Commonly, the percentage of water to snow is called the “snow ratio.” An old rule of thumb was that for every 10 inches of snow, there would be 1 inch of water (10:1). However, this is far from the norm, and recent studies indicate that a 12:1 ratio might be more representative (on average) for Iowa. This said, there are so many variables that can affect the ratio of liquid water to snow that using a rule of thumb is usually off the mark. In fact, the snow ratios can change dramatically within a snow event itself. Some of the variables that come into play include…

• Depth of the warm (above-freezing) layer from the surface into the snow-producing cloud. The warmer it is (closer to freezing), the lower the ratio will be.
• Amount of ice in the snow-producing cloud. If there are more supercooled water droplets in the cloud, snow ratios will be lower. If there’s a higher amount of ice crystals, snow ratios will be higher.
• If it’s windy, snowflakes can fracture, losing their “lacy” structure and leading to lower accumulations (lower snow ratios).
• Deep cold, in general, promotes higher snow ratios (fluffier snow).

Snow ratios are crucial in converting liquid precipitation forecasts into snowfall totals, which is key for accurate snowfall forecasts. Snow ratios in Iowa can vary from roughly 5:1 (heavy, wet snow) to 30:1 (very fluffy, light snow) or more.

Forecasts take into account the expected temperature profile through the atmosphere. Meteorologists start with a forecast for liquid precipitation from weather models. The exact ratio can be hard to predict since it depends on rapidly changing atmospheric conditions. Even within a single storm, different areas might have different snow ratios due to variations in temperature or moisture. Forecasts are often adjusted in real-time as new data comes in, particularly from the intensity of snowfall on radar or from initial snowfall reports, which might indicate if the initial ratios were too high or too low.

Understanding snow ratios helps not only in forecasting but also in preparing for the impact of snow on travel and daily life. If you’re dealing with a specific forecast or event, remember that these ratios might be adjusted or specified by our meteorologists to better reflect expected conditions, and stay up to date at WHO13.com!