"Why is there air?" - Bill Cosby

Air- It's invisible most of the time unless you live in a smoggy area like Southern California. When driven hard by high and low pressure systems, wind is turned into a destructive force you have to witness to believe. Just ask the folks who live in tornado ridden areas what the wind is capable of.

In this photo the asphalt road has been torn away by a twister.

THE EARTH'S ATMOSPHERE

Earth's atmosphere is made up of various gases in a surprising thin layer that encompasses the planet yet provides an amazing amount of regulation to the little rock we call home.


The atmosphere is made up of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.

The planet's atmosphere is around 300 miles (480 km) thick, but most of the atmosphere (about 80%) is within 10 miles (16 km) of the surface of the Earth. There is no precise place where the atmosphere ends and space begins; it just gets thinner and thinner until it melds into space.

Air Pressure as measured in pounds per square feet
At sea level, the air pressure is around 14.7 pounds per square inch. As your altitude increases the air pressure decreases and so does the pressure wind provides on your boat's sails. At an altitude of 10,000 feet, the air pressure is 10 pound per square inch which is a reduction of nearly 30%. The folks sailing at Lake Dillon in Colorado can attest to what a difference altitude makes on not only the air density but also the loss of temperature as you climb up in the atmosphere. The adiabatic lapse rate averages around 3.2F for every 1000 feet climbed.

The Layers of the Atmosphere:

Image courtesy of www.answers.com

oExosphere: This is the outermost layer of the Earth's atmosphere. The exosphere generally lives in an area from about 400 miles (640 km) above the Earth's surface to about 800 miles (1,280 km).

oLonosphere: The ionosphere begins around 43-50 miles (70-80 km) above the surface of the earth and continues for hundreds of miles (about 400 miles = 640 km). It contains many ions and free electrons. The ions are created when sunlight hits atoms and tears off some electrons. This is the area of the atmosphere that the brilliant "Auroras" light shows occur.

oMesosphere: The mesosphere is characterized by temperatures that quickly decrease as altitude increases. The mesosphere extends from between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.

oStratosphere: The stratosphere extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface and contains ozone which is critical for filtering out ultraviolet radiation from the sun. The cirrus family of clouds reside in the lower parts of the stratosphere.

oTropopause: this is the boundary layer between the stratosphere and the troposphere. Very little temperature change is found within this layer regardless of altitude.

oTroposphere: We live in the troposphere. This first 11 miles of the atmosphere is where all of the fireworks we call weather take place. Hurricanes, tornados, monsoons and thunderstorms all live in this lowest layer of the atmosphere.

WIND

As sailing mariners we need wind to power our boats through the water. Breeze in the average range (Here in the US that is 8-12 knots) keeps the sea surface relatively calm and provides a modest amount of power and a manageable power resource for sailors. As anyone who has sailed a centerboard boat with no self righting keel attached can tell you, just a 25% increase in velocity from 12-16 knots provides a tremendous increase in power. Why?

The following explains how the power of wind increases with velocity: Let's assign one unit of energy to each knot of wind. To determine the amount of power the wind is providing you multiply the number of knots by itself (square) as read by an anemometer (wind meter) and assign the product of your math equation to the wind as the "power value". For example- Ten knots of wind = 100 units of wind power (10 x 10= 100). Twelve knots of wind gives you a wind power product of 144 units of wind power. Now apply the equation to the increase mentioned above (12 knots versus a 25% increase in wind speed to 16 knots) and you find that the wind "power value" has increased by nearly 44%! What an amazing difference in strength! This is why finding modest increases in wind speed on the course can outweigh small shifts in wind for high performance boats and a premium is put on finding "pressure" these days for boats that exceed their theoretical hull speed.

So where do I find the wind? On a buoy course (micro climate area) you'll first need to find what is driving the pressure that day and then use logic to stay in the best pressure as conditions change. On many lakes there is better pressure near the shoreline than in the middle of the lake as the earth's surface heats upfrom exposure to sun energy.. If the surface of the earth is higher than the water temperature of the lake or ocean nearby, the breeze travels from the body of water toward the void created by the rising air over the ground which is being heated by the sun. The opposite is true at night. As the sun goes down and the surface of the Earth cools the wind will change direction when the temperature of the land drops below the temperature of the water. Folks who live on the Gulf Coast often see a brisk wind early in the morning blowing "offshore" and then a shutdown around mid morning as the land heats up. Some of the best racing I have ever done commenced at 8:30AM in Pensacola Bay and then a "brunch break" around 10:00AM while the breeze turned around. This type of pressure is known as a "convective gradient".

When the winds are being primarily driven by a cyclone or frontal boundary then the direction and pressure are driven by the juxtaposition of the high and low pressure centers and then factor in the amount of Coriolis effect that will bare on the system to give you the direction. The wind speed at the surface will be determined by two things-

1. How tight and how large (Pressure change)) the gradient is and
2. Topographical features like mountains or man made structure like building.
While it does not seem like there is very much air "pressure" on we humans here at the surface of the Earth one need only free dive in a lake or the ocean to a depth of 33 feet (one atmosphere) to understand that we do in fact live under a fair amount of air pressure. With this concept in mind I can tell you that air at the surface does not like to go up and over a topographical (surface) feature. It would much rather go around that feature. A good example of this occurs in large cities with tightly packed sky scrapers. If wind blowing toward a city is measured at 15 knots where there are no buildings, it can be rapidly accelerated to over 30 knots as it tries to find its way through the city instead of simply rising and going over it. Where two peaks or buildings come together forcing the air between them a "Venturi effect" takes place.

The same thing happens but to a lesser degree around points of land. The air would much rather go around the rising point of land than go over it. Los Angeles harbor is a great example. Winds typically blow from 285 degrees in the harbor but as you get closer to Point Vicente you enter an area known for years as Hurricane Gulch (Cabrillo Beach) where wind speeds increase by almost double the rest of the harbor when winds are from a favored direction. This often changes or "bends" the wind direction locally.

Points of land and other obstructions can bend the wind. Even modest ten foot high break walls can bend the breeze. A nice lift can be found close to a break wall if the wind is from the proper direction.

Change in direction with the passing of a frontal boundary-

If you live in the northern hemisphere and you know a cold front is coming your way you can pretty easily deduce the time of its passing and figure out where you need to be on the course to best take advantage of the coming changes. It is no more complicated than doing a simple time distance calculation by using either radar if there is precipitation in the front or a water vapor satellite map if precipitation is absent. As the front passes over you there will generally be a pronounced shift to the right and the air temperature will change. Sometimes subtly and sometimes dramatically depending on the air masses involved. I will cover more on this next issue of Weather and the modern mariner. Comments?

10/05/06