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I'm a pretty intelligent guy (I like to think) and I work with a pretty intelligent guy. Today, I said cyclones can't form close to the equator, thinking cyclones need the Coriolis effect. He was quite adament that they could and sent me this:

(Tropical Cyclone) Vamei formed and reached tropical storm strength at 1.5º N

But I came back with this:

Hurricanes and cyclones can't actually form within 4 degrees of the equator, because the Coriolis effect is just too small

Do cyclones (or hurricanes) form within 4 degrees of the equator?

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  • I've made a small, but significant change to your question: we can't really answer whether they theoretically can form there but only whether they practically do from there. If they do form there, then they certainly can - not so clear otherwise...
    – Sklivvz
    Apr 2, 2014 at 9:31
  • "I said cyclones can form" should probably be "I said cyclones cannot form"?
    – Suma
    Apr 2, 2014 at 9:40
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    It seems the articles linked pretty much sum up the knowledge and show you are both right. Cyclones do not normally form near the equator, but rarely they do: "the probability for a similar equatorial development was at least once every four centuries."
    – Suma
    Apr 2, 2014 at 9:42
  • Looking at the second article, it seems that the 4 degrees limitation assumes no nearby land to affect wind patterns.
    – Ofir
    Feb 17, 2015 at 8:55
  • Anyone reading back at this... the question Impossible or improbable? Hurricane crossing the equator at Earth Science SE should give very important insight on the topic too.
    – JeopardyTempest
    Aug 6, 2018 at 6:00

1 Answer 1

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Can they form near the the equator? Yes. Cyclone Agni from 2004 is probably the best example of this: according to the India Meteorological Department, it was 1.5 degrees north of the equator when it became a tropical cyclone, and before it reached that strength, the circulation center may have crossed the equator twice.

Is it normal? No. There are any number of sources that will tell you that tropical cyclones normally form at least 5 degrees poleward of the equator (eg. this NOAA page (warning: contains Geocities-style web design)); most of these trace back to work by William Gray, but the general situation can be summarized by the following image:

Tropical Cyclone tracks from 1945 to 2006, showing a distinct gap at the equator Image by Wikipedia user Citynoise using NOAA data.

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