Fog!                                                                          10/13

    1.  Ice-crystal process:  Does the ice-crystal process occur on any surfaces?  Does it occur on anything other than
         surfaces?  Can you clarify the process by explaining it in a different way?
            
              The ice-crystal process refers to the fact that in a saturated environment (cloud) at temperatures below freezing, ice crystals grow
                 at the expense of surrounding super-cooled water droplets.  This occurs because, at a given temperature, condensation and
                 deposition rates are comparable, but the evaporation rate tends to be greater than the sublimation rate.  Thus the evaporation
                 rate can exceed the condensation rate (producing net evaporation) in the same environment where the deposition rate exceeds
                 the sublimation rate (producing net deposition).  Or, stated differently, at a given sub-freezing temperature the equilibrium vapor
                 pressure over water (the vapor pressure at which the evaporation rate equals the condensation rate) exceeds the equilibrium vapor
                 pressure over ice (the vapor pressure at which the deposition rate equals the sublimation rate.

                 The ice-crystal process occurs anywhere the temperature is below freezing and the atmosphere is saturated or close to it.  In these
                 circumstances ice crystals grow at the expense of super-cooled water droplets whether those ice crystals be aerosals or attached to
                 a surface (rime).

    2.  Why does it seem to be warmer when it is snowing [than in between snowstorms]?

              It can snow at any temperature if conditions allow a descending atmospheric ice particle to reach the ground before it melts.
                 Since a wet snowflake will be at the wet-bulb temperature, anytime the wet-bulb temperature is at or below freezing, even when
                 the dry-bulb temperature is above freezing, snow can occur.  The lower the relative humidity, the greater the difference between
                 the wet-bulb and dry-bulb temperatures and the higher the dry-bulb temperature at which snow can occur.  Snowfall at higher
                 temperatures is also more likely in a strong down draft where a melting snowflake will reach the ground more quickly before it
                 entirely melts.

                 Heavy snow is less likely in very cold temperatures than near freezing because the water content of very cold saturated air is less
                 than that of warmer saturated air.  However the principal reason that it seems to warm up to snow is probably the fact that winter
                 snows are usually associated with large cyclonic (low pressure) storm systems which have counterclockwise, inward circulation
                 patterns.  Since these storms track generally from west to east, the winds ahead of the storm are from the south and warmer than
                 the north winds which tend to blow after the storm.

    3.  What is "sensible heat?"

              "Sensible heat" is a technical term we shall not use in Physics 137.  The glossary definition from the American Meteorological
                 Society can be accessed by clicking on the term.  We do use the term "sensible temperature" which refers to the temperature
                 perceived and which depends not only upon the air temperature, but also the temperature of the remainder of one's
                 environment, the wind speed, the relative humidity and one's level of physical activity.

     4.  Please help with the equations and concepts on questions 3 & 5 [assignment #6?].

              Please visit the on-line homework keys for detailed solutions of these and all other completed homework assignment questions.