October 7, 2022
Ian's Fury - all photos courtesy of NASA GOES image viewer
Last weekend's terrible hurricane was easily seen from spacecraft and made public on NASA's website: GOES image viewer (https://www.star.nesdis.noaa.gov/GOES/index.php). This website takes you to a whole array of images from different wavelengths of light. This image combines nighttime images taken with infrared light that can see the tops of clouds at night as well as the city lights. Some of the clouds are thin enough that the infrared cameras can see the city lights through the clouds for much of the southeast coastal states. The hurricane shows its well-formed "eye" at its core as well as the tops of numerous thunderstorms formed off the coast over the Atlantic. Click on the image for an animation and an explanation of the animation properties.
Why does a hurricane rotate counter-clockwise? This is a phenomenon called the Coriolis effect. The Coriolis effect is a rotational force on moving objects due to the rotation of the Earth. When looking down on the Earth from above the poles, we would see the Earth rotating counter-clockwise with a period of one day per rotation. The Earth would appear to rotate half the rate of the hour-hand on our 12 hour am/pm common clocks. This doesn't seem very rapid compared with the rotational speed of a merry-go-round exercise toy on a typical playground that rotates typically at a much shorter period of about 5 sec per rotation. Thus when we see children (and adults) playing on these rides, they appear to be going around quickly compared to an hour hand on a clock or the Sun's rate of marching across the sky. If you try to walk across the rotating platform - always hang on to the rails on this equipment! - you will feel an extra force pushing you sideways! It is rather difficult to walk on the rotating platform compared to walking across a stationary floor. This sideways force on a person walking across a rotating platform is called the Coriolis force. It arises because the speed of any part of the platform is faster on the outer rim than when near the center. That is because the perimeter of the disk has to travel further in one cycle (all around the circle) then near the center (around a much smaller circle).
Imagine the platform rotates counter-clockwise (toward the right as you are facing the center). If you step toward the center you step onto part of the platform that is also moving to the right, but at a slower speed. If you were to step from a moving walkway onto a slower moving walkway, your momentum would deflect you in the direction of the higher speed from the walkway you were stepping from. You would be thrown to the right. If you were tired of riding near the center and wanted to ride at a faster speed, you would step outwards toward the perimeter to a portion of the that was moving that was moving faster than where you stepped from. But in going outward, the perimeter is moving to the left faster than from near the center, thus when stepping outward you feel yourself being thrown to the right. Thus if you try to walk across a counter-clockwise rotating platform you will always be deflected toward your right. This rightward "force" occurs for which-ever direction you try to walk across the platform. This is the Coriolis effect.
A hurricane begins with a warm spot in the ocean that heats up the moist air above it. The rising air forms thunderstorms the grow to become more intense and coalesce into a mega storm. The rising air from the growing storms forms a massive updraft and very low barometric pressure. Surface air comes from the surroundings to replace the air at the base of the updraft - often from several hundred miles distant. The Earth (northern hemisphere) is like a disk (warped into an inverted bowl) that is rotating slowly (1 revolution per day) in the counter-clockwise direction looking from above. Like a child playing on the rotating disk at the playground, the wind is forced toward the right as it tries to move toward the low pressure at the center of the storm. The wind heading north toward the storm center is deflected toward its right (east). The wind heading south from the north is also deflected toward its right which is the west. The combined effect of winds trying to reach the center of the storm from all directions is to force the winds to spiral around the storm center. In a hurricane, fueled by warm ocean temperature and high humidity the the spiraling effect is magnified by more and more moist wind spiraling toward the center and building more and more powerful updrafts and we have a "runaway" effect with stronger and stronger spiraling winds until we get a huge storm several hundred miles in diameter and an eye where no winds penetrate because they were deflected upwards in the eye-wall of the storm.
In the video the extreme perimeter of the hurricane reached the Carolina's as can be seen in the daylight satellite image at left. In the video we see the outer limits rotating to the left, contrary to all the discussion about rotating right. The explanation is that these clouds that cover the Carolina's consist of the outflow of the hurricane. The updrafts that fed the hurricane emerge from the top of the hurricane clouds and move outward, north away from the rapidly spinning center of the storm. This outflow, that travels outward from the center is again deflected toward it's right, but instead of a cyclonic counter-clockwise rotation is directed clockwise as in an anti-cyclone. A high pressure system where wind is directed away from a center.
It is a common folk myth that water drains from basins, bathtubs, sinks, and toilets always spin counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere like cyclones and hurricanes. Cyclones and hurricanes do rotate clockwise in the southern hemisphere because the inverted bowl of the southern hemisphere rotates clockwise when looking down on it from over the south pole. Some tour guides have even "demonstrated" this effect to tourists visiting tropical countries straddling the Earth's equator. However, the Coriolis effect depends on the rotational velocity and the velocity of the material on the disk on which it is riding. The rotational velocity of the Earth is only 1 rotation per day. In contrast, when a sink or basin is drained the angular speed for the water left over from the sloshing of the water from filling or washing in it is much greater than the angular rotation rate of the Earth. With hurricanes and cyclones, the distances involved are so much greater than the distances in a basin, bathtub, or flushing toile. The vortex in the drain depends on which direction the water flows from the last "slosh" or by jets in the toilet. The rotation rate of the Earth is too small for it's Coriolis effect on small household items. The Coriolis effect on the Earth is observed only for large scale motions (distances of several hundreds of miles).
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
Comments
Post a Comment