I.  Air masses are large bodies of air (in the troposphere) with similar characteristics of temperature and vapor content.
    A.  Maritime air masses (m) originate from over ocean and generally have high water vapor content.
    B.  Continental air masses (c) originate from over land and generally have very low water vapor content.
    C.  Tropical air masses (T) originate from lower latitudes and are generally warmer.
    D.  Polar air masses (P) originate from higher latitudes and are generally cooler.

II.  Weather fronts are boundaries where cold and warm air masses meet
    A.  A cold front occurs when cold air pushes out warm air (note symbols/color of each front)
        1.  Intense rains, thunderstorms.
        2.  High, vertical cloud structures, caused by steep angle that warm air must climb
        3.  Rains generally situated on or slightly behind front.  Once front passes, clear skies.
    B.  A warm front occurs when warm air pushes out cold air
        1.  Generally gentle/steady rains
        2.  Wider cloud structure caused by warm air gradually rising (low angle) over cold air.
        3.  Rains, clouds generally situated ahead of the front.
    C.  A stationary front occurs when neither warm not cold air can push out the other.  Boundary remains stationary.
    D.  An occluded front occurs when a cold front overtakes a warm front and causes the warm air masses to be wedged upwards.
        1.  A combination of warm and cold fronts; some thunderstorms, followed by gentler rains
        2.  Some vertical cloud structure; some broader, layer clouds.
        3.  Rains situated on and on either side of front.

III.  A mid-latitude cyclone is a weather system that most affects the lower 48 states during the winter season.
    A.  The sub-polar low (Polar Front) advances to lower latitudes during the winter months from its position at ~60^oN latitude
    B.  From a stationary front, a wave forms along the Polar front that sets into motion a warm front followed by a cold front.
    C.  The collective movement of the system is counterclockwise -- typical of a cyclonic system in the N. Hemisphere.
    D.  The cold front starts to overtake the warm front closest to the low pressure system; this initiates an occluded front.
    E.  As the occluded front continues to develop, the mid-latitude cyclone gradually dissipates.
    F.  Mid-latitude cyclones generally move from west to east across the U.S.  The winter storms that affect us in California usually develop as mid-latitude cyclones in the northern Pacific.

IV.  Hurricanes

V.  Tornadoes