New Haven is turning out to be quite a dramatic place so far.  Last Tuesday, I was working at my desk when suddenly my chair started wobbling, and then my desk started wobbling, and then it felt like my entire 6th-floor room was wobbling.  I thought my head had gone funny, but realised it was an earthquake.  A very calm, elegant one as earthquakes go: shaken by a few seconds of strangeness and then back to normal – except I was left feeling awed by the fact that the earth had moved underneath me, and had an unusual sense of the epic scale of our planet, the vastness of this rock on which we wander around from shop to shop and fret about which classes to choose.

Now it’s not the earth moving but the air, howling through the streets, with gusts every now and then that shriek as if with delight.  A Category I hurricane has sustained winds of around 75mph: imagine zooming down a motorway and sticking your head out of the car window.  Hurricane Irene has been downgraded to a tropical storm, which is still impressive because at 41 degrees North of the equator we’re well out of the tropics.  But from the safety of my room, it just feels like I’m back in Edale on a typical summer’s day.

The rain began last night and is still puddling the streets, though there’s no sign of flooding near me.  All the shops here are shut and there’s no public transport, although from my window I did see one brave soul battling down the road on his bicycle.  We’ve been advised not to leave our buildings until given the all-clear by the Yale Alert system, and luckily we still have electricity on campus, though 700,000 houses across Connecticut are without power.  Apparently hurricanes are usually followed by a heat wave, so I feel for the people who won’t be able to cool down their homes with air con.

As well as giving us packages of food to last us through the day, Davenport College gave us a really good low-down on how hurricanes work (which I’ve supplemented here with info from the National Oceanic and Atmospheric Administration), which delighted the geographer in me!  ‘Hurricane’ is the name given to tropical cyclones in the Atlantic; elsewhere, it’s called a typhoon.  To form, it needs warm sea surface temperatures of more than 26 degrees C, because below this temperature, the atmosphere is relatively stable so you don’t get strong enough upwards convection.  Moisture evaporates from the ocean surface; warm air rises and as it rises it cools, so the moisture condenses into water droplets.  This condensation process releases energy, which drives the circulation of the cyclone.

As soon as a hurricane moves over cooler waters, or moves over land, it loses the driving power of the warm water, so starts to calm down (as this map demonstrates).  Irene has got so far north because along the USA’s east coast runs the Gulf Stream, which transports warm water from the Gulf of Mexico northwards.  In the northern hemisphere, hurricanes spin in an anti-clockwise direction, so the winds pile up the seawater on the eastern side of the storm and can cause a storm surge, which is why they were worried about serious flooding in New York City.  Apparently in New Haven we’re protected from storm surges by Long Island, which has borne the brunt of the weather.

Lastly, tropical cyclone scientists cannot yet say whether or not hurricanes are getting stronger and more frequent due to climate change.  It’s hard to make out long-term trends with the evidence currently available, so they can’t draw firm conclusions.  But they point out that it is likely that as tropical sea surface temperatures increase, cyclones will have higher peak wind speeds and rainfall.  And as sea levels rise, the risk of storm surges will be amplified.