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Snow can disappear straight into the atmosphere in hot, dry weather

Creeks, rivers and lakes that are fed by melting snow across the U.S. West are , much to the worry of farmers, biologists and . This is not surprising in California, where snow levels over the previous winter were . But it is also true across Colorado and the Rocky Mountains, which in general received a . You'd think if there was normal amount of snow you'd have plenty of water downstream, right?

Over a century ago, snow scientist James Church at the University of Nevada, Reno, began examining how the amount of fed by the melting snow. But as hydrologists have learned over the many decades since, the . Surprisingly, there is a lot researchers don't know about .

Of course, a dry winter will result in . But there are other reasons snow from the mountains won't reach a river below. One growing area of research is exploring how can lead to chronically dry soil that sucks up more water than normal. This water also refills the groundwater below.

But another less studied way moisture can be lost is by . Just as the amount of snow varies each year, so too does the loss of water to the air. Under the right conditions, more snow can disappear into the air than melts into rivers. But how snowfall and loss of moisture into the air itself relate to water levels in rivers and lakes is an , particularly in drought years.

Losing moisture to the air

There are two ways moisture can be lost to the atmosphere before it reaches a creek or river.

The first is through . When water absorbs enough energy from the Sun, the water molecules will change into a gas called water vapor. This floating water vapor is then stored in the air. Most of this evaporation happens from the surface of lakes, from water in the soil or as snow melts and the water flows over rocks or other surfaces.

Another way moisture can be lost to the atmosphere is one you might be less familiar with: sublimation. Sublimation is when a solid turns directly into a gas—think of dry ice. The same can happen to water when . When the air is colder than freezing, sublimation happens when molecules of ice and snow absorb so much energy that they skip the liquid form and jump straight to a gas.

A number of atmospheric conditions can lead to increased and and eventually, less water making it to creeks and streams. Dry air can and pull more moisture from the ground into the atmosphere. High winds can also blow moisture into the air and away from the area where it initially fell. And finally, the warmer air is and more Sun that shines, the more energy is available for snow or water to change to vapor. When you get combinations of these factors—like —evaporation and sublimation can happen quite fast. On a dry, windy day, . That translates to about one swimming pool of water for each football field-sized area of snow.

Sublimation is mysterious

It is relatively easy to measure how much water is flowing through a river or in a lake. And using , hydrologists can get decent estimates of how much snow is on a mountain range. Measuring evaporation, and especially sublimation, is much harder to do.

Today researchers usually estimate sublimation indirectly using . But there are lots of . Additionally, researchers know that the most moisture loss from sublimation above the treeline—but . This further adds to the uncertainty around sublimation because if you don't know how much moisture a system started out with, it is hard to know how much was lost.

Finally, weather and depths . All of this makes measuring the amount of snow that falls and then .

When scientists have been able to measure and estimate sublimation, they have measured moisture losses that range from a , depending on the climate and where you are. And even in one spot, sublimation can depending on snow and weather.

When moisture is lost into the atmosphere, it will fall to the surface as rain or snow eventually. But that could be on the other side of the Earth and is not helpful to drought-stricken areas.

Important knowledge

It is hard to say how important loss of moisture to the atmosphere is to the total water cycle in any given mountain range. —especially at high elevations —can help researchers better understand what is happening to the snow and the conditions that cause losses to the atmosphere.

The amount of water in rivers—and when that water appears—influences agriculture, ecosystems and how people live. When there is a water shortage, problems occur. With climate change leading to and , filling a knowledge gap of the water cycle like the one around sublimation is important.