Update: October 4, 2006 
HERE COMES THE RAIN!
By Pam Eastlick for THE DEEP on line

Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond. 

Have you noticed that it’s been raining a lot?  If you look at the satellite photo, you’ll see that our whole area is completely covered with clouds.  The reason is complex, but that cloud band is the Intertropical Convergence Zone, also referred to as the ITCZ and as the equatorial convergence zone.  And what generates it is something you probably won’t be able to see this week; the Sun.

To help you understand the ITCZ and our weather I want you to do something.  Right now, point to the Sun’s position in the sky at noon.  Go on, you can do it!  Just take that index finger and point!  Now, let me get my special star lady psychic powers going.  Your image is slowly coming into focus.  There!  You’re pointing straight up, aren’t you?

Well, I hate to tell you this, but you’re wrong.  But don’t feel bad.  I didn’t need special psychic powers to know the answer because virtually everybody points straight up when asked to identify the Sun's position at noon.  In fact, the Sun is NEVER directly overhead at noon for most of the world's inhabitants and only rarely straight overhead for the rest of them.  In the northern hemisphere, north of the Tropic of Cancer (23ºN) the Sun is always located in the southern half of the sky.  In the southern hemisphere, south of the Tropic of Capricorn (23ºS) the Sun is always in the northern half of the sky.  In the temperate zones (above 23º north and below 23º south) the Sun is NEVER straight overhead. 

This happens because Earth is tilted 23º as it spins.  This tilt causes the seasons.  When the northern hemisphere is tilted toward the Sun, the Sun is higher in the sky and more direct sunlight falls on the Earth.  The northern hemisphere has summer.  When the northern hemisphere is tilted away from the Sun, the Sun is much closer to the southern horizon and the Earth cools off.  The northern hemisphere has winter.  Above 67º north, the Sun disappears entirely and it's dark all the time.  The converse happens in the southern hemisphere.

The area between 23ºN and 23ºS is called the tropics and there the Sun is always high in the sky, the land is always warm and there are no seasons.  In the tropics, the Sun is straight overhead for 2 days a year.  The Sun is straight overhead at the equator on 22 March and 23 September, the spring and fall equinoxes.  The Sun is straight overhead at 23ºN on 21 June, the first day of summer in the northern hemisphere.  The Sun is straight overhead at 23ºS on 20 December, the first day of summer in the southern hemisphere.

Here on Guam, the Sun is straight overhead at 12:20 p.m. on or about 20 April as the Earth tilts toward the north from its spring equinox position (with the Sun over the equator).  From that point on, the Sun is in the northern half of the sky over Guam, something that NEVER happens in the mainland US.
The Sun reaches its northernmost point on 20 June over the Tropic of Cancer and the Earth tilts toward the south again.  On Guam, the Sun is overhead at noontime again, on or about 20 August on its way to its position above the equator on 23 September.  The Sun is north of straight up over Guam for four months each year. 

When the Sun shines straight down on the ocean, it heats the air and water. Hot air rises and when it

reaches the much colder air at higher altitudes it cools and forms the huge cloud systems that characterize the ITCZ.  Of course, there are other factors at work.  The water retains the Sun’s heat long after it no longer shines straight down on the area.  The Sun was directly overhead at the equator last month (on 23 September, the autumnal equinox) and it’s already shining straight down in the southern hemisphere.  But a combination of ‘left-over’ heated water and cool air already pushing south from the rapidly-cooling far north, has caused the ITCZ to form in all its glory right on top of our heads.

It’s not a bad place for it to be because the ITCZ also spawns something we really don’t like: typhoons.  If all those clouds begin to swirl, then there can be people in trouble, like the recent victims of Typhoon Xangsane in the Philippines and Vietnam.  Typhoon Xangsane formed up over us! The equatorial convergence zone is the original and ultimate jet-setter as it quite literally ‘follows the Sun’.  And right now, it’s in the process of moving south of us as it follows that retreating Sun.  And that is not such a good thing.

The spinning Earth causes all northern hemisphere typhoons to track north.  Oh, they may go south for a while because of prevailing local wind conditions, but in general they travel north.  (Southern hemisphere storms track south).  And if the ITCZ moves to the south, the typhoons it spawns travel north . . . . in our direction.

So don’t curse the rain!  At least there’s no 150 mph wind blowing it sideways!
Grab your umbrella and splash on over to the Deep Website at www.thedeepradioshow.com to learn more about weather and many other topics.  Enjoy!