Image courtesy of Dione Rossiter

From: Dione Rossiter
Iquique, Chile, October 16, 2008

What is a PDI?

Hola from Chile!

I arrived in Iquique (pronounced like the letter "E", the word "KEY", and then the letter "K") only a few days ago and have been getting ready for the field mission. Iquique is a town in northern Chile where the Twin Otter airplane will be flying lots of different instruments, including our own, through the atmosphere.

Our instrument is called the Phase Doppler Interferometer, nicknamed the PDI. The PDI contains a laser, a laser beam splitter, lots of little mirrors, and a sensor (kind of like a very fancy eye). The laser in the PDI starts off as one beam. Then, the beam splitter breaks it into two separate beams. Little mirrors are used to reflect the beams in different directions but eventually, the two beams are crossed (like an X). The sensor looks directly at the point where the two beams cross each other. A pattern of light is formed at the center of the X, which an eye cannot see but the sensor can.

When tiny things (including water droplets from a cloud) move through the center of the X, the pattern changes. The sensor notices this and sends information about the changes to a computer. Because the changes in the pattern depend on how big the water droplet is and how fast it is moving, the computer can calculate the size and speed of the droplet. I use this information to try understand how the water droplets that make up clouds are related to the Earth's climate.

You can split one laser beam into two by shining a laser pointer at different angles into a glass of water. Of course, this should be done under adult supervision. :)

Have fun trying the experiment!


Postcards from the Field: Climate Science from the Southeast Pacific

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