ON WINDWARD SIDE
IN SOUTHERN CALIFORNIA
An understanding of orographic lifting seems to date back to the ancient Greeks. It was looked at by
observing the conditions of ground fog and mists forming and looking at the atmosphere temperature
change with height.
"Similarly, the majority of springs are in the neighborhood of mountains and high places, and there
are few sources of water in the plains except rivers. For mountains and high places act like a thick
sponge overhanging the earth and make the water drip through and run together in small quantities
in many places. For they receive the great volume of rain that falls… and they cool the vapor
as it rises and condense it again to water." In pondering the origin of springs and rivers
around 340 B.C., Aristotle writes in Meteorologica.
Research efforts appear to have stalled thereafter, and understanding remained little changed. Expressed
by Francis Bacon in 1622:
"Windes do contract themselves into rain… either being burthened by the burthen itselfe, when the
vapours are copious, or by the contrary motions of windes, so they be calme and milde; or by
the opposition of mountains and promontories which stop the violence of the windes, and by
little and little turn themselves against themselves; or by extreme colds, whereby they
are condensed and thickened" (cited by Middleton 1966).
Los Angeles is located at 34.21 N. - 118.25 W. The weather in Southern California is influenced by a
sub-tropical high located over the Pacific Ocean. From April to September we have a very warm, dry,
and stable climate. From October to March we have what is known as our wet season. When the wet
season arrives the North Pacific high decreases. When this happens, synoptic conditions appear
with moist onshore flow bringing precipitation. This is when we get almost all our annual rainfall.
Southern California has a very interesting topography, which include large mountain complexes, low
lying coasts and interior plains and high desert plateaus.
The main mountain ranges for Los Angeles are the San Gabriel Mountains and the San Bernardino Mountains. We
also have different surface types: ocean, highly urbanized land areas, zones of intensive agriculture,
forests and deserts all found within Southern California.
From October to March the Southern California climate finds itself with heavy and long rain events. During
this wet season the winds over the Pacific Ocean develop a westerly component. In the zone between the coast
and the coastal ranges the wind anomalies are small. But on the eastern side of the coastal ranges the
onshore flow reappears. This sets up barrier jets parallel to the coastal mountain ranges in the
coastal zone. The onshore flow is forced to rise above the topography. There are four types of
lifting. They all usually work in conjunction with one another.
1. Convective lifting - caused by unequal heating of the surface. A parcel of air near the ground
will be warmed more than the air around it. The parcel expands and rises toward a lower density layer.
2. Frontal lifting - when two air masses meet that are unalike, they will not mix. A zone called a
front is formed between them and the warmer air rises over the colder air.
3. Convergence lifting is not as common as the others - when air converges (come together)
there will be uplift.
4. The topic we will focus on will be orographic lifting - the word orography which refers to
the uneven shape of the earth surface.
For air to move upward there has to be some type of mechanism to cause it to rise. In orographic lifting
a parcel of air encounters a physical barrier such as a mountain, so it is forced to rise over it since
it can not go around it. Orographic lifting is a mechanical process. The mechanical lifting causes
the parcel to undergo a physical change.
Atmospheric pressure decreases with altitude. So a rising parcel is subjected to pressure around it. The parcel
expands, during expansion the molecules have to work. Energy reduces the kinetic energy (the energy
a parcel possesses as a consequence of its motion) of the molecules and the temperature of the parcel
decreases. No heat is lost or gained from the environment surrounding the parcel. The process is
called Adiabatic. Adiabatic is defined as the internal changes within a gas during expansion or
contraction when no energy is removed from or added to the gas. The adiabatic process applies
to both rising (wet adiabatic) and sinking (dry adiabatic) parcels of air.
In the first law of thermodynamics it tells us that the temperature of a gas may be changed by the
addition or subtraction of heat, a change in pressure or a combination of both. In the adiabatic
processes, the addition and subtraction of heat may be disregarded and expressed simply as:
(Change in temperature = constant x change in pressure).
As the air rises and cools, orographic clouds form and serve as the source of precipitation, most of which
falls upwind of the mountain ridge. Some fall short of the ridge and is sometimes called spillover. In
Southern California that is why we see more rain in the windward side of mountains, in cities such as
Burbank and Pasadena.
The wind direction will determine which side of a high elevation region experiences orographic lifting. Weather
in the mid-latitudes tends to move from west to east. It is common for the western side of the high elevation
regions to have significant orographic lifting.
They estimate that rainfall rates are typically doubled or tripled because of orographic lifting. If the
atmosphere is too stable or the flow is not strong enough, air may be unable to ascend over the range, and
at lower levels the flow may become blocked. Flow may get diverted around the mountain or it may
stagnate. This blocked air can cause ascent further windward of the range and can also enhance
the lifting (and hence the precipitation) that does occur. Another part of the atmosphere response
to orography is the possible triggering of unstable convection.
The existence of a rain shadow across a mountain range whose axis lies perpendicular to the prevailing winds
is one of the most confident expectations in atmospheric science. For large mountain ranges, precipitation
maximizes over the windward slopes, and for smaller hills the maximum tends to occur nearer the crest.
So as we see orographic lifting is an interaction between land and surface and the atmosphere. From the
ancient Greeks till now over millions of years patterns of orographic lifting has controlled patterns of
erosion and rock formation acting to shape and form the mountain ranges.
The Relationship Between Upslope Flow and Rainfall in California Coastal Mountains
By A.B. White
Local Regimes of Atmosphere Variability:
By Alex Hall
Lessons of Orographic Precipitation from the Mesoscale Alpine Programme:
By Richard Roturno and Robert Honze
Jeff Haby Instructor at Mississippi State University
By Lutgins, Tarbuck
Climatology, An Atmospheric Science
By John E. Oliver and John J. Hidore