The Santa Ana Winds
Factors Combine to Cause Wind Direction
Determining what causes wind direction in the dynamic troposphere of the Earth is not as straightforward as it may
seem. Despite this, scientists have a good grasp of how factors come together to create both wind speed and
direction. Regardless of the geographic scale and area, there are three primary forces which contribute to the cause of
wind direction. Certain forces play bigger roles depending on scale and area, but this will give the basics to
understanding what causes wind direction no matter the situation.
Pressure
Blowing from high to low pressure, air always attempts to balance out the pressure levels. A highpressure system next to a low-pressure system will cause wind direction to flow clockwise and
outward toward a low-pressure system. The low-pressure system is what causes wind direction to
flow counterclockwise and inward. This is also referred to as a cyclonic flow. Daytime sea breezes and nighttime
land breezes commonly occur along coasts. One of the prime drivers of wind direction is atmospheric pressure,
essentially the weight at a given point of the overlying column of air. Low pressure is often caused by solar heating,
as warmer air ascends; cooled, descending air creates an area of high pressure. Winds flow generally from high to
low pressure, essentially to replace the "loss" of air in the latter situation. In addition to helping drive prevailing
winds, heat and pressure differences cause variations in local wind direction. For example, "sea breezes" and "land
breezes" form because of the differential heating of land masses and large water bodies. During the day, the land
surface absorbs heat more rapidly than the water surface and heats the overlying air, which rises; at the height of this,
usually in the afternoon, winds travel from the higher-pressure water body inland. At night, the opposite happens-the air over the water retains more heat than the fast-cooling land--and a "land breeze" heads sea- or lakeward.
Coriolis Effect
Winds, though, are partly shunted off direct courses between high and low pressure by the rotation of the Earth. This
discrepancy of direction is called the Coriolis Effect. The planet rotates from west to east (hence the "rising" of the
sun in the east and its "setting" in the west). In the Northern Hemisphere, the Coriolis Effect causes out-rushing
winds from a high-pressure cell--the anticyclone--to blow in clockwise fashion, while in-rushing winds spiral
counterclockwise around the low-pressure cyclone.
Friction
The final cause of wind direction is friction. Surface-level winds are mostly influenced by friction, as this is where
wind encounters varying surfaces. If the wind blows into a building, it must cause a change in direction. It could rise
above the building or go around it in either direction, but it will cause a wind direction change. Slowing down of
winds by a rougher surface will also decrease Coriolis deflection, and acceleration over a smoother surface will
cause the opposite.
Topography
Canyons and valleys often experience up- and downslope winds because of differential heating.
At the surface of the Earth, topographic variations may affect wind direction. This factor doesn't operate exclusively
of pressure influences. For example, in mountainous regions winds will switch from blowing up- and downslope
depending on the time of day. This has to do with differential heating, pressure and air-parcel weights: At night,
heavy cold air rolls down into the valley bottoms; during the day, heating of surrounding slopes draws winds out of
the bottoms.
Adiabatic Heating and Cooling
The cooling process that leads to cloud formation is quite different from that associated with the other condensation
forms that we have already examined. The cooling process that produces fog, frost, and dew is either radiation or
advection. On the other hand, clouds usually develop from a cooling process that results when a parcel of air on
Earth’s surface is lifted into the atmosphere. The rising parcel of air will expand as it encounters decreasing
atmospheric pressure with height. This expansion allows the air molecules to spread out, which causes the parcel’s
temperature to decrease. This is known as adiabatic cooling and occurs at the constant lapse rate of approximately
10°C per 1000 meters (5.6°F/1000 ft). By the same token, air descending through the atmosphere is compressed by
the increasing pressure and undergoes adiabatic heating of the same magnitude.
What is a Santa Ana?
The Santa Ana is a dry, sometimes hot and dusty, wind in southwestern California that blows westward through the
canyons toward the coastal areas. Santa Anas are a seasonal phenomena, occurring mostly during fall, winter and
spring, tending to peak in December. The wind usually has its origin when cold air spills southward into the Great
Basin, trapped between the Rockies to the east and the Sierras and Southern California coastal range to the west.
This cold air mass is characterized by unusually high pressure near the land surface. Winds are driven into Southern
California when the pressure of this interior air mass exceeds the pressure along the California coast. Winds are
often strongest in mountain passes which are ducts for the continental air flow. Because the air over the higher
elevations of the Great Basin sinks as it flows into coastal California, it is heated adiabatically, and temperatures are
often quite warm. This continental air mass is invariably dry, so humidities in Santa Anas are low, often less than
25% relative humidity. Santa Anas have occurred irregularly over the time period since about 1950 when we have
collected detailed wind and humidity observations, with some months experiencing Santa Ana conditions 30% the
time, and other months less than 5% of the time.
Santa Anas have several colorful nicknames including "devil's breath".
What is the historical record of Santa Ana events?
Santa Anas typically occur from October into March. Many associate Santa Anas with autumn because at that time
the winds often spread wildfires across areas that have gone months with little or no rain
We use an index of three different weather parameters to detect Santa Ana conditions. These parameters are:



the surface pressure difference (must be relatively high) between weather stations over the Great Basin and
those sited along coastal Southern California
at the coastal Southern California stations, the humidity of the air at or near the surface (must be relatively
low)
the direction of the wind at or near the surface (must be in the sector between north-northwesterly and
easterly)
What atmospheric circulation features are associated with Santa Ana events?

Any low-pressure system in the Pacific off the California coast may change the stability of the Great
Basin High. The Great Basin High winds then turn southward along the eastern slopes of the Sierras.
The low-pressure system over the Pacific literally sucks the winds through the mountain passes of
Southern California toward the coastal areas.
Article: The Santa Ana Winds
Name: ____________________________
1. What is air “attempting” to do as it blows?
2. Describe how air flows around a high pressure system.
3. Describe how air flows around a low pressure system.
4. What is atmospheric pressure essentially?
5. Describe (in detail) what causes a land and sea breeze.
6. How does the Coriolis Effect change wind direction?
7. Describe 2 ways that friction affects winds.
8. How might winds change throughout the day between a mountain and a valley?
9. Describe the cooling process that leads to cloud formation.
10. What is the process form question 9 called?
11. What is adiabatic heating?
12. What are the Santa Ana winds?
13. Describe the atmospheric characteristics that lead to the Santa Ana Winds.
14. What time of year do the Santa Ana wind usually occur?
15. What weather parameters are used to detect the Santa Ana Winds?
16. What are the atmospheric circulation features associated with the Santa Ana winds?
Article: The Santa Ana Winds
Name: ____________________________
1. What is air “attempting” to do as it blows?
2. Describe how air flows around a high pressure system.
3. Describe how air flows around a low pressure system.
4. What is atmospheric pressure essentially?
5. Describe (in detail) what causes a land and sea breeze.
6. How does the Coriolis Effect change wind direction?
7. Describe 2 ways that friction affects winds.
8. How might winds change throughout the day between a mountain and a valley?
9. Describe the cooling process that leads to cloud formation.
10. What is the process form question 9 called?
11. What is adiabatic heating?
12. What are the Santa Ana winds?
13. Describe the atmospheric characteristics that lead to the Santa Ana Winds.
14. What time of year do the Santa Ana wind usually occur?
15. What weather parameters are used to detect the Santa Ana Winds?
16. What are the atmospheric circulation features associated with the Santa Ana winds?