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Boston Weather: 37.0°F | A Few Clouds

Weather

What is Truth?

By Timothy R. Whitcomb
STAFF METEOROLOGIST

As a scientist, I used to like to harass philosophers who, instead of looking for answers, would instead decide that something could not be known. I dismissed what seemed to be an act of academically throwing up my hands. I have come to learn, though education and experience, that I needed to give the philosophers more credit, at least regarding the weather.

Truth comes in many guises claimed by religious and secular alike. On a more practical note, though, what is the temperature outside? Your thermometer says that it is 46 F. Does that mean the true temperature is 46? I doubt it — that thermometer you have in your hand was calibrated during the Nixon administration and the measurements oscillate wildly over even the smallest time period. You can’t say for certain that it is 46 degrees — you can, however, guess based on what you know about the error characteristics of your instrument that the temperature is much more likely to be 46 degrees than, say, 15 or 20. The “correct” measurement is then not a single number, but a probability distribution function (PDF).

If we can’t know what the current temperature is, how can we possibly forecast what the temperature will be tomorrow? News sources continue to give you a high temperature without any indication of the uncertainty inherent in the estimate. Although it is possible to advance PDF’s forward in time, it is prohibitive in terms of the required computing power. Modern forecasting techniques use “ensemble forecasts.” Instead of forecasting the entire probability distribution, we instead sample the PDF and integrate those samples forward to approximate the PDF at some future time.

Providing probabilistic information is a much more intellectually honest forecast. Telling you that it is going to be 80 and sunny tomorrow while neglecting to tell you that there’s also a sizeable probability of 60 and rainy just seems wrong. Admittedly, this is a bit of hyperbole, but still, there is a big difference between a confidence of 2 or 3 degrees or 10 to 15 degrees.

Now, watch as I go against my convictions.

Extended Forecast

Tuesday: Rain likely. High 63 F. Tuesday night: Rain likely early. Low 35 F.

Wednesday: Highs near 58 F. Wednesday night: Low 40 F. Gusty winds.

Thursday: Some afternoon rain. High 62 F.

Thursday night: Low 37 F.