Thursday, June 30, 2005

The sun as a microwave source

The Sun is a convenient source of radio waves. You can see this yourself by watching weather radar around sunrise or sunset. Radar works by emitting a very short pulse of microwave radiation. Then it listens for the weak returns for a short time (since a radar beam is line of sight, there is no point is listening for returns past 200 miles or so because of the curvature of the earth, or roughly 1 millisecond). The time it takes to hear the return gives you the "range" or distance to the object that reflected the beam. When the sun is low on the horizon, the radar receiver picks up the radiation as if it were a continuous return, and it's visible on all of the weather radars.

The image is of the radar from St. Louis last night at 8:28 PM CDT. You can see how the weather radar interprets a continuous return as if there were stronger reflectors further out, and how far north the sun is at this time of year near the summer solstice.

Sunday, June 26, 2005

Colin is here

Colin is spending the summer and next year in Santiago, Chile. I pulled up a recent TERRA image of Chile & surrounding areas:

If you have a computer with a lot of memory and a great Internet connection, try this one:

These images were taken on 06/24/05 at 14:55 UTC, about three days ago.

As usual, wikipedia has a link to great canonical satellite image of the city:

A really high-res image of part of the city is here:

Planetary conjunction

Mercury, Saturn, and Venus bunch up low in the evening sky.

Saturday, June 25, 2005

Don't always assume

For some reason I trust newspapers' web sites to provide me with news that is important to me--like important local news that may affect me personally. Like a big transformer fire, or such, that kills power to the local Internet POP and destabilizes routing. But, the Chicago Tribune didn't do that yesterday evening; instead I first saw the story on and linked there was a local story from a TV station.

Wednesday, June 15, 2005

Time to melt some bricks

Sam bought a surplus fresnel lens for me. It's June, mostly clear, and it's time to melt some bricks.

Tuesday, June 07, 2005

Plants in the near infrared

I always had thought that the bright white appearance of foliage in the near infrared was due to chlorophyll fluorescence--indeed, in a college class on plant structure and function, I recall examining a large beaker filled with chlorophyll dissolved in some alcohol, illuminated with a strong beam of light, that was deep red when viewed to the side.
You can see a spectrum of various vegetation here. Note how strongly plants begin to reflect at 700nm, which is near the far-red limit of vision. (You can see light beyond 800nm, if it's bright enough, but that's another post of OSHA violations and laser regs).

However, this remote sensing page shows that the high albedo of plants in the near infrared is actually just due to the cellular structure of the leaf. Plant structures are essentially transparent in the near-infrared, and the light is efficiently scattered in the air spaces beyond the first layer of cells. This has been compared to the mechanism that causes snow to be so white and reflective for a substance that is made up of transparent pieces. Newer leaves are not as thick and full of air gaps as mature leaves, and so should reflect less. The spongy mesophyll (what a great term, in my opinion), this interior area, allows for proper interchange of gases, and access to it is controlled by the stomata. Here is a (large) photomicrograph of a leaf cross-section:

I took most of these photos over a month ago, when Chicago was experiencing spring, so the trees had small leaves. In many cases, the wind is blowing the trees around and ruining the multi-second exposures!