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 EASIER TO MEASURE AND FORECAST: RAIN OR SNOW?

METEOROLOGIST JEFF HABY

This writing focuses on trying to answer the question of which is more difficult to measure and forecast: rain or snow.

Rain measurement: Rain is fairly easy to measure and all rain basically has the same density, thus 0.5” measured at one station with the same type of device will be the same amount of water as 0.5” measured at another station.

Rain forecast: Rain can be easy or very difficult to forecast depending on the situation. It is more difficult to forecast the amount of rain at a point location if it is a convective rain event. One location may get 1.0” while another location nearly gets very little. This is one reason why the rain chance is typically expressed as a percentage that a point location will receive measureable precipitation.

Snow measurement: Snow can be more difficult to measure than rain and the density of snow in two different situations or even in two different locations in a forecast region can be vastly different. For example, 10 inches of snow in one event could melt down to 1” liquid while in another event 10 inches could melt down to 0.5”. Also, the inches of snow measurement can vary significantly even within a few feet of each other due to drifting, compaction of the snow over time, places of wind favored deposits, snow blowing off elevated surface to the ground, and the surface on which the snow accumulates. Measuring at several places in an open area will help minimize this measurement problem.

Snow forecast: Just like rain, snow amounts can vary dramatically over the forecast area depending on the temperature profile, the density of the snow, elevation, the intensity of the snow and other factors. Typically a range of expected snow amounts at a location will be given rather than trying to forecast an exact inch total due to the variation in accumulation that typically occurs across the forecast area. In a snow forecast situation, the forecaster also has to determine if the precipitation will be all snow or a wintry mix and also the forecaster has to determine the form of precipitation when the precipitation event has the highest intensity. For example, 80% of the precipitation in an event falling as snow and 20% as rain can produce more snow than 80% of the precipitation falling as rain and 20% falling as snow.

The snow measurement and snow forecast will often be more difficult than the rain measurement and rain forecast due to the extra factors that have to be taken into consideration.