One of the most important isotherms in meteorology is the lower tropospheric zero degree Celsius isotherm. It is important to precipitation type forecasts and infrastructure that is vulnerable to freezing. The zero degree isotherm is a boundary between surface wintry precipitation and rain (or melting wintry precipitation). When using the graphical forecast models, the zero degree isotherm can be located on the surface, 1000 mb and 850 millibar progs. This can help a forecaster determine precipitation type and the accumulation potential. The zero degree isotherm moves primarily by: (1) thermal advection (2) evaporative cooling (3) radiational cooling Freezing temperatures can do significant damage to crops and water infrastructure, especially if the temperature drops well below freezing at a time of the year below freezing temperatures are not expected. If the temperature just briefly drops below freezing, usually damage is minimal. A freeze warning should be issued for those nights temperatures drop well below freezing and for at least several hours. These are termed "hard" freezes. The use of wind machines and the latent heat release of condensation via spraying water can be used to avoid a light freeze. |