2 July 2014
You may need to click on the image above to really see the ripples across the top of Arthur, so do that first, before I tell you why they are there.
You are looking at gravity waves, but a better way of understanding it is to compare it to the ripples you see after you throw a rock into a still pond. The rock disturbs the water and makes waves that slowly dampen out at the water reaches the level it started out at. The same thing is happening here, except the air is rising and falling instead of water.
Imagine you are a blob of warm air rising through the core of Tropical Storm Arthur. You are warmer than the air around you, and therefore lighter and buoyant. You rise like a hot air balloon until you reach the stratosphere, but then things change. The ambient temperature begins to rise in the stratosphere (although it’s still very cold) and suddenly our blob of air finds that it is now colder than the air around it. Now it begins to fall, and remember from high school physics that all objects fall at the same rate, not matter how heavy they are. Our blob of air falls and suddenly finds itself warmer than the air around it again and it bounces back up. It continues to do this but each time it rises and falls a little less as air resistance dampens the motion.
A blob of air will cool about 10 degrees C for every 1000 meters it rises unless it is saturated, and then the condensing water will add in some heat and it will cool at about 5.5 degrees C/1000 meters. This difference in cooling rates make rain and thunderstorms possible on Earth. You can see gravity waves in severe storms here courtesy of my friend Metorologist Brad Panovich.
That is what you are seeing here. Just like ripple on a pond. Gravity waves are a common feature of the atmosphere and can cause thunderstorms and sudden strong wind events.
Thermodynamics is cool isn’t it!