Much Ado about Dew Point

If you've read any of my blogs this spring, you'll likely have noticed that I often refer to dew point, and rarely mention relative humidity.  Some of you may wonder why that is.  And does it relate to the physical formation of dew when you're hoping to bale your alfalfa at 4am?  

This blog will be a little long and detailed.  You'll probably want to sit down with a cold drink to read this one.

There are several ways to measure and express the moisture in the air.  The partial reason for this is that warm air can hold more moisture than cold air.  So some measurements of atmospheric moisture are relative (e.g. relative humidity, mixing ratio) while others are absolute (e.g. dew point, specific humidity, precipitable water).  We'll start with the absolute ones first.  These represent that actual amount of moisture in the atmosphere.  

Specific Humidity

I haven't mentioned this one yet, and it's not used all that much.  But it's important because it is the basic definition of atmospheric moisture.  Let's say you have a box of air that is completely dry.  Now you add 1 ounce of water into the box.  After it all evaporates, the Specific Humidity is 1 ounce.  It doesn't care how warm or cold the air is, or how much moisture it could hold.  It just tells you the weight of the water vapor in the box of air.  That's why it's called an absolute measurement.

Dew Point

This is also a measure of the actual amount of moisture in the air, but it's strangely expressed as a temperature.  And as you might guess, it's the temperature at which dew will form.  Lets say that box of air with 1 ounce of moisture calculates to a Dew Point of 50F.  Let's further say that the temperature of that box of air is 70F.  Now you set the box of air outside in the evening.  As that air cools, the Dew Point won't change even as the air temperature descends.  Remember, the Dew Point measures the amount of moisture in the box, which hasn't changed.  When the air temperature reaches 50F (i.e. the same as the Dew Point), dew and/or fog will form.  I won't go into the physical reasons why.  Just remember that Dew Point is the temperature at which dew will form.

On a weather map, Dew Point will look like the image below, with high values (e.g. 60s and 70s) in the tropics and eastern US, and low values (30s and 40s) in the western US and near the poles.

Now let's talk about relative moisture.  Relative Humidity (or RH) measures how much moisture a box of air holds relative to the amount that the box could hold at the given temperature.  So from our previous example:

So in the first case, the box of air only held half as much moisture as it could have (i.e. RH was about 50%).   After the air cooled to 50F, the RH was 100% since the Dew Point is also 50F.  You couldn't add any more water vapor to that box of air.  It is saturated.  But you also didn't change the amount of moisture in the box of air.

The problem with RH is that it depends on two things: temperature and moisture.  You can change both values and the results can be unpredictable. And we've all experienced this.  On a hot summer day (e.g. 90F) in the Inland Northwest, the Dew Point is usually around 40F, which calculates to an RH of only 17%.  Very dry.  But in the winter, we often have days where the air temperature is 34F and the dew point is 33F, which means the RH is 96%.  Very damp.  But the winter air actually has less moisture in it than the summer air, regardless of what the RH is telling you.  That's because the Dew Point in the summer (40F) is higher than in the winter (33F).  And that's why your skin is so dry in the winter but not in the summer.

OK, so what does all of this have to do with hay weather?  If you want a light dew for baling, then you want the air temperature to reach the dew point temperature, or at least close to it.  Then dew will form on your windrow (which will have a similar temperature to the air).  Sounds simple enough, right?

Here's a chart of a typical day of Temperature (blue), Dew Point (purple) and Relative Humidity (green).  Starting from the left, you can see that the temperature dropped rapidly in the evening, which caused the RH to climb rapidly.  But also notice that the Dew Point rose a bit in the evening due to a rain shower.  Then just after midnight, the temperature gradually fell to the Dew Point temperature, which means the RH was up to near 100%.  But notice something interesting from about 1am to 6am: the Dew Point falls from the upper 50s to the lower 50s.  Where did that moisture go?  It went into the making of dew.  The moisture was removed from the air.  Right after sunrise, the Dew Point went back up into the upper 50s.  Why?  That was the dew evaporating back into the atmosphere.  Also notice that the Dew Point falls during the afternoon.  This is due to mechanical mixing of the air, bringing dry air from aloft and mixing it with the moister air near the surface.

So why are some night heavy with dew, some light, and some with no dew at all?  This all depends on several factors.

• Dew points in the 50s (or 60s) will make a heavier dew than Dew Points in the 40s.
• The temperature has to be able to cool to the Dew Point.  If you have clouds or wind that keep the temperature up overnight, you won't get much if any dew.
• If the air temperature reaches the Dew Point (RH near 100%) for only an hour or so, the dew will be lighter than if you have several hours of RH near 100%.

A hay farming friend recalled a few days last August where he slept in his truck for a few nights, hoping for some dew that never came.  He was finally able to bale on the morning of the 5th.  Let's take a look at those days to see what happened.

On the 1st of August, the temperature climbed to near 100F.  The Dew Point started around 50F in the morning, dropped to around 40F in the afternoon, and climbed back into the upper 40s overnight.  But notice how on the night of the 1st, the temperature only dropped into the 60s and upper 50s.  No where near the Dew Point.  As a result, the RH was only around 60%.  The reason?  Probably the breezy winds (grey line) that kept the temperature from cooling.

On the 2nd and 3rd, the temperature was a bit cooler, but the dew point was in the lower 40s and 30s and a dry cold front had moved through.  Again, the wind prevented the nighttime temperatures from coming anywhere close to the Dew Point (blue line doesn't touch the purple line).  In fact, it's not until the morning of the 5th that the blue and purple lines come close together, and the RH climbs above 75%.  A similar thing happens on the morning of the 6th, with several hours of RH > 75%.  But note how the wind on the 5th and 6th was very light.

So the takeaway from this case study.  Although the dew point was highest on the 1st, the atmosphere stayed too warm overnight (due to wind) for the temperature to come close to the dew point.  This pattern repeated itself for the next few nights.  Finally after the 4th, the wind decreased and the temperatures were much cooler overnight and dew could form.

If you have any questions about this topic, or would like a forecast of what day has the best odds of some dew, feel free to write me at inlandnorthwesthayweather@gmail.com