I just saw the local weather report. They said that the current temperature is 45°F, and that the dew point is 27°.
If I understand that correctly, it means that the current humidity is enough that if the temperature would drop to 27°, dew would begin to form.
In other words, the dew point is a description of how humid the air is. There’s not enough humidity to form dew at 28°, but more than enough to form dew if it would cool down to 26°.
If that is a correct understanding of “dew point”, then my question is: Why bother? Why not just tell us the relative humidity? If it’s not cold enough to form dew right now, do I really care how cold it needs to get for that to happen? And although it might get colder in the next few hours, it is also possible that the humidity could change, making the 27° figure totally useless.
It appears that my understanding of “dew point” is inaccurate. Can someone tell me the Straight Dope?
Well as the temperature drops, the humidity does change. It gets more and more humid as the temperature and dew point get closer together. So humidity is not as useful to know as the dew point.
Also, you can easily work out the likely cloud base if you know the temperature and dew point. Cloud base in thousands of feet = (temp °C - dew point)/2.4. If you just know the temperature and humidity then its not so easy to work out.
Ultimately it is more important to know what outside temperature cloud will form at rather than some vague indication of how muggy it is.
I thought that’s why they developed the idea of “relative humidity”, as opposed to plain “humidity”.
For what purpose? If it is going to be very muggy, I need to wear lighter and looser clothing. But clouds (I presume you mean fog?) don’t occur that often around here. And when it is expected, the weather report will explicitly warn us, “Fog is expected tomorrow morning.”
Relative humidity is useless to me. Dew point provides a lot of data in one number. 65 and above is a muggy day. Above 32 means little chance of frost. Etc.
Mrs. FtG needs to know it this time of year for deciding whether she can make a batch of egg white cookies.
Dew point is the temperature at which water vapor condenses (given constant pressure, etc., etc.). It does not have anything to do with “dew” as the term is used colloqially.
I’m not sure if you are understanding relative humidity correctly. Relative humidity is the amount of water vapour in the air as a percentage of the total amount the local airmass can hold. As the temperature falls towards the dew point, the relative humidity increases towards 100% when the temperature equals the dew point. It is actually the dew point that remains much the same while the R/H changes.
The dew point is a far more useful number than relative humidity. Relative humidity only tells you if it is muggy or not. The dew point and temperature together tell you how muggy it is (if they are close together, it’s humid, if they are separated, it’s not). As well as that, the dew point will tell you what altitude the cloud base will be, this is useful information for pilots and anyone else with a professional interest in the weather. It tells you at what temperature surface cloud (or fog as you point out) will form. Given the dew point and some local knowledge, you can make a good prediction on whether there will be fog. Once again it is good information for those with an interest in the weather.
I think you are probably forgetting that there are plenty of people who’s lives revolve around the weather, who have some knowledge of it, and who like to have more than just dumbed down information presented on the TV.
Because weather can be difficult to predict, some people like to be given more of the basic information so they can then make their own assessment.
Relative humidity, when compared to a dew point, is relatively useless.
This info also helps to decide if you are going to bale hay, which lane you want at the drag strip and how you set the blower and fuel mixture. Also makes a lot of difference in auto racing.
Take off distance for aircraft is affected by the actual humidity also.
Like 1920 said, lots of folks need and want to know the actual numbers.