Relationship between wet bulb T and dew point?

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  • same or greater or lesser or none?


  • Good question! In the old days, sailors would use the wet and dry bulb thermometers and note the average of their difference on a piece of paper. They would then take a bucket of sea water and note the temperature on the same diagram.

    If they were sailing say, toward land, the projection of the two lines would tell them when they can expect to meet fog.

    Now, when both bulbs are at the same temperature, the relative humidity is then 100 percent and at dew point. We all agree on that.

    But is the wet bulb temperature the same as the dew point temperature? The answer is: not exactly. You need to use a psychometric chart to find it out.

    A similar problem is found for aviation: we get our weather forecast, called TAF, with ambient air temperature and dew point temperature. From that, we can read the difference called the spread and estimate the ceiling of the clouds but doing: 2 degrees Celsius (3 F) per thousand feet. So, if the dew point temperature is 4 C lower than ambient air, then the ceiling can be expected at about 2,000 ft. But there is a slight mistake because the dew point is also proportional to the air pressure and at 2,000 ft, it is lower than at ground level.
    However, for all practical purpose, knowing that the ceiling will be around 2,000 ft is enough for us. I would know then that I can forget to fly from coast to coast, here in Norway, where I should climb over 6,500 ft!







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