Step One – A Fertile Question
It’s only in the last couple of years that I have finally arrived at the lesson on hurricane formation that I am happy to deliver. Sure, cross sections where students can identify the features and the pre-requisites for their formation are fine, but I wanted to delve deeper. I want students to know the genesis of a hurricane from its humble beginnings as a series of small thunderstorms in equatorial Africa to the phenomenal hurricanes that news stations are so happy to send reporters out in to. To begin, I ask what Mark Enser would term a fertile question – ‘Why is the United States interested in thunderstorms in Africa?’
Step Two – Planning the Page
Again, I would always suggest using a student exercise book to plan out drawings and explanations. You then know what will fit on a page (even for the student who has massive writing). I’ve decided that a birds-eye and cross sectional view are to be used as between them I show all the intricacies and features a hurricane has. I’ve decided to use a page and utilise the margin as my equator for the birds-eye view and have the cross sectional view above.
Step Three – Planning Stages
I’ve decided that five stages, from thunderstorms in Africa to the dissipation of the hurricane over the Southern States of the USA, will allow for sufficient understanding and not over-complicate the delivery. Every stage is drawn out over the whole page so that I can see what a student book will look like at the end. It also allows me to judge the size of drawings and how much detail I can show as well as the space I can devote to explanation.
I would draw the birds-eye view first then follow this with the cross-sectional view. I would then spend some time explaining drawings and would be questioning them as to why thunderstorms would be forming here before writing down the brief notes to explain the diagrams.
Step 4 – Thunderstorms over Africa
The key points I would want students to know are:
- Maximum insolation due to curvature of the Earth causes rapidly rising air and the formation of thunderstorms.
- The absence of shear winds in the upper atmosphere means that the thunderstorms are not broken up.
- Easterly winds move the thunderstorms out into the Atlantic.
The second section involves drawing one enlarged thunderstorm out at sea, both in birds-eye and cross-sectional view.
Step 5 – Agglomeration
Again, the drawings would be explained and I would want students to know:
- Easterly winds have moved the thunderstorms out into the Atlantic.
- The absence of shear winds means that the thunderstorms haven’t broken up.
- The warm (greater than 26.5 degree Celsius) and deep (greater than 50m depth) ocean water fuels the thunderstorm further.
- The rapidly rising air condenses and releases heat, which further fuels that thunderstorm and causes the increased size.
The third stage then introduces spinning. Both the birds-eye view and cross-sectional view illustrate the thunderstorm increasing in size. The birds-eye view adds to this by showing the thunderstorm path moving away from the equator, which causes the spinning.
Step 6 – Spinning Around
From the drawings and explantion I would want students to know:
- The thunderstorm has increased in size as it is being fuelled by warm, deep oceans and heat released by condensation.
- The thunderstorm starts to spin anti-clockwise as it moves away from the equator in the northern hemisphere.
The fourth stage introduces the typical cross-section of a hurricane as well a birds-eye view that shows the spin direction as well as the eye and eye wall. This is the section that is the most complicated to draw so I would spend a but of time on this.
Step 7 – Cells
From the drawings and explanation I would want students to know:
- The energy of the thunderstorm has now caused wind speeds to exceed that needed for it to be classed as a hurricane.
- The thunderstorm has increased in size to the extent that there is now bands of warm, rising air and cold, descending air creating bands of cumulonimbus clouds.
- The centre of the hurricane has an eye where cold air quickly descends causing an eye of cloudless sky.
- The eye is surrounded by a rotating spiral of rapidly rising warm air, called the eye wall.
The final step shows the dissipation of the hurricane and its return to a thunderstorm over land.
Step 8 – Dissipation
Again, this would be explained and students would be expected to know the causes of hurricane dissipation, which came up as an exam question a few years ago in an AQA GCSE exam:
- Hurricane moves over land and is no longer being fueled by warm, deep oceans.
So, that’s how I explain the formation of hurricanes. This, like with all explanations I draw, is something that I will revisit each year and develop. Let me know what you think and if it’s been of any help.