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TROPICS IN DEPTH: All about Shear

We've taken you through the embryonic stage all the way to a full blown major tropical system. But shear is an important factor during a storm's entire lifespan.
Credit: WTLV

JACKSONVILLE, Fla — Before we get into the details of this week's Tropics in Depth, it's probably best to start off with the basics of shear.

Shear (or wind shear) is defined the change in wind speed and or direction across a relatively small distance in the atmosphere. Shear can also be defined on a larger scale, but we will get into that in a bit.

There are two main types of shear -- speed and directional. Speed shear is a change, usually an increase, of wind speed with height. Directional shear indicates a change of wind direction with height.

The type of shear environment is important for not only tropical forecasting, but also when meteorologists are trying to determine the type of thunderstorms to expect. Could storms become strong to severe? Can we expect downbursts or tornadoes? The questions go on.

When we talk about wind shear and hurricanes it's often because the shear is helping to tear apart a storm. However, there is another side to shear and, if conditions are right, it could actually aid in tropical cyclones sustaining themselves for a longer term.

Let's start with the classic major hurricane. With a twist in the cyclone, counterclockwise winds on the surface becomes clockwise on the top of the storm. This is what allows the system to breath. However, wind shear disrupts this process and inhibits vertical development.

So, what about the Category 1's and 2's? They can handle some shear as long as the system is still able to ventilate. Either light, weak shear or diffluent shear are examples of this. Diffluent shear is like water flowing around a rock and thus the flow is forced to spread out.

Finally, there's also shear on a global scale. Across the planet, meteorologists watch for larger cycles that propagate in the 30 to 60 day time frame. An example of this is the MJO, or Madden-Julian Oscillation.

The MJO is the largest element of variability in the tropical atmosphere. It's defined by an area of atmospheric circulation and deep tropical convection, of thunderstorm activity. Cycles like the MJO increase and decrease lift in the atmosphere within the tropics. This pattern shifts across the globe makes some areas more prone to tropical development than others in that 30 to 60 day time frame.

For example, the MJO has enhanced tropical activity across the Pacific region so far this season. In the meantime, the Atlantic basin has been much quieter with suppressed activity. This pattern is expected to switch in the coming weeks as the MJO shifts our way.

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