Fresh snowfall has again covered large areas of the drought-stricken Great Plains and Midwest. Farmers have eyed the snowfall with hope, but also first-hand knowledge of what happens to snow covering frozen ground.  

"This snow is a huge blessing, and our subsoil has a lot more room to store moisture," said a Kansas farmer on Agriculture.com after last week's big snow in the state.

"Snow will only help the ponds, as we have a very good layer of frost," wrote another farmer. 

This week's heaviest snow occurred in some of the neediest areas. Coverage included northwest Texas, west central Oklahoma and far south central Kansas, said Don Keeney, MDA EarthSat Weather.

"The snow is now melting across the southern Plains, which will help to improve soil moisture for wheat," Keeney noted in a e-mail newsletter on Friday.  

More moisture in the form of snow is on the way. "Additional snow is expected to cross the northern Plains into the Midwest later this weekend into next week, which will maintain extensive snow cover there. This will also improve soil moisture once it melts," Keeney said.

But, that large blanket of white across the country is mostly a mirage in terms of recharging soil moisture. By the time the soil thaws, most of the moisture will be gone, thanks to runoff and sublimation, says Matt Helmers, an Iowa State University agricultural and biosystems engineer.

"Things are still frozen underneath in many areas, so that once it warms up we will probably lose the majority of the water to runoff, rather than infiltration," Helmers told Agriculture.com. "We may also lose some snow to sublimation, which doesn’t help our soil moisture."

What is sublimation?

It's a process in which ice and snow bypass the liquid stage and turn directly into a gas, or vapor, according to Helmers. (Evaporation occurs when a liquid changes to its vapor form.)

With sublimation,when the air is dry enough and it's sunny, ice and snow molecules get converted directly to water vapor, and the snow pack recedes without ever becoming water. Sublimation is most likely to occur in the middle of the day and later in winter.

"There are benefits of the snow [for streams, ponds and reservoirs], but probably not as much for soil moisture and to replenish the bank for the crop this year," Helmers said.