Snow Melt

[kentish maid]

How come snow melts on the ground at different rates?

I have witnessed snow melting at different rates on a flat, equal patch of grass with no underground heating influences, yet areas of snow melt before others. why is this? Its also been the same on concrete and other material like shed roofs.

I would have thought that as it lays level, it would melt at the same time???

Anyone know why this would be?

[Terminal Moraine]

How come snow melts on the ground at different rates?

I have witnessed snow melting at different rates on a flat, equal patch of grass with no underground heating influences, yet areas of snow melt before others. why is this? Its also been the same on concrete and other material like shed roofs.

I would have thought that as it lays level, it would melt at the same time???

Anyone know why this would be?

The most likely reason for this, Emma, is due to minute differences over the surface of the grass area. Although the surface lookes even and flat there will be small differences in grass length and small surface undulations, even on a bowling green like surface.

When the snow falls there will be areas where the snow is in direct contact with the earth at the base of the grass stems and other areas where there is an insulating gap between the two as the snow is supported on the grass blades.

If the ground is not frozen hard prior to the snowfall, the areas of snow in direct contact with the soil surface will begin to thaw slowly, as this process continues the snow will sink slightly in this area and this will alter the surface albedo, allowing this area to absorb slightly more solar insolation ( even if the sun is not shining) and thus accelerate the thawing process.

If the ground is frozen hard beneath the snowfall there will be no thawing from beneath but the minor irregularities of the snow surface itself will result in small differences in albedo. Initially any differences in the rate of thaw due to these minor irregularites will be imperceptible but as time progresses those areas absorbing slightly more insolation will constantly thaw slightly more quickly than other areas until the effect becomes noticeable.

Once a hole appears in the snow sheet, however small, there is a rapid increase in albedo around that area and the snow thaws very quickly in comparison to a n area with complete cover. A good example of this is to observe the effect of walking across a pristine snow cover on a lawn. Even if the footsteps do not penetrate through to the grass the rate of thawing around the steps will be much greater than that over the undisturbed area due to an increase in albedo; eventually this will result in a thawed track through an otherwise even cover of snow.

The same is true of concrete, tarmac, shed roofs etc, it is the very minor irregularites of the surface or the snow and the surface beneath which result in an irregular rather of thawing.

I've gone on a bit, hope this is some help.

T.M

[kentish maid]

The most likely reason for this, Emma, is due to minute differences over the surface of the grass area. Although the surface lookes even and flat there will be small differences in grass length and small surface undulations, even on a bowling green like surface.

When the snow falls there will be areas where the snow is in direct contact with the earth at the base of the grass stems and other areas where there is an insulating gap between the two as the snow is supported on the grass blades.

If the ground is not frozen hard prior to the snowfall, the areas of snow in direct contact with the soil surface will begin to thaw slowly, as this process continues the snow will sink slightly in this area and this will alter the surface albedo, allowing this area to absorb slightly more solar insolation ( even if the sun is not shining) and thus accelerate the thawing process.

If the ground is frozen hard beneath the snowfall there will be no thawing from beneath but the minor irregularities of the snow surface itself will result in small differences in albedo. Initially any differences in the rate of thaw due to these minor irregularites will be imperceptible but as time progresses those areas absorbing slightly more insolation will constantly thaw slightly more quickly than other areas until the effect becomes noticeable.

Once a hole appears in the snow sheet, however small, there is a rapid increase in albedo around that area and the snow thaws very quickly in comparison to a n area with complete cover. A good example of this is to observe the effect of walking across a pristine snow cover on a lawn. Even if the footsteps do not penetrate through to the grass the rate of thawing around the steps will be much greater than that over the undisturbed area due to an increase in albedo; eventually this will result in a thawed track through an otherwise even cover of snow.

The same is true of concrete, tarmac, shed roofs etc, it is the very minor irregularites of the surface or the snow and the surface beneath which result in an irregular rather of thawing.

I've gone on a bit, hope this is some help.

T.M

No thats great. It has been on my mind for a while (sad i know) but that explains it really well, especially the footprints.

Thanks

[Dave J]

That's a great explanation T.M.

Thanks.

[Terminal Moraine]

[

Once a hole appears in the snow sheet, however small, there is a rapid increase in albedo around that area and the snow thaws very quickly in comparison to a n area with complete cover. T.M

[Coopsy]

[

Once a hole appears in the snow sheet, however small, there is a rapid increase in albedo around that area and the snow thaws very quickly in comparison to a n area with complete cover. T.M

I just noticed my error here when scanning through a few posts. The word 'increase' in this sentence should, of course, be 'decrease'.

Increased albedo =higher reflectivity, decreased =lower.

T.M

TM - If you ware reading this, rather than start a new thread I thoguth I would ask a quick question about snow melt.

It snowed here the other day and the temp was mimus 30ºC, but the snow still metled on a hill facing the south. I know its the sun but I thought that at minus 30ºC it wouldnt have any effect. Any ideas?

[Terminal Moraine]

TM - If you ware reading this, rather than start a new thread I thoguth I would ask a quick question about snow melt.

It snowed here the other day and the temp was mimus 30ºC, but the snow still metled on a hill facing the south. I know its the sun but I thought that at minus 30ºC it wouldnt have any effect. Any ideas?

A difficult one, Coops, without knowing a few more details.

Was the -30c recorded in a valley overnight after the snowfall or was it during the snowfall? Any idea what the max' temp' was on the day when the snow melted and how deep was the snowfall?

I'm assuming it was only a light fall of snow and after an overnight min' of -30c it was a cloudless, calm day to follow. Under these conditions, on a slope almost at right angles to the incident rays of the sun and at your latitude (on a par with northern France ) a combination of sublimation in very low humidity and strong sunshine would account for this.

A thin cover of very light and fluffy snow would readily evaporate into very dry air and once any darker material beneath was exposed, even in very small amounts, this would rapidly heat up in the sunshine and melt any snow in direct contact.

This process would increase almost exponentially the more earth/stone etc was exposed until the snow cover was gone. For maximum effect the slope would need to be at about 20 degrees or steeper.

I've made a lot of assumptions here, please correct me if I'm wrong.

T.M

[Coopsy]

A difficult one, Coops, without knowing a few more details.

Was the -30c recorded in a valley overnight after the snowfall or was it during the snowfall? Any idea what the max' temp' was on the day when the snow melted and how deep was the snowfall?

I'm assuming it was only a light fall of snow and after an overnight min' of -30c it was a cloudless, calm day to follow. Under these conditions, on a slope almost at right angles to the incident rays of the sun and at your latitude (on a par with northern France ) a combination of sublimation in very low humidity and strong sunshine would account for this.

A thin cover of very light and fluffy snow would readily evaporate into very dry air and once any darker material beneath was exposed, even in very small amounts, this would rapidly heat up in the sunshine and melt any snow in direct contact.

This process would increase almost exponentially the more earth/stone etc was exposed until the snow cover was gone. For maximum effect the slope would need to be at about 20 degrees or steeper.

I've made a lot of assumptions here, please correct me if I'm wrong.

T.M

You sound pretty spot on. the slope is at about 30 degrees and the temp was minus 30 but the air was very dry, the sunshine abundant and the snow cover les than 2 cm. but i still thought that it was to with air temps so guess thats one myth shattered

thanks for the info