Converting September’s Temperature Anomaly to Celsius

A temperature can be 69.4°F, while its difference from a historical average is 4.6°F. Both use Fahrenheit degrees, but converting them to Celsius requires different arithmetic.

NOAA's September 2026 US climate summary, published October 8, reports those two numbers for the contiguous United States. The first is the monthly mean temperature; the second is its departure from the 20th-century average. The region excludes Alaska and Hawaii.

The difference of 4.6°F is about 2.6°C. Here is how to check that result without accidentally converting it as a thermometer reading.

Convert the difference

A Celsius degree spans the same temperature interval as 1.8 Fahrenheit degrees. For a difference, scale by five ninths:

Difference in °C = difference in °F × 5 ÷ 9.

Open the Standard Calculator and enter 4.6 × 5 ÷ 9. It gives approximately 2.5556, so the reported departure is about +2.6°C. Keep the plus sign in mind: the month was above that reference average.

There is no subtraction of 32 in this operation. We are measuring the length of a temperature interval, not locating a temperature on the scale.

Convert the actual temperature separately

For an absolute reading, the Fahrenheit and Celsius zero points differ. The familiar conversion is:

Temperature in °C = (temperature in °F − 32) × 5 ÷ 9.

Use the Unit Converter, choose Fahrenheit to Celsius, and enter 69.4. The result is approximately 20.7778°C, or about 20.8°C.

The tool's Fahrenheit-to-Celsius mode is for absolute temperatures. Do not enter 4.6 in that mode to convert the anomaly: it would calculate about −15.22°C, which is the Celsius equivalent of an actual reading of 4.6°F. That is a different question.

Check it using two endpoints

Subtract the published departure from the published mean:

69.4 − 4.6 = 64.8°F.

That reconstructs a reference temperature from rounded values. Convert 64.8°F using the Unit Converter: approximately 18.2222°C. Now compare the converted temperatures:

20.7778 − 18.2222 ≈ 2.5556°C.

The subtraction of 32 occurs in both endpoint conversions, so it cancels when you take their difference. Only the scale factor remains. Work with full precision internally, then round the final difference; these extra decimals explain the calculation, not additional precision in NOAA's report.

Why a percentage is unhelpful here

Dividing 4.6 by 64.8 would produce a percentage based on Fahrenheit's zero point. Doing the same comparison in Celsius gives a different percentage. Such a percentage is not a meaningful way to describe this climate anomaly.

Use a temperature difference with its scale and baseline. Similarly, a national monthly average is not the temperature at every home, the highest temperature that month, or a forecast for tomorrow.

Try your own interval

For an invented 9°F increase, enter 9 × 5 ÷ 9: the difference is 5°C. For a 9°F decrease, the difference is −5°C. If you instead have two actual readings, convert each in the Unit Converter and subtract, or find their Fahrenheit difference and scale it in the Standard Calculator.

Source and image note

Source: NOAA NCEI, Monitoring the U.S. Temperature and Precipitation in September 2026, October 8, 2026, checked October 20, 2026. The hero image is an original conceptual illustration, not a NOAA instrument, chart or photograph of the reported month.