Dry conditions mean less humidity
One of the most enjoyable aspects of living in the Inland Northwest is the climate. For snow lovers, there is ample white stuff (usually) for the various types of winter recreation. In the summer, we see drier conditions, which leads to plenty of days with blue skies and warm temperatures.
One of the factors that makes the warm season so comfortable, is how dry it is. I am speaking from the perspective of a Midwestern transplant, having spent most of my younger years in Oklahoma and Texas. While I enjoyed the fact that spring and summer started quite a bit earlier in those areas – with blooming flowers as early as February – there was a price to pay come July and August. Not only would it get hot, but it would be hot and muggy. The air was so laden with moisture that your body’s main mechanism for cooling itself – sweating – did not work.
Sweating works by cooling your skin through evaporation. Evaporation has a cooling effect because it takes heat energy to evaporate the water from your skin, in other words heat turns the liquid water into water vapor. This only works, however, when the air is relatively dry and has the capacity to hold more water vapor. When conditions are not only hot, but the air has a high moisture content, there is less evaporation. The sweating mechanism is less effective at cooling the body, and you feel hotter and more uncomfortable. You have probably heard the saying “it’s not the heat, it’s the humidity.”
Humidity is a much misunderstood term. Every day, on most weathercasts, you will see a value for “relative humidity” given as a percentage. Most people would assume that a relative humidity of 100 percent would be “muggy,” while a 20 percent value would be dry. Though the relative humidity figure does convey important information about fire danger during the summer, let me explain why it is not a good indicator of comfort or “mugginess.”
The word “relative” in “relative humidity” means that the value is dependent on another factor, in this case temperature. A relative humidity of 90 percent at a temperature of 85 degrees is very different from a relative humidity of 90 percent at 40 degrees. Relative humidity is not an absolute measure. Let me use the analogy of a thimble and a large pitcher. If you filled both containers to 90 percent full, it is obvious that the pitcher will contain much more liquid than the thimble. Similar to this is the fact that warm air has the capacity to “hold” more water vapor than cooler air. Relative humidity tells you nothing of how much moisture is in the air, only how close the air is to being saturated, or “full” of water vapor.
A much better indicator of moisture in the atmosphere is the dew point. The dew point is simply the temperature at which water vapor would condense into liquid, or “dew.” Dew point is not a relative term, and so a 30-degree dew point means the same thing whether the air temperature is 40 degrees or 90 degrees. The dew point can never be higher than the air temperature. As a general rule, most people start to get uncomfortable when the weather is warm and dew points are in the mid 50s or higher. It is not uncommon east of the Rockies to have dew points in the 70s and even in the lower 80s.
In contrast, average dew points across this area in July are only in the mid-40s. A dip in Lake Coeur d’Alene on a scorching day will not only cool you off while you are in, but the rapid evaporation may even give you a refreshing “chill” when you step out.