No...The Air Isn't Actually "Heavier" During Humid Summer Days, But Why Does It Feel That Way?

         We've all heard our friends, family, and even meteorologists discuss how "heavy" the air feels during humid summertime afternoons in RVA. How many of us have also jokingly said that we could "wear the air" on those particular days? Well I hate to burst your bubble....but you've been lied to...  warm, humid air is actually less dense or "lighter" than drier air. How can that be?! Well...

        This is due simply to the presence of water vapor, which has a smaller molecular mass by particle than the same particle of dry air. This can be explained by a simplified example: Let's assume for a moment that dry air is only composed of pure oxygen molecules (O2). O2 has a molecular weight of ~32 amu (16x2). Water vapor, H2O, is composed of 2 Hydrogen molecules and 1 Oxygen molecule for a total amu of ~18 ((2x1)+16=18 amu). But what about Nitrogen (N2), which makes up around 78% of the atmosphere? Nitrogen gas is lighter than Oxygen at ~28 amu (14x2)...but even N2 has a larger atomic mass than a molecule of water vapor. 

        As water vapor molecules make up a higher percentage of a unit of air, the total mass of this air begins to lower. Conversely, with such little water vapor in drier air, this unit of drier air contains little "lighter" molecules of water vapor, increasing its mass. So why does the air feel "heavier" during humid days when its actually lighter by mass than the same amount of dry air? 

        Our bodies cool ourselves via evaporative cooling. When sweat collects on our skin and evaporates, this leaves our skin feeling cooler. However, when the air contains more water vapor, especially when the air is nearing the limits of being able to "hold" water (saturation), less evaporation takes place. This leaves the sweat collecting on our skin instead of evaporating to cool our bodies, leading to this sticky feeling and a feeling of the air being thick. This is also the reason why heat + humidity is a dangerous combination.  

Comments

Post a Comment

Popular posts from this blog

Navigating Hurricane Erin...A Guide to the Steering Currents Aiding Her Maiden Voyage Across the Atlantic

Image
Atlantic Hurricane Season Has Officially Awoken With Monster Category 5 Erin...  Satellite Imagery From NHC showing Erin's "symmetry" and clear "pinhole" eye, key features in the most powerful of hurricanes.             Erin continues her rath of terror across the fish colonies in the Atlantic as Erin rapidly strengthened overnight into a Monster Category 5 hurricane. As of this post, The National Hurricane Center (NHC) says Erin is currently a 160 MPH, 915 mb Category 5 puffy Cinnabon of terror, making it the first Category 5 hurricane this early in the Atlantic hurricane season. While the current thinking is for Erin to be "fish food" and remain far enough out to sea, a complex series of atmospheric conditions will drive its future track, which still remains unknown. This "unknown" means we will need to track Erin closely over the next several days as sudden changes in this setup and its future strength will all play a part in its future ...
Read more

Midwest Mischief as Shortwave Trough and Strong Low Pressure Team Up To Deliver Multiple Days of Trouble

Image
 Oh She Trouble...    Ahhh.. Yes, the good old fashioned severe weather season has returned. After a rather quiet April, it seems Mother Nature needs to remind folks who's boss as she takes aim at the Midwest and MidAtlantic today, tomorrow, and on Saturday. In particular, its looking like tomorrow may bring her worst as parts of Kentucky, Tennessee, Illinois, and Indiana and others prepare for damaging winds, large hail, and tornadoes. The Storm Prediction Center (SPC) has outlined these areas in a Moderate Risk (Level 4/5) for the possibility of widespread, long lived severe weather with portions of the MidAtlantic (including Virginia) being outlined in a Slight Risk (Level 2/5).  SPC Day 2 Categorical Risk. Severe Weather is most likely over the Moderate Risk area, which encompasses Missouri, Illinois, and Kentucky  The Setup:  A potent shortwave trough and occluding, strong low pressure will slowly eject over the Plains and upper midwest over the next 4...
Read more

Beware the Inverted-V and Here's Why...

Image
           With the warmer temperatures and severe weather season well underway, you may have heard me or other meteorologists mention the term "Inverted-V."  Unless you happen to be a weather geek like me or decided to google it after reading this, you've probably thought to yourself "is he talking about a mega wedgie"?            No I swear, the Inverted-V isn't another name for a wedgie...it is another way meteorologists describe an atmosphere that features relatively drier air near the surface. More specifically, the name originates from weather soundings that feature larger temperature-dewpoint (dewpoint depressions) spreads near the surface compared to the atmosphere, which resembles an inverted-v shape.  An Inverted-V Weather Sounding (Depiction of Atmospheric Conditions) Courtesy Of  wasatchweatherweenies . Red line is temperature and green line is dewpoint. As seen in the sounding, the red and gree...
Read more