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Grade: 3/5
1. Salinas Valley Climatology
Salinas is located at approximately 36°40'N, 121°36'W, about 25 mi
east of the of Pacific Ocean. It lies between the Gabilan and Santa Lucia mountain
ranges of western CA, runs SE-NW along the Salinas River, and is a prime location for
agriculture. The following is a brief climatic summary.
The valley has a Mediterranean climate, i.e., summers are relatively cool
and dry, while winters are mild and wet with occasional migratory storm systems (Ahrens
1991). The Pacific High, which dominates the region almost year round, is a semipermanent pressure system because it remains nearly stationary over the northeastern
Pacific area. During summer and part of autumn, migratory lows are diverted northward,
keeping most of the western US rain-free. During winter and spring, it is displaced to the
south, allowing more storm systems to affect much of the western states.
Places with Mediterranean climates accumulate 30 to 90 cm of rainfall per
year Ahrens (1991). January is the wettest month for Salinas, with about 6.78 cm of
rainfall. August is the driest, with an average of about 0.1 cm. A typical rainy season lasts
from November to April.
The Pacific High gives the coastal regions of the western states a pleasant
climate in terms of temperature. The clockwise flow of this system is responsible for the
California Current, which brings cooler water from the northern Pacific southward along
the western North American coast. Because of this pattern, summers are relatively cooler.
During the winter and spring months, the Rocky Mountains and Sierra Nevada provide a
barrier for the West Coast from polar and arctic air, producing generally mild winters.
Average summer temperature in Salinas is about 72°F, with September warmest with a
mean of 74°F. Average winter temperature is about 40°F, with January usually the
coldest with average temperatures dipping in the upper 30°Fs.
The California Current also produces coastal upwelling, another factor that
keeps temperatures down, especially along the coastal areas. In addition to radiatonal
cooling, upwelling produces a surface-based inversion. The combination of the Pacific
High, California Current, upwelling, and surface inversion produces a coastal marine
layer, a shallow layer of stratiform clouds over coastal areas. A subsidence inversion, a
downward compression of air, acts as an upper-level cap to the marine layer. However, at
any given time of the year, the marine layer and/or fog (sea fog or radiation fog) can
affect the Salinas Valley, especially overnight when maximum cooling occurs. Types of
fog that affect the region include advection and valley radiation. Afternoon heating
usually dissipates radiation and advection fog. During summer, advection fog is common,
with
advection and radiation fog affecting winter unless a passing frontal system dissipates it.
In conclusion, the Salinas Valley has a Mediterranean climate and is
protected from cold polar air from the northeast. Although rare, temperatures have been
known to dip below freezing without warning. Cooler, ocean water interacting with
warmer land areas produces on-shore flows that keep relative humidity levels up during
the warm season
2. Atmospheric Circulations
Earth’s axis is angled at a 23.5° tilt in relation to its orbital path. The tilt
remains stationary as the planet revolves around the sun and rotates about its axis. One
revolution takes approximately 365.25 d, while one rotation takes approximately 23 h and
56 min.
Atmospheric circulation begins with heating differences by the sun. The
sun’s rays are nearly perpendicular at the equator and are angled at the poles. Because of
this, it is much colder at the poles than it is at the equator. Cold air exerts higher pressure
than warm air. Cold air is denser and tends to sink, while warm air is less dense and tends
to rise. The higher density of cold air makes it heavier, creating a stronger, downward
force. Because cold air is more compact, high pressure is created and is expressed by
p = F/A ,
where all symbols are expressed in Appendix A.
As a cold air parcel sinks and hits the surface, it diverges and spreads
outward. A warm air parcel rises, and to fill the void at the surface, air converges. Since
air is colder at the poles, the sinking motion and surface divergence creates a net
southward movement of air towards the equator. Air rising at the equator adiabatically
cools and expands until it reaches the tropopause. At this point, air diverges and either
flows southward or northward toward either pole. This net circulation is known as the
single cell theory, assuming a uniform surface (all water planet). This cell is known as the
Hadley cell, named after George Hadley, an 18th century, English meteorologist Ahrens
(1991). This cell assumes no planetary rotation. Because of this, coriolis force is ignored.
The forces in the single cell theory are gravitational force and pressure gradient force
(PGF). PGF is expressed by
F/m = -(1/ρ)[(dp/dx)(dp/dy)(dp/dz)] .
This force is proportional to the gradient of the pressure field, not to the pressure itself
Holton (2004).
In the three cell theory, planetary rotation is considered. As a result, the
Hadley cell breaks into a series of smaller cells because of Coriolis force. This is an
apparent force that results from planetary rotation. Moving objects, such as a moving air
parcel, are deflected to the right in the northern hemisphere and to the left in the southern
hemisphere. This force is represented by
f = -2Ώ(u)sinφ and f = 2Ώ(u)cosφ .
Since moving objects are deflected to the right or left, the single cell theory is no longer
valid, since straight-line flow is not possible. In general, each cell is divided about every
30° in latitude by semi-permanent pressure systems, where 0° is the equator; 30° is the
subtropical high; 60° is the subpolar low; and 90° is the polar high. The cell nearest the
pole is the Polar cell; the middle cell is the Ferrel cell, named after William Ferrel, an
American meteorologist; while the Hadley cell is nearest the equator. Since circulation is
governed by the sun, these cells follow the seasonal patterns, due to Earth’s tilt.
Jet streams are regions of high winds (50 kt or greater) near the
tropopause, moving in an eastward direction. As air from the equator rises and diverges
aloft, the northward or southward movement towards the pole is affected by coriolis. In
the northern hemisphere, air is deflected to the right. The eastward flow results in the
subtropical jet stream (STJ). The STJ is strongest during the northern hemisphere
summer, due to the conservation of angular momentum. This law states that if an object
rotates about an axis, then the angular momentum with respect to a point on the axis is
related to the mass of the object, the velocity, and the distance of the mass to the axis. In
other words, as the radius of the STJ decreases due to the northward migration during the
northern hemisphere summer, its angular momentum increases.
Upper-level divergence, coriolis, and a strong, thermal contrast between
the Polar cell and Ferrel cell result in the polar front jet stream (PFJ). The PFJ is strongest
during the winter, since temperature gradient between the equator and pole peaks during
this time. Temperatures are relatively constant at the equator year-round, while they drop
at the polar region during the winter, producing a stronger gradient. The PFJ is the
boundary between cold and warm air masses, the location of frontal boundaries.