The ability of H2O to change states of matter (solid, liquid, gas) at Earth's surface is one of the unique features of its atmosphere.

I.  (See Fig. 17.?) Terms to describe changing states of matter:
    A.  Melting
    B.  Freezing
    C.  Condensation
    D.  Evaporation
    E.  Sublimation: solid --> gas
    F.  Deposition: gas --> solid
  
II.  Heat that is involved (absorbed or released) in changing the state of matter of a substance is called latent heat.  Heat that is involved in changing the temperature of a substance is called sensible heat.

III.  Water vapor has a maximum limit of ~4% concentration in the atmosphere, at higher temperatures.
    A.  The maximum amount of water vapor held in the atmosphere at a given temperature is the saturation mixing ratio.
            1.  Quantities of water vapor in the atmosphere are typically expressed in grams of water vapor per kilogram of atmospheric gas.
            2.  With increasing temperatures, the saturation mixing ratio increases; falling temperatures --> saturation mixing ratio also falls.
            3.  When the amount of water vapor in the air equals the saturation mixing ratio (at given temp), no more water vapor can be taken in to the atmosphere.
    B.  The actual amount of water vapor in the atmosphere is called the mixing ratio (sometimes called "specific humidity").  The mixing ratio does not have to equal the saturation mixing ratio.
    C.  The relation of the mixing ratio to the saturation mixing ratio is called relative humidity (also R.H.).
            1.  relative humidity = mixing ratio                    x 100
                                                 saturation mixing ratio
            2.  R.H. = 100% means that the air is saturated.  No more water vapor can be evaporated into the air at that temperature.
            3.  A low R.H. (for instance, 20%) might be indicative of dry air.
            4.  RH changes w/ temperature throughout the day.
                    a.  As the temperature goes up (late afternoon), RH typically goes down.
                    b.  As the temperature goes down (overnight), RH goes up.
    D.  The temperature that the air typically falls to in order to reach 100% RH is called the Dew Point Temperature (or just "dew point").
            1.  As the name suggests, this would be the temperature at which dew (condensation) would form.
            2.  Dew point temperature is typically reached overnight, when temperature naturally falls.  That's why we see dew or condensation on the ground

    E.  What 100% RH means
            1.  That air is saturated w/ water vapor
            2.  That the dew point has been reached
            3.  Possibly that clouds have formed (especially when temperature drops below dew point)
            4.  Possibly fog
    F.  Why does high RH feel so uncomfortable?
            1.  When we exert ourselves, we sweat.  
            2.  Ideally, sweat evaporates, taking w/ it the (latent) heat for evaporation from the surface of the skin.
            3.  When RH is very high (at or close to 100%), the air cannot take up any more water vapor.
            4.  Sweat stays on the skin and does not evaporate
            5.  Heat also stays w/ the skin
          6.  So one feels hot and sticky, even when the temperature might not seem so high.  Next time your friend or relative in Houston or New Orleans tells you how aweful things are at 85^o, please be understanding.

IV.  What causes clouds to form?
    A.  Clouds are masses of suspended droplets of water.  It is a liquid state of matter.
    B.  When air masses rise and cool, excess water vapor condenses to form clouds.

VI.  Fog -- low altitude clouds
    A.  Advection fog -- also "coastal" or "San Francisco fog"
    B.  Radiation fog -- also "Tule fog"
    C.  Upslope fog -- "Sierra Nevada fog"