Why is humidity important for meteorologists?

Water vapour is always mixed with air throughout the atmosphere. When this water becomes condensed, it forms many types of weather phenomena; clouds, rain, hail, snow, fog, dew... and the list goes on!

I see. But how is humidity measured?

It's quite difficult to measure humidity without sophisticated apparatus. The traditional way meant that a comparison had to be drawn between 'wet' and 'dry' thermometers. Most of the stations now, including us at BSMO, have a box which calculates the humidity, making it much easier! Humidity is measured in grams per cubic metre.

Now I'm confused! I thought that humidity was expressed as a fraction or percentage on TV broadcasts.

It can be. The percentage is the fraction of the total possible amount of water that potentially could be in the volume. For instance if the maximum level was 10 grammes per cubic metre, and it was measured to be 5 grammes per cubic metres, then there would be a relative humidity of 50%.

Does the maximum level of humidity vary depending on temperature?

Yes it does. This maximum level is called the saturation level. For instance, at zero degrees Celsius, the saturation level is at 5 grammes per cubic metre, but at room temperature, saturation is at about 9 grammes per cubic metre. This means that any air will be become saturated if it is cooled enough. Beyond this, excess water is condensed. This is the reason why the warm air breathed by people in a cold car leaves misty windows!