UKC

Obviously we as living creatures feel the windchill effect on our extremities so one would assume that the exterior surfaces of snow and ice(or water) would also be altered by that same effect.Is this true?

Does water get chilled by wind chill or will it only freeze if the air temperature surrounding it is 0 degrees or below?

It's quite common to see snow fall on unfrozen turf, which in turn can freeze but then create an unbonded pack as the ground temperature is insulated and is actually warmer than the air temperature on the outside of the snow pack. But.....if temperatures are a couple of degrees above but wind speeds are high, will the crust freeze and if so would this then conduct through the snow pack and meet the conducted warmer temperatures from the un - frozen ground.........thus creating molecular changes that result in better bonding of layers???

Thermal dynamics are not my specialty before you ask although I do work with very high temperature engineering. Air speed or wind speed would not affect heat transferral at high temps apart from the distribution of that heat so I'm guessing that at cold temps the same rule would apply???

In reply to smuffy: I've pondered this myself, surely feeling colder is being colder, or is it a case that the wind whips away heat faster, in hot countries it can feel cooler in the wind but its still the same temp .....

In reply to smuffy: Wind chill only has an effect if the object being cooled is hotter than the air cooling it (its forced convection), as a human body is. Once it is at the same temperature then it won't get colder than the air. Since snow will always be very close to the air temperature wind chill will have a minimal effect, so yes it will only freeze if the air temperature is 0 or below. This ignores radiative effects, which can be quite strong and explain why it is possible to get frosts when the air temperature is above freezing.

In reply to smuffy: for water to change state, ie liquid to ice, the temperature needs to be below zero. Wind chill is a kind of measure of heat loss from animals and says with this wind speed and temperature it will actually feel like a lower temperature.

HTH Davey

In reply to Mountain Llama: > for water to change state, ie liquid to ice, the temperature needs to be below zero

I thought the temperature just had to be below the dew point for frost to form?

If we ignore evaporation (for the sake of argument) then the effect on the the snow/ice from the wind will only increase the rate of cooling until equilibrium is reached between the two. I.e. the ice/snow will not cool to below the air temperature - i.e, no windchill.

If we include evaporation, then (due to the loss of latent heat due to the evaporation) the snow/ice could cool to slightly below the air temperature.

Obviously there's a few simplifications (ignoring the effect of the ground temperature etc.)

In reply to smuffy:

Wind chill is surely just a 'rate of cooling' (think blowing hot soup). The effect of windchill is speeding the cooling of an object through convection(?)

The situation you describe above with snow fall above un-frozen ground, any increased rate of cooling on the top surface of snow (with warmer ground temperature below) is exactly the situation which develops hoar crystals in the snow pack that causes instabilities. I would imagine that knowing this, there is no such conductive action creating better stability. In fact the opposite is true in that you need to reduce the temperature gradient (from above) to better bond the layers.

In reply to iksander: Frost is formed a bit like dew on the grass. First the temperature of the water vapour needs to be below the dew point so water droplets form and provided the contact surface is below zero then frost will form, else dew.

In reply to smuffy:

The forced convection answer is correct. The reason humans are affected by wind chill is that our bodies are very bad at measuring temperature. We use a measure of heat flux (rate of temperature change) instead. I.e. we feel the wind chill effect because the heat is leaving our bodies faster than usual for the given air temperature. if a piece of wood and a piece of metal have been sitting next to each other in a room you know they are going to be the same temperature, and yet the metal will feel colder than the wood. Exactly the same thing.

In reply to smuffy:

In still air, a relatively gradual temperature gradient will develop in the layer of air surrounding a surface, whereas, in a wind, air warmed/cooled by heat flow from/too from the surface is constantly blown away and replaced by more air at the ambient temperature, maintaining a steep temperature gradient. Thus a surface will cool/warm to the ambient temperature more quickly in wind. Note that the final temperature reached by the surface will still be the ambient air temperature; only the rate at which it is reached varies,

In reply to Robert Durran:

in clear skies heat radiation can be lost to space so that the ground is cooler than the ambient air temperature. The wet bulb effect due to evaporation also allows this to occur. this is why neve can form from unconsolidated snow on shady aspects when its a few degrees above freezing but the weather is high pressure.

In reply to Bob_the_Builder:

I used to work in a cryogenics lab. During cooling there wasn't much to do but play with the snow that built up on cables and stuff. The snow always melted instantly on the metal workbench and slowly in your hand, even though your hand was at a much warmed temperature. It was a nice demonstration of the above effect