19 November 2019

A Thunderstorm You Can Set Your Clock (or Compass) By

Posted by Dan Satterfield

It’s a thunderstorm that’s so common it has a name:


From September- March on Tiwi Island near Darwin, Australia, you can set your clock by it, because it explodes each afternoon around 3 PM.  Tiwi Island is about 70 km north of Darwin, and the shape of the island plays a vital role in making Hector form.  The island has just the right shape to allow for afternoon sea breezes to converge. This lifts the warm and humid air into the unstable tropical airmass surrounding the island and in minutes an intense thunderstorm forms.

Hector is so reliable that back in World War Two pilots called it “Hector the vector”! In the days before satellite-based GPS the pilots used it as a reliable navigation beacon. The storm over Tiwi often reaches a height of over 20 km and pilots at high altitudes, can see it from many hundreds of miles away.

The Australian Bureau of Meteorology posted a tweet about Hector exploding right on time this past weekend and it has a nice explainer video to go along with it (Click the image to see the video):

You can see a satellite loop from the BOM Tweet here. Notice the incredible outflow boundary the storm produces. This blast of cool air is like a mini cool front and shoots off to the west. Many times these boundaries will kick off new thunderstorms and the winds can be very strong as these blasts of rain-cooled air pass a location. More about outflow boundaries from Dennis Mersereau here.

Hector’s predictability makes it perfect for the study of deep convection in the atmosphere, and numerous papers have been written about it. Researchers have even used high-resolution numerical models to simulate the conditions that cause Hector and the models do indeed produce a huge thunderstorm. Those studies have shown the shape of the island is a big factor and that Hector forms in the unstable atmosphere at the Intertropical Convergence Zone makes the atmosphere unstable.

To get a thunderstorm, you need an unstable atmosphere and something to lift warm and muggy air into it. The sea breezes merging together on Tiwi island do the heavy lifting and the unstable atmosphere in the warm season does the rest. Two good pieces of published science to learn more are here and here. The European Geophysical Union talked about it in 2010 and there is a good summary here as well.

You can see hector on this sector from the Japanese Himawari Weather satellite. It is quite active this time of year and usually develops around 3 PM local time which is about 05-07 GMT.