Units of measurement

1. Why do we use Celcius rather than Fahrenheit?

The Celsius scale is the World Meteorological Organization standard for temperature measurement and is used throughout the world by the meteorological community for global exchange of information.

2. How do you convert Celsius to Fahrenheit?

  • From Celsius to Fahrenheit - °F = °C × 9/5 + 32
  • From Fahrenheit to Celsius - °C = (°F - 32) × 5/9

3. How do you convert from millibars to inches? 

Multiply the millibar value by 0.02953 to get the value in inches.

4. How do you convert from miles per hour (mph) to knots (kt)?

  • To convert from miles per hour to knots multiply the mph value by 0.868
  • To convert from knots to miles per hour multiply the knot value by 1.15

5. What are hectopascals?

The SI unit for pressure is a pascal. The worldwide meteorological community uses the hectopascal, i.e. a hundred pascals, which is the metric equivalent of a millibar. However, millibars (and inches) are still used in some public forecasts in the UK and USA.

6. What is UTC?

UTC is the abbreviation for Coordinated Universal Time and it is equivalent to GMT. The Royal Observatory website has more information on the history of timekeeping. It also has a description of local time.

7. How do I get a setting for my barometer?

Atmospheric pressure varies over time and space and also varies with height. Since the altitude of the barometer normally stays constant (the station height) a correction is made to the reading to make it equivalent to the mean sea-level reading. This is done so that readings from different locations can be compared, with differences due to height being removed. Aneroid barometers are normally adjusted to mean sea-level values - read the barometer's instructions to see how to adjust the instrument (normally by a screw on the back). Mercury barometers cannot be adjusted (don't tamper with them because a mercury spillage is a health hazard).
To get the value for your barometer, choose a high-pressure day, pressure values are not changing very much - you can watch the TV forecasts for such a day. Go to the Met Office's observation page and choose the station nearest to your location - on a quiet weather day the distance away from you will not be significant. Adjust the barometer to the station's pressure value. You can check your barometer on other days but will have to compensate for fast-changing pressures or distance if the pressure is low or changing fast. The observation includes information about how the pressure is changing.

Last updated: 24 April 2014