Atmospheric Circulation
Zachary Carson-Gilmore
October 12, 2013
Embry-Riddle

Atmospheric Circulation
On a nice summer evening as you walk across the beach heading home, you hear the waves crashing on the beach and the warm wind on your face. Have you ever wondered how wind is created and how we know it’s with us?
There has been a time when I wished there was a breeze outside to dissipate some of the heat on the flight line, where it was so hot I couldn’t even touch the aircraft to work on it. On the other hand, I have wished that the wind would go away when we couldn’t get clearance to do any test flights because of high winds. Atmospheric circulation and how it affects aircraft performance is important to know and understand because of the problems it can create with an aircraft, but also how it can potentially benefit the airframe. But first we need to know what atmospheric circulation is and how it is created to understand the positives and negatives against aviation.
“Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the atmosphere with different intensities at the equator, the middle latitudes, and the poles” ("Atmospheric Circulation", n.d.). Electromagnetic radiation is absorbed by the earth’s soil and by bodies of water. This reaction creates a heating effect and this effect is intensified during the summer months because the earth absorbs more energy when the rays hit perpendicular. The soil and water bodies have different densities and absorb the suns energy at different rates and water tends to reflect more of the rays that hit it. This causes the land to be warmer than the water. “The warmer air over the land rises, and the cooler air over the water blows in toward the shore. This is called the sea breeze. At night, the process is reversed. Land and the air above it cool quickly without sunlight, whereas water and the air above it maintain a higher temperature. The warmer air over the water rises, and the cooler air over the land blows out over the water. This is called the land breeze. Thus the different properties of land and water affect atmospheric circulation” ("Why Does Air Move? Atmospheric Circulation", n.d.). Thermal differences cause wind and high and low pressures which help create the air circulation.
Pressure plays another roll in wind generation. High pressure air tends to flow toward low pressure areas to equalize. There are three wind cells between the equator and each pole. They are the trade winds, prevailing westerlies and the polar easterlies. The trade winds are created because the air at the equator is heated and rise. The rising air then creates a low pressure area at the equator. That rising will go to the top of the troposphere and head to the poles where it then cools on the way down to about 30 degrees latitude for the north and south. Most of the air will head back to the equation because air flows to warm areas and to areas of low pressure. The same concept happens for the prevailing westerlies except these winds happen between 30 degrees and 60 degrees latitude. The polar easterlies do not flow back to the equator. They move to the pole cool and head back to 60 degrees latitude to warm and head back to the poles (Atmospheric circulation, n.d.). Knowing the basics of how air circulates and how wind is created will allow us to understand what effects it has on an aircraft.
Airplanes use airflows to create lift. The wing is shaped to create almost a straight line on the bottom and a longer round bubble on top of the wing. This creates different distances of travel for the air to move over the wing. “Bernoulli’s principle, which states that “high flow velocity gives a low static pressure”, is in line with this concept. The higher the velocity of airflow over airplane wings, the lower would be the static pressure exerted on the wing” (Ali, 2011). This is the principle that allowed so many to fly. Basically if two molecules hit the wing at the same time, one goes on top and one goes on the bottom of the wing, the top molecule will have to move faster to meet the other molecule at the other end of the wing. This causes a high and low pressure side of the wing and in return creates lift. This is what creates the benefits of having wind associated to aircraft. Pilots will always want to land and take off into the wind because it increases the air flow over the wing. The more wind over the wing the shorter take off/landing distances and the more weight an aircraft can be loaded with. Another advantage is a tailwind. Depending on what direction you are traveling, a tailwind contributes to faster speeds and less fuel used. Once you are in the air you could be pushed to your destination faster making the gas consumption less since Mother Nature is providing some thrust. The downside to wind would be a head wind. This will slow down an aircraft and cause the fuel consumption increase because the drag on the aircraft has increased.
Wind affects aircraft in many ways. So for, I have outlined some of the benefits to aircraft performance from wind. However, there are some affects that pilots need to cautious of while flying. Cross winds will push an aircraft off course. Pilots will need to compensate for lateral travel in which case the airplane will move off course, especially in long travels. Different altitudes have different air circulation and therefore could cause turbulence at different altitudes. Pilots need to be aware that cool sinking air can cause an airplane too loose altitude. Wind flowing around airport structures potentially causes turbulent air around the particular runway and pilots need to be cautious of the hazards this creates especially at low speed flight during landing ("Wind and Currents | About Flight.com", n.d.).
Atmospheric circulation and wind is a powerful force that should not be taken for granted. Managers and pilots need to understand the hazards of wind and how to take steps to safely fly aircraft they are entrusted with. From normal circulations to turbulent air, weather is everywhere. You cannot escape wind but you can understand it and take steps to harness the environment and let it help you get to your destination faster or load up more fuel and power through your headwind.

References
Ali, J. (2011, November 29). Airplane Wings: How Lift is Created During Aircraft Flight and Takeoff. Retrieved October 12, 2013, from http://www.decodedscience.com/airplane-wings-how-lift-is-created/6595
ATMOSPHERIC CIRCULATION | VERONETWORK. (n.d.). Retrieved October 12, 2013, from http://veronetwork.wordpress.com/2013/05/18/atmospheric-circulation/
Atmospheric Circulation. (n.d.). Retrieved October 12, 2013, from http://www.scienceclarified.com/As-Bi/Atmospheric-Circulation.html
Effect of Wind | About Flight.com. (n.d.). Retrieved October 12, 2013, from http://www.aboutflight.com/handbook-of-aeronautical-knowledge/ch-15-navigation/effect-of-wind
Kurtis, R. (2008, May 6). Heat Transfer on Earth by Ron Kurtus - Succeed in Understanding Physics: School for Champions. Retrieved October 12, 2013, from http://www.school-for-champions.com/science/heat_transfer_earth.htm
Why Does Air Move? Atmospheric Circulation. (n.d.). Retrieved October 12, 2013, from http://www.concord.org/~btinker/GL/web/air/motion/atmospheric_circulation_t.htm
Wind and Currents | About Flight.com. (n.d.). Retrieved October 12, 2013, from http://www.aboutflight.com/handbook-of-aeronautical-knowledge/ch-11-weather-theory/wind-and-currents