Dew-point depression
= Dew-point depression
{title2=$T-T_D$}
The difference between air <temperature> and its <dew point>. For unsaturated air it is positive. Near terrestrial surface conditions, a closed ascending parcel's <lifting condensation level> is approximately $125\,\mathrm{m\,K^{-1}}$ times this difference, because its <temperature> decreases about $8\,\mathrm{K\,km^{-1}}$ faster than its <dew point>.