What to do with the Dew Point?

The Western Star - - WEATHER - CINDY DAY Chief Me­te­o­rol­o­gist Cindy Day

It’s been quite a sum­mer! The heat wave that blan­keted At­lantic Canada was one for the record books; many towns and cities set records for con­sec­u­tive days with tem­per­a­tures above 25 de­grees. If you were to read this a few years down the road and didn’t re­mem­ber ex­pe­ri­enc­ing it, you might be tempted to say, “that sounds pretty nice.” But the air tem­per­a­ture was only a small part of the story.

I talked a lot about the hu­midex. The hu­midex is a mea­sure in­di­cat­ing the level of dis­com­fort the av­er­age per­son is thought to ex­pe­ri­ence as a re­sult of the com­bined ef­fects of the tem­per­a­ture and hu­mid­ity. The hu­midex should not be re­ferred to as a tem­per­a­ture since it is not mea­sured. It’s a cal­cu­lated value.

Back to the dew point. It is a tem­per­a­ture and an im­por­tant one. It’s the tem­per­a­ture at which the air be­comes sat­u­rated. In other words, the dew point is the point at which the air must be cooled in or­der for the air mass to be­come sat­u­rated – as­sum­ing there is no dras­tic change in air pres­sure. At that point, the air con­denses into wa­ter droplets, which we see as fog. It is re­ally a true mea­sure of how much wa­ter vapour is in the air... the higher the dew point, the more wa­ter there is in the air, and the lower the dew point, the drier the air mass is.

The dew point is a good tool to use all year long. In the win­ter, the lower the dew point, the drier the air mass is, and the more likely pre­cip­i­ta­tion will fall as snow, even if the tem­per­a­ture ini­tially is above freez­ing. That’s be­cause the air tem­per­a­ture will cool as the pre­cip­i­ta­tion starts to evap­o­rate.

It’s hard to imag­ine snow right now but just for kicks, I went back over the monthly sum­maries and found that for many of us, the first light dust­ing of snow came dur­ing the first week of Novem­ber last year – 12 weeks for now. ;)

Foggy morn­ing art. Lew Turner was taken by these del­i­cate beads of dew on the spi­der’s web at sun­rise.

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