Total Totals Index
tc500mb = temperature at 500hPa [°C]
tc850mb = temperature at 850hPa [°C]
dewp850mb = dew point temperature at 850hPa [°C]
'define ttindex = tc850mb+dewp850mb-(2*tc500mb)'
tc500mb = temperature at 500hPa [°C]
tc850mb = temperature at 850hPa [°C]
dewp850mb = dew point temperature at 850hPa [°C]
'define ttindex = tc850mb+dewp850mb-(2*tc500mb)'