During a typical day, when is the relative humidity highest? Lowest?

Question

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Answer 1

To come up with a rule of thumb, let's assume we're talking summertime, when we're most concerned about the comfort impact of high relative humidity.  Let's also assume that the moisture in the air (actual humidity) remains the same throughout the day.  The percentage of total moisture the air can support (relative humidity) will vary with temperature (and pressure, but let's ignore that for the moment).  So, at the point of time when the temperature is the lowest the relative humidity will be highest.  And the reverse is true, also.

So the rule of thumb is that relative humidity will bottom out around 2-3:00pm, when the temperature high occurs on average.  It will peak sometime during the nighttime hours, usually early in the morning, when the temperature low occurs on average.  

That's why there is often dew after sunset and fog early in the morning.  These phenomena are both caused by relative humidity rising to near 100%, which causes the moisture in the air to condense (become visible as fog) and precipitate (dew).

Keep in mind, though that the "real world" is considerably more complex.  Local and seasonal weather patterns and microclimates can mean that humidity varies considerably during the day, and that the temp high and low may not occur at 3pm/3am.  Also, changes in atmospheric pressure caused by a number of factors can impact humidity (less dense air--air at lower pressure--can contain less moisture)  So the "rule of thumb" may be different depending on the season of the year and where you live.  

For example, if you live near a lake, sun-heated landmass causes rising air in the afternoon, which causes a "lake breeze."  This can serve to bring in moisture (and lower-temperature air) from over the lake, causing late-afternoon humidity or even fog.

And incoming fronts and weather systems, of course, can affect humidity drastically.  So if you live in south-central Florida, for example, again you may find that afternoons are more typically the most humid part of the day because of the frequent thunderstorms there.

Hope this helps!

Answer 2

simple observation and logic would indicate that relative humidity is highest in the early morning, after temperature is at its lowest and moisture in the air condenses (=dew).  lowest would be when temperature is highest and moisture is held in the air after evaporating all day.