Isentropic lifting is a term that can be encountered in forecast discussions. Isentropic lifting results when air rises due to air from one air mass flowing over another air mass. Typically, it will occur when air from a maritime tropical type air mass (warm and moist) flows over a colder air mass such as a cool maritime, cool continental, or polar air mass. The precipitation from isentropic lifting typically occurs above and into the cool air mass such as on the cool side of a warm front. The precipitation can fall as various forms of precipitation type including rain, snow, freezing rain or sleet depending on the temperature profile. The precipitation tends to be moderate to light in intensity but can be heavier when strong lifting occurs. The air that rises from isentropic lifting rises at an angle. The cool air will be shallow at the warm front boundary but becomes deeper when moving farther away from the warm front in the cool air. The warm and moist air flows over the slope of this cooler air. The rising air causes the warm and moist air to cool adiabatically and causes the relative humidity to increase. After the air is saturated, continued lifting condenses out moisture that produces clouds and precipitation. Isentropic lifting is common in the same places and same seasons where frontal boundaries are present. Mid-latitude locations are popular locations for fronts and low pressure systems. If the low pressure system is able to draw in warm and moist air into the system then isentropic lifting with significant precipitation will often occur as this air flows over the warm front. |