Offshore Winds in California During the Dry Season*

(Santa Ana in southern California; Diablo in north-central California)

(Note: Santa Ana/Diablo Wind Pattern can occur during the Fall, Spring or Winter, but only in the late Summer and Fall does it occur when vegetation is dried out from the normal summer dry season, making fire hazard extremely high.)

I. Are "Hot" Because

  1. (For Metr 201 and 356) At the Surface
    1. Are in contact with a relatively hot surface (deserts to the east)
    2. Are sinking to sealevel and therefore are warmed compressionally.
  2. (Advanced: For Metr 201 only ) Aloft: Associated with strong subsidence of already warm air from the upper to the lower troposphere (air is warmed futher by compressional heating);

Air that is warm to begin with is further warmed by compression.

  • II. Are "Dry" Because
    1. (For Metr 201 and 356) At the Surface
      1. Associated with "desert" air chracterized by low dew points and a great difference between temperatures and dew points.
      2. Associated with sinking to sealevel, thus further warming the air, increasing the difference between temperature and dew point and lowering relative humidity.
    2. (Advanced: For Metr 201 only ) Aloft:  Associated with warming, sinking air  (dewpoint stays the same but the temperature increases, thus lowering the relative humidity).

    Air with little water vapor to begin with is warmed furtther, thus, further lowering relative humidity.

    III. Are associated with very strong surface wind speeds because

    1. Surface pressure differences between the interior and the coast are often extremely great (dynamic highs tend to be strong);
    2. Wind direction is at right angles to the mountain ranges and tends to be "funnelled" by mountain passes and canyons.  (Air "accumulates" east of mountains (higher pressure) and a net deficit of air occurs west of mountains (lower pressure).  Thus, in the locations where air has an opportunity to "break" through or over mountains, very high wind speeds can occur, often  far in excess of  what may be evident from a glance at pressure differences seen on the large-scale weather map.)

    Very fast wind speeds occur downwind of major canyons and over ridges.