I took some Canal water temperatures on a rainy Friday (13 January 2012, 9 am) and got these temperatures (in Celsius):

Rainwater puddle at Union and Bond Street: 5 degrees street storm flow at 2nd street and Canal: 5 degrees Middle of Canal at Union Street Bridge: 7 degrees (south of main RH35 CSO overflow) Carroll Street Bridge, at OH-005 CSO outlet 6.75 degrees Gowanus Canal water overflowing onto 2nd St 6.5 to 6.75 degrees

diluted sewage water from RH-35 Overflow: 7.25 degrees (at the end of Bond Street at Canal)

Canal water 30 feet north of RH 35 overflow: 6.5 degrees (upstream from outgoing tide)

16 January 2012

I took some further temperature calibrations on a sunny day, at noon, with air cold enough to freeze puddles. (- 4 degrees celsius)

The water in the Canal at the 2nd Street dock was +/- 2 degrees celsius.

At the Denton's Pond outflow, where fresh water is running in out of pipe (could not be actually observed, as pipe was under high tide water) the water temperature was between 2 to 2.25 degrees celsius.

Difficult to determine exactly, as I only had a glass thermometer - a digital one would give better fractional readings.

At Union Street Bridge, water was 1.5 degrees Celsius, and at Butler end of Canal, surface water was showing a mushy freezing (0 degrees Celsius)

Canal water was generally 5 to 6 degrees warmer than outside air.

For reference, cold water from my tap on 13 January was 10 degrees

This means that for color painting to be effective we need to compress the color range as follows:

pale green: starts at 1 degrees ... and ramps up

dark green: 2 degrees

blue: 4 degrees

red: 6 degrees (anything above)

orange : 8 degrees

yellow: anything above 8 degrees

But note that temperatures on Arduino script would have to be adjusted on a DAILY basis, based on temperature sample for that day - it seems like water temperature differences hover between +/- 1 to 2 degrees in the water itself, based on water depth and inflows, and to catch these fluctuations with different colors you have to set the center point of the ramp at +/- 5 degrees above that day's air temperature, otherwise the firm flashlight will stay on just one color.