Extremely cold air like we have this week can cause lots of creaking and snapping sounds, but on rare occasions, it can cause a boom or bang. If you heard one, you may have heard a cryoseism, or frost quake.

There are few things that need to be in place for frost quakes to happen:

• First, the ground has to saturated with water. Soil moisture in Iowa is running very high because of the wet fall and early winter.

• Second, the temperature has to have a big drop in a fairly short amount of time. Temperatures on Monday were in the lower 20s and are now some 40 degrees colder just a couple of days later.

• Third, the snowpack shouldn’t be very deep since that can insulate the ground. Snow depths in the region are generally around a foot, which may be deep enough to limit frost quakes.

When the moisture in the ground freezes, it expands and puts pressure on the things around it. If that stress gets strong enough, it may only be able to relieve that stress in a sudden, explosive way as a frost quake.

Frost quakes can be strong enough to make the ground to shake over a very small area and even crack at the surface.