Snow

[gazzawazza]

This is a topical one which has puzzled me for years. When snow lays even quite thickly, usually, the first parts to thaw on roadways and pathways are metal lids on the ground, such as hydrant lids and other service covers.

Why? I do have my own theory but does anyone know the reason for definite?

[j2buttonsw]

Don't they just route warm (er) air from below the ground?

[sddsddean]

The first ones to show are foul and surface water sewer manhole covers. The sewers themselves (especially the foul) will be warmer than the outside snow/ground as soil etc is a very good heat insulator. The covers are therefore 'heated' from below. Things such as Fire Hydrant covers etc will be marginally warmer (for the same reason) but not as warm as a sewer as there is nothing down there decaying etc. Also, of course, foul sewers take all our 'used' hot water. I once used this phenomenen on a road I was building. We accidentally tarmaced over a manhole and weren't sure where it was. Two mornings later the tarmac road was covered in frost, except for a neat 2' diameter circle!

[gazzawazza]

Yep, thanks both of you, this is more or less what I had suspected. I can definitely see that sewers would generate heat, and even water mains should be at a depth to prevent the water from being affected by the cold. Therefore the air/gas below the covers will be warmer than the snow, and the metal lids act as a conductor of heat to speed the process and melt the snow.