Wednesday, December 16, 2009

It's that time of year again

I am not talking about the Christmas season, although we all had a lot of fun choosing presents for Twelve Days of Christmas: An Aviation Gift Guide, but about the start of the cyclone season over the tropical part of Australia. The cyclone season Down Under usually runs from December to April.


To kick off the season we have TC Laurence, currently in the Broome area in the north-western corner of Australia, about 15 degrees latitude south. Broome, which is as far from Sydney as Vancouver is from the Gulf of Mexico or Paris is from Greenland, is the region in Australia most prone to cyclones.

Although tropical cyclones very rarely move as far south as Sydney, the influence of tropical cyclones in the north-east can be felt in the south in the form of heavy rain falls. There's a great map here with the track of all cyclones around Australia for the last hundred years or so.

The aviation weather forecast for the Broome area (Area 69) spells out what tropical cyclone means in graphic details:
OVERVIEW:
TROPICAL CYCLONE LAURENCE 933HPA NEAR 15.4S 124.2E AT 00Z, MOVING SW
AT 05 KNOTS [REFER TO SIGMET].
FREQUENT EMBEDDED THUNDERSTORMS, SEVERE TURBULENCE AND WIDESPREAD
RAIN WITHIN 120NM OF TC LAURENCE. AREAS OF RAIN WITH OCCASIONAL
THUNDERSTORMS REMAINDER EAST OF YBRM/PRES. ISOLATED THUNDERSTORMS AND
SHOWERS REMAINDER.

WIND:
3000 5000 7000 10000 14000 18500
VRB/20 VRB/20 VRB/20 VRB/20 PS10 VRB/20 PS04 VRB/20 MS03
REMARKS: [1] WINDS ALL LEVELS CLOCKWISE AROUND TROPICAL CYCLONE.
[2] WINDS INCREASING TO 60 KNOTS WITHIN 120NM OF
TROPICAL CYCLONE INCREASING TO 110 KNOTS TOWARDS TC
CENTRE.

You will notice of course that winds blow clockwise around a low in the Southern Hemisphere, in opposition to how things are done north of the Equator. Blame my illustrious fellow Frenchman for that.

Interestingly, this is why this weather phenomenon is called a cyclone: a cyclonic air flow is an air flow that rotates in the same direction as the Earth. As the Earth rotates from East to West, the rotation is clockwise when seen from a point above the South Pole, and anti-clockwise when seen from a point above the North Pole. To add to the confusion, cyclones are known as hurricanes in the US and typhoons in Asia.

Depending which of the club aircraft I would choose to fly close enough to the center of TC Laurence, with a headwind of 110kt, I could either hover in the Archer (cruise speed 115kt), or even fly backwards in the Warrior (cruise speed 95kt). Not for long obviously, but a sure way to add my name to the shortlist for the Darwin Awards.

1 comment:

sylvia said...

LOL, I got a bit frightened reading that last paragraph - Darwin award indeed!