reports only those.