Fog!                                                                          10/8

    1.  What does the term "stable" mean in the phrase "stable atmosphere?"
            
              Any physical system is in a state of equilibrium if it is not changing.  Its equilibrium is said to be "stable" if, when given a small
                 perturbation (slight disturbance), it tends back to its initial equilibrium condition.  It is "unstable" if the perturbation causes it to
                 tend even further from its initial condition.  In the atmospheric context a stable air parcel, when give a tiny downward or upward
                 perturbation, returns to its initial level, whereas an unstable parcel continues to move, upward or downward, at an accelerating
                 rate in the direction of the perturbation.  The stability of the atmosphere is determined by the value of the environmental lapse
                 rate.  Conditions which determine the lapse rate will be discussed in class on Friday.

    2.  What is "latent heat?"

              "Latent heat" refers to the energy released when water condenses from water vapor or when water freezes to form ice.  Or, if the
                 change is in the opposite sense, latent heat is the energy absorbed by melting ice or evaporating water.

    3.  Is Utah Valley's atmosphere stable, conditionally unstable, or unstable?

              The state of the atmosphere is continually changing among these conditions and is often in a different state at different levels, all
                 because the environmental lapse rate, which determines stability, is constantly changing.  Only rarely is the atmosphere unstable
                 for dry parcels, usually in a limited layer near the surface when it is heated by sunshine.  It is often conditionally unstable in layers
                 of various thicknesses.  Cumulus clouds in the atmosphere are a sign that the atmosphere is conditionally unstable in the layer in
                 which those clouds occur (but, if the air is dry, it can be conditionally unstable without any clouds present).  A stable atmosphere is
                 common, sometimes from top to bottom.  A stable atmosphere may be clear or may contain stratus layers where the air is saturated
                 or close to it.  Thick, conditionally unstable layers, if the air is moist, often produce precipitation.  In fact a thick, conditionally
                 unstable layer is a necessary condition for heavy precipitation.  Light precipitation may also fall from a conditionally unstable
                 layer, but sometimes falls from a stable layer of stratus clouds.

    4.  What is an inversion and what are its consequences?

              An inversion is the condition in which the atmospheric temperature increases with increasing altitude.  The stratosphere and the
                 thermosphere are in perpetual states of inversion.  In an inverted condition, the atmosphere is extremely stable.  That is why
                 clouds do not build into the stratosphere (with the rare exception of overshooting cumulonimbus clouds which get there by sheer
                 inertia).  Shallow inversions occur at the surface on almost every clear, calm night, but these usually "burn off" when the sun strikes
                 the surface, except sometimes in the winter, when the low angle of the sun prevents significant surface heating.  Deep inversions
                 are often associated with long-time periods of stagnant weather conditions, with few clouds (except on the surface) and extensive
                 trapping of pollutants near the surface.  Surface clouds (fog and smog) are often associated with persistent inversions.

    5.  Are there high level clouds in the atmosphere?

              Above the troposphere there are almost no clouds, the only exceptions being overshooting cumulonimbus clouds which sometime
                 penetrate the lower stratosphere, and nacreous and noctilucent clouds, poorly understood, very thin clouds well above the
                 troposphere which seem to be unconnected to weather conditions.  Nacreous clouds form in the stratosphere more than 30 km
                 (100 000 ft) above the surface.  Noctilucent clouds are even higher, in the mesosphere at altitudes above 75 km (250 000 ft).  Both
                 these cloud types are usually observed at twilight at very high, sometimes polar, latitudes where they become visible in the
                 sunlight when the lower atmosphere becomes dark.