Posted January 24, 2008
A cold front will move across the state today as the leading edge of an Artic air mass. High pressure in the Midwest was at the core of this artic air mass. At 8:00 this morning, the temperatures in Minnesota ranged from -35° to a balmy -8°!
What I remember most about walking to the bus stop on these frigid mornings as a child in Minnesota, were the sounds. The loud crunching of the snow under foot, clearly hearing what my friends at the bus stop were saying, even though is was still 2-3 blocks away, and we could hear the bus a mile away. Although sound travels slower in cold air that warm, cold air is more efficient in carrying the sound waves. Colder air is denser. Sound waves are more "efficient" in dense air. Plus, cold air is usually associated with inversions (the air warms as you go up) which also helps sound travel to your ears better. Sound waves tend to be dispersed in all directions, but with a strong inversion, the sound waves will be forced to travel more horizontally as the inversion keeps the sound waves from moving upward into the warmer air. So if you combine these two factors, you tend to hear sounds a bit better in colder weather.
Another fond memory is standing out on a frozen lake where the ice is 2-3 feet think. On a cold night as the temperature drops, you hear, and feel, the ice moving against itself. Ice expands and contracts with changes in temperature. The amount it can change is moderated by the mass of water underneath. The ice gets under great strain and then a crack starts and fires across hundreds of yards or even miles of ice. Depending on where you are listening with respect to the crack, the sound varies from a sharp "crack" to an intense rumble. It is a very eerie sound that echoes up and down the lake.