What causes Lake Effect Snow?

Most people know that it snows a lot in upstate New York. Many upstate New Yorkers are also aware that it doesn't snow evenly across the region - instead snows tend to be very localized, hitting hardest in certain favored areas to the lee of Lakes Erie and Ontario. What even most New Yorkers don't know is just HOW MUCH it snows in these certain favored areas. Part of the reason for this is that not many people live in these areas - it actually snows so much that these regions are fairly unpopulated. Buffalo and Syracuse are both fairly snowy big cities (~1 million people), but just an hour's drive from either city you can find regions which average more than twice as much snow each winter.   

Much of this snow results from an odd atmospheric phenomenon known as the Lake Effect. During winter, cold arctic air masses sweep across the Great Lakes, gaining heat and moisture from the relatively warmer lake surface below. The air may warm by several degrees and the air's water content may double during its trip across the lake.

In general, if you take a parcel of air and warm it so that it becomes less dense than its surrounding environment, buoyancy will cause that air parcel to rise. Air will then flow in from the sides to replace the air that has risen, generating a complete circulation. This is known as the solenoid effect - a common example of the solenoid effect in operation is the nearly daily sea breeze which tends to cool areas near the coast on a hot summer's day. Lake Effect Snow involves more than just a solenoid effect, however - convection is also involved.

Convection involves buoyant forces run amok. To understand convection, one must know a little bit about the vertical structure of the atmosphere. Normally, the lower part of the atmosphere becomes colder the higher you go (up until the top of the troposphere). There are several reasons why this occurs, but one way to think about it is that the energy of a dry air parcel can be thought as partitioned between thermal and potential energy. Thermal energy is determined by the temperature of the air, while the potential energy is determined by the height of the air (and the strength of the gravitational force). If a dry air parcel rises, its thermal energy is converted into potential energy at the rate of g/cp (the acceleration due to gravity divided by the heat capacity of the air at constant pressure). If this were the only factor (i.e. we assume that no heat is added during this process - an adiabatic process), the air will cool at the rate of about 5.5 degrees Fahrenheit per 1000 vertical feet. This value is known as the dry adiabatic lapse rate - the rate at which temperature "lapses" in a process which does not add heat to the parcel. Air which sinks will also warm at the rate of 5.5 degrees per 1000 feet.

The normal environmental lapse rate for the lower troposphere is more like 3.5 degrees per 1000 feet - it differs from the dry adiabatic lapse rate for a number of reasons, but probably most important is the effect of water. Air always contains some water vapor as well as small particles called aerosols. At the microscopic level, water vapor molecules are constantly condensing onto the aerosols (and any other surface), and liquid water is constantly evaporating from those surfaces. When the temperature drops below a certain point, more water vapor condenses than evaporates from the surface - the balance tips in favor of net condensation. When water condenses, it releases the latent heat

This warming and moistening of the near-surface air is precisely what is needed for the atmosphere to become convective (the relatively warmer, moister air near the surface rises because its density is less than the air surrounding it - when its moisture condenses, latent heat is released, making the air even more buoyant - if a parcel is displaced vertically and its buoyancy force is such that it keeps rising, then convection occurs - the parcel rises through the surrounding environment ). Under just the right atmospheric conditions (strong instability generated by very cold air and warm lakes, with little variation of wind direction with height so that the wind is all blowing the same direction), the convection can organize into a linear band.