How we read the water temperature from space
The bay temperature on the briefing isn't measured in the water at all. It's read from orbit — a satellite, hundreds of kilometres up, taking the temperature of the sea without ever touching it.
Everything warm glows
Everything warm gives off infrared — a kind of light our eyes can't see, but an instrument can. The warmer something is, the more it glows. Warm water glows a little brighter than cold. A satellite carrying an infrared camera reads that faint glow coming off the sea and turns it into a temperature, accurate to a fraction of a degree.
It only reads the very top skin of the water — the top fraction of a millimetre — which is close enough to the water you'd capsize into.
A temperature map of the whole sea
NOAA, the American ocean and weather agency, runs a product called CoralTemp. It was built to watch coral reefs for dangerous heat, but to do that it turns the satellite readings into a daily temperature map of the entire ocean — cut into squares about 5 km across. Port Phillip is just a handful of those squares.
We take the square over Port Melbourne
To get our number, we pick the square sitting on the water off the beach and read its temperature. Because that square is inside the bay, it reads bay water — which runs warmer in summer and cooler in winter than the Strait outside, as the shallow bay heats and cools faster. Pick a square out at the Heads and you'd be reading the ocean, and get the wrong season.
It's a long-running NOAA satellite product — we just read the square nearest the club.
What about clouds?
Infrared can't see through cloud, so on an overcast day a square can come back blank. The map fills those gaps by blending the last few days and several satellites, so there is always a number, redrawn once a day. That's why the reading is a day or two old and a 5 km average rather than this-minute, this-spot — which is enough to decide whether you want a wetsuit.
From orbit to the slipway
The sun warms the shallow bay. The water glows, faintly, in infrared. A satellite passing overhead reads the glow. NOAA turns it into a 5 km map of the sea. We read the one square over Port Melbourne. And it tells a sailor whether to pull on a wetsuit before launching.
A number that has travelled from space to the slipway — to keep a crew warm.
Where this comes from
The bay temperature is NOAA Coral Reef Watch's daily 5 km CoralTemp sea-surface temperature, read at a satellite grid point off Port Melbourne. For why the temperature matters, see the water-temperature page.