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Kuchera ratios are used in meteorology to estimate snowfall amounts based on the temperature profile in the atmosphere. They provide a more accurate prediction of snow-to-liquid ratios compared to the standard 10:1 rule, which assumes 10 inches of snow for every inch of liquid water.
Steps for Kuchera Ratio Calculations:
Gather Atmospheric Data: Obtain vertical profiles of temperature and humidity from numerical weather models (e.g., GFS, ECMWF, NAM). This is often visualized in a Skew-T diagram.
Identify Critical Layers:
Look at the temperature profile throughout the atmosphere. Snowfall efficiency depends on whether temperatures are conducive to dendritic growth, typically between -12°C and -18°C.
Check for layers above freezing, which could cause melting and affect snow ratios.
Estimate Snow-to-Liquid Ratio:
The Kuchera method calculates a dynamic snow-to-liquid ratio based on the temperature and saturation levels at various atmospheric layers.
Ratios are higher (e.g., 15:1 or more) in colder, fluffier snow conditions and lower (e.g., 8:1 or less) in wetter snow.
Calculate Liquid Precipitation Amount:
Determine the total precipitation amount forecasted in liquid form (usually in inches).
Apply Kuchera Ratios:
Multiply the liquid precipitation forecast by the Kuchera snow ratio for each time or grid point to estimate total snowfall. Models often automate this step.
Example:
If a model predicts 1 inch of liquid equivalent precipitation:
A Kuchera ratio of 12:1 gives 12 inches of snow.
A ratio of 8:1 (wet snow) gives 8 inches.
A ratio of 15:1 (light, fluffy snow) gives 15 inches.
Kuchera ratios consider temperature dependencies, making them more reliable for variable snow conditions than static assumptions.