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How to spot a thunderstorm in the making

What creates a thunderstorm?  And how can you pick one in the skies above you?

New contributor Tim Gunn walks us through the basics of the clouds overhead and what you should be looking out for.

A storm is the result of the atmosphere trying to solve a problem it has: That is positive and negative electrical charges which have arisen because of the differences in temperature and moisture.

A lightning strike is the atmospheres way of temporarily neutralizing the differentiation between +ve and -ve charges.

A storm will initially start off as a small Cumulus Humilis cloud (latin translation: Heap Humble)

As further daytime surface heating is provided the cloud will start to get slightly larger – bigger than the Humilis. (On most days this is where the clouds life will stay and dissipate).

If cooler temperatures exist and further land heating and/or lift is provided the cloud will start to turn into a Cumulus Congestus (latin translation: Heap Tower).

If you see this cloud late in the day it most likely will not develop any further as the supply of warm air will start to go as the sun goes down.

However, if you see this cloud in the morning or in the afternoon with a rather crisp cauli-form look to the top it can very well develop to the next stage.

Note: This is the cloud Weather Presenters often use for the term “Heat or Convective Showers”.

Now if deeper atmospheric instability exists (very cold air on top of the cloud) and a good amount of moisture is in the air the cumulus cloud will now transform into the stage of Cumulonimbus.

Cumulonimbus Calvus (latin translation: Column Rain Bald) is the generally the first stage of a Thunderstorm.

This cloud may or may not have lightning at this stage but will most likely have heavy rain and/or some hail.

The next cloud associated with a thunderstorm is called Cumulonimbus Incus (Column Rain Crown)
This Cloud is often what people call the “Thunderhead”:  This is when the cloud has developed so much that it cant develop any higher as it has hit the top of tropopause the bit where all of our weather is located in the Atmosphere.

This cloud can contain – but is not limited to – Lightning, Hail, Rain (could be very heavy) and Gusty Winds.  It is also possible that this cloud could produce a tornado but since this is NZ the risk is low – but never rule it out as the weather can always throw you something you would never expect.

When you see this cloud it means that it is in its “Mature” phase so this when possible severe conditions will be most likely to happen.

There are more clouds (related to storms) that I could talk about but I think that’s enough for one post.

If you would like to ask any questions please post in the comment section below.

Tim Gunn has had a fascination with the weather since he was very young.   “When I was around 6 to 8 years old I made up a play weather company called Metro Weather.  I constantly went outside and gave out a forecast to my mum and dad and issued a thunderstorm warning whenever I heard thunder.
I typed this up on an old typewriter and then handed it out to my parents saying shut down the computer and TV etc!”  (Sounds a bit like Philip Duncan’s weather obsession).

I have learnt so much since then and even today the weather still fascinates me – in fact even more!

The shear power of thunderstorms, lightning, tornado truly puts me into this weathery type mood that I cant get out of.  I also chase storms when they arise if they are worth the petrol to get to them.

You can obviously tell I LOVE storms – and I look forward to answering your questions.

Best regards,



JohnGaul on 30/11/2010 9:06am

Interesting and informative but thunderstorms in Canterbury can develop a bit different to that as the majority of thunderstorms here are frontal ones relying on a convergence sort of effect ie. resistence of a change of airmass.


WW Forecast Team on 30/11/2010 10:00am

 Yes this mainly relates to more diurnal heated type thunderstorms.

But the same concept applies to frontal thunderstorms as they still need a form of heat to produce them.

Cold Air on top and Warm air down below.

ChrisMB on 30/11/2010 5:44am

Thank you for sharing that – a very good, thorough and informative post!

I must say, I learned that if the Cumulus Congestus occurs late in the day, a thunderstorm is unlikely..didn’t know that 🙂

…and the extra latin names were helpful – I’ve always stopped at Cumulonimbus…now I can sound smart with Cumulonimbus Calvus.

Thanks heaps 🙂


WW Forecast Team on 30/11/2010 6:17am

 No probs!

I am glad you found it of use.

Yea Cumulus Congestus genrally require a good amount of heat to produce good updrafts to go past any stable layers of air above it to transform it into a Cumulonimbus, this isnt to say a thunderstorm will not develop from this after dark it just means its unlikely.

Next post I will make will be clouds Warning Signs…..

AmySam on 29/11/2010 8:02pm

Thanks for that Tim, that was very interesting! I, too am a huge lover of storms and thunderstorms, my fiance thinks I am crazy and all my friends laugh as I predict rain for the upteenth time!
Now I have even more ammo to predict the weather- THANKS!
I am also a huuuge fan of weatherwatch, love the lightning tracker and check in with you all every morning!
Keep up the great work…and if you ever need an intern….

WW Forecast Team on 30/11/2010 6:23am

 Thanks for the comments 🙂

Yea Storms are truly fascinating!

Ah yes I can relate to the friends situation, hahaha

I randomly say stuff what is about happen and they must think what am I on about.

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