Ask Brandon: Why Does Our Weather Move West to East but Hurricanes Travel from East to West?
We were recently studying weather in my 8th grade class. Through student’s independent inquiry projects, we discovered that hurricanes travel from east to west, but upon watching a recent weather video, we found that weather travels typically from west to east. My students wanted me to ask why weather changes direction at the equator?
– Marie Gillespie, Pierre, SD
Dear Ms. Gillespie’s 8th Grade Class,
Before we get into the reasons why weather moves in certain directions in Earth’s atmosphere, there are two general rules to discuss. First off, when it comes to the weather is that wind moves from high pressure to low pressure. The second is that of the Coriolis force, which is a result of Earth’s rotation. In the Northern Hemisphere the Coriolis force turns air to the right, while in the Southern Hemisphere the Coriolis force turns air to the left.
There are actually six different zones in Earth’s atmosphere, three in each hemisphere. Hurricanes form in the tropics which are called the “Trade Winds” located between the Equator 0° and 30° latitude in both hemispheres. In these locations the air movement is from east to west. This area is also the location of the Hadley Cell (see picture below), where air rises at the Equator and sinks around 30° latitude.
Experts at NC State University explain it like this, “At this latitude surface high pressure causes the air near the ground to diverge. This forces air to come down from aloft to “fill in” for the air that is diverging away from the surface high pressure. This causes a strong temperature gradient between the two different air masses and a jet stream results. At the 30° latitudes, this jet is known as the subtropical jet stream which flows from west to east in both the Northern and Southern Hemispheres. Clear skies generally prevail throughout the surface high pressure, which is where many of the deserts are located in the world.”
Hurricanes form in the warmest ocean waters and move away from the Equator towards the poles. If a hurricane lasts long enough, it will reach 30° and move into the “Westerlies”
The “Westerlies”, which is located between 30° and 60° latitude in either hemisphere, is where a major of the United States is located in (Aside for extreme southern Florida) and is why our weather primarily moves west to east. Due to the sinking air at 30° latitude, the air continues traveling northward toward the pole and the Coriolis force bends it to the right (in the Northern Hemisphere). This area is part of the Ferrel Cell (pictured above)
While hurricanes generally move east to west, when one reaches 30° latitude it will begin to “recurve” or change its direction, and start moving to the East. The picture below is an example.
Here is an example of a hurricanes track that changes directions as it reaches the “Westerlies”. This example is of Hurricane Bill from 2009.
There is one final cell called the Polar Cell, which is located between the poles and 60° latitude. With high pressure at the poles, and low pressure at 60°, the air moves away from the Pole towards 60°, and turns right due to the Coriolis force, meaning air moves from the east to the west in these locations. Thank you for the question and hopefully this helps you understand how our weather is formed!