SnowFlurry 16 2019/20 | When rain turns snow into more snow

Rain in winter

In the Alps, rain is quite normal in winter, even well above 2000 meters. This has always been the case in weather conditions with very warm air masses. Unfortunately, however, rain events tend to increase in height and frequency in winter due to warming. Time to take a closer look at the effects on the snow cover.

Rain: snow eater or snow multiplier?

Rain is rightly considered a "snow eater". Relatively high temperatures and intense solar radiation can hardly bring as much energy into the snow cover as rain. And when it rains, the snow thaws. A distinction is made between "thawing" and "melting". Briefly summarized: When thawing, the snow disappears tens of times faster than when melting. Find out more here.

For lower-lying areas in particular - i.e. where people live - the statement of the snow eater can basically be confirmed. The snow cover there is usually only very thin and all layers of snow are usually very warm. This means that the snow temperature is only just below 0°C. If rain falls on a warm, thin layer of snow, it melts rapidly and the snow depth and snow mass decrease. The rain quickly penetrates deep into the snow cover and the meltwater makes its way through the snow cover to the ground, where it eventually runs off or seeps away. The amount of liters of water then roughly corresponds to the amount of kilograms of snow that are lost.

However, this statement is only partially true. This is because there is a thick and cold blanket of snow, especially on the higher mountains, at least in the height of winter. In addition, it usually only rains briefly there and then usually snows, while the same precipitation event in the valley is all rain. The rain on the mountains in high winter usually only soaks the top few centimetres of the snow cover. Only some of the rain and meltwater from the surface layers then penetrates deeper into the snow cover. Vertical paths are formed through which the water runs deeper down. On hard layers - such as older rain or wind crusts - the water accumulates and then finds it difficult or even impossible to get any further.

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A blanket of snow that is at least superficially soaked.

Lukas Ruetz

But it doesn't necessarily need hard layers in the snow cover. In most cases, there is simply not enough water that could seep in deeper and only the layers near the surface remain moist. This causes them to settle heavily, meaning that the snow depth decreases. Nevertheless, the snow becomes "more". While the height decreases, the snow density increases. This means that there is still the same amount of snow at a lower snow depth. The rain and snow melt water remains in a moist layer of snow for a while and then freezes again. This means that all the water turns back into ice/snow. The mass of the snow cover has thus increased exactly by the amount of rain that has fallen, while the snow depth has decreased. The snow therefore becomes more, while the snow depth decreases.

In practice

Rain is always bad overall. Especially in terms of snow quality. First of all, the snow becomes sticky and sticky and can no longer be skied properly even with the best wax. The risk of avalanches almost always increases (significantly) when it rains. The water weakens the bond between the snow crystals.

As soon as the water on or in the snowpack freezes again, you often get broken snow. Only when it rains very heavily is there a thick, load-bearing layer of harsch. In the height of winter, this rarely firns up due to the weak radiation. Subsequently, however, weak layers often form around the melt crusts in the snow cover because the temperature difference to the neighboring layers is large.

Water from rain and melting snow accumulates on hard layers in the snow cover. — Water from rain and melting snow accumulates on hard layers in the snowpack
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Lukas Ruetz

The insanity of "removing heavy snow from roofs after rain"

After a rain event in winter, the maximum amount of weight on house roofs is equal to the amount of rain that has fallen. Where else would the mass come from? Our feeling that the snow on the roofs "becomes much heavier" does not refer to the mass of the snow cover, but to its density. A fictitious example: there is one meter of snow on a house roof with a density of 200 kilograms per cubic meter. This means that there are 200 kilograms of snow on one square meter of roof. It rains 20 liters (= 20 kilograms of rain) per square meter. The snow cover is so cold that the rain cannot seep through and the melt water does not run off via the gutter but remains within the snow cover. However, the snow depth decreases from 1 m to 50 cm because it settles massively due to the rain. There are now 200 kilograms of snow + 20 kilograms of rain as water within the snow cover on one square meter = 220 kg/m². However, this is no longer a snow depth of 1 m but now 50 cm. We have the feeling that the snow is twice as heavy. However, it is actually only about twice as dense, only the little bit of rainwater has been added in terms of mass or weight. However, the density has more than doubled from 200 kg/m³ to 440 kg/m³.

When the snow cover is already heavy and wet and rain falls on it, more water runs out from under the snow than came in through the rain. On the one hand, the excess rainwater comes out at the bottom and on the other hand, the rain will melt additional snow, which also leaves the snow cover as water. The load on the roof is then reduced by rain because the snow mass decreases.

Even if we have the feeling that the snow is too heavy for the roof due to the rain - the roof is only loaded by a maximum of the liters = kilograms of rain that fall from the sky. And that's usually not that much.

Note: Rain on a snow cover is negative in terms of snow quality and avalanche risk. On the mountains, however, the snow mass usually increases as a result - as if it had simply snowed.