Interesting Weather Information

Thursday, July 18, 2013

It's Not The Heat ... It's the Humidity: Some Things You May Not Know About Atmospheric Moisture

It's not the heat, it's the humidity, a bit of folk wisdom I have heard countless times from as far back as I can remember.  And of course, within limits it's true.

The human body sheds heat mainly by the evaporation of sweat. While you may perspire, I sweat and though it may not be a pleasant sight a sweat drenched athlete's body is doing what it evolved to do when rising internal temperature triggers the sweat glands.

Each drop of sweat (a salty solution) that evaporates takes with it excess heat. The drier the air the faster the evaporation and the greater the heat loss.  So a heat index of 100° when the temperature is 93° means your body loses heat as if the temperature is 100° not 93°. High humidity hinders heat loss because it slows evaporation. As a result the "feels like" temperature, in this case heat index, communicates how much the heat loss is hindered.

Four Ways Humidity May Affect Your Daily Life?

1. High humidity can accelerate mold growth and some molds release spores when the humidity increases. Most allergenic molds release spores when humidity is low. Do you know why? A dry spore can travel farther than a spore moistened by a humid atmosphere that weighs more. However if you are sensitive to mold spores released in humid weather as humidity increases you may get the sniffles.

Mold spores. Image courtesy

2. Your hair lengthens as humidity increases. In fact for years most instruments that measure humidity used human hair. A bundle of human hair is linked to a mechanical arm and levers scale up and down motion so it can draw a graph on a rotating drum in the hygrometer.

But there is more to this than than you probably may have imagined.

According to   The Atlas of Human Hair Microscopic Characteristics, straight hair generally has a round cross section while curly hair is flatter in cross section. The flatter the cross section the curlier the hair.

So when humidity increases round, straight hair strands lengthen and the effect of the humidity may make your hair go limp and lose body. If you have curly locks, that is strands with a flat cross section, humidity may give you the frizzies as your hair curls even more.

3. Did you ever reach for a potato chip that has been outside for a while on a hot, humid, summer picnic afternoon?  You anticipated that satisfying "crunch" but your chips had lost their chomp and instead you reeled back startled by the salty, spongy silence.

Blame the salt! It is what scientists call hygroscopic or water attracting. The sodium and chlorine of table salt carry positive and negative electrical charges while one end of a water molecule is slightly positive and the other slightly negative. Opposites attract and that satisfying salty sprinkle captures humidity, free wheeling molecules drifting through the space surrounding the chip. The water molecules are absorbed by the starch of the potato chip. The result is a mealy mouthful.

4. Humidity, also known as Earth's most abundant greenhouse gas is not only important as an element of global warming but locally it drastically effects night time temperatures.

The high deserts of California may drop to near freezing overnight even in summer and soar to 100° during the day.  It cools so much at night because of a lack of humidity.  The same factor that makes the dry heat of the day tolerable brings a shiver at sunrise. Earth heat escapes easily to space through an atmosphere that lacks water vapor because of a weak greenhouse effect.

Thursday, July 11, 2013

Microburst near Hilliard, OH 10 July 2013

A National Weather Service damage survey completed July 11, 2013 concluded a microburst caused straight-line winds estimated at 95 mph based on their damage survey.Six individuals were injured during this event.

A microburst occurs when a thunderstorm collapses. The collapse can be caused by a variety of factors that cause the warm, moist inflow that drives the storm to be cut off or weaken dramatically. With the updraft weakened or stopped there is no upward directed force to hold the cloud mass aloft.

Updrafts can also be weakened or made to collapse by evaporative cooling of rain and melting of hail falling from the storm. The cooled air is more dense than before cooling and the updraft energy becomes insufficient to maintain the storm.

The entire cloud mass crashes to the ground over several minutes. As the water drops and everything else within the cloud accelerate towards the ground under the influence of gravity a rapid burst of air is pushed ahead of all the cloud material.

When the air hits the ground it fans out and surface winds can exceed 100mph. Damage can be extensive at ground level and even worse is that the change of wind direction and speed can be a great hazard to aviation.

Artist's depiction of a microburst over an airport.  As the air crashed to the ground it fans out and wind speeds can exceed 100 mph. Aircraft that approach the microburst experience a sudden and violent change of vertical wind. On approach or takeoff the aircraft is close to the ground an may not have time to adjust flaps and air speed to avoid a crash. Courtesy: NASA.

A 3D (volume render) view of the thunderstorm that caused the Hilliard micoburst rendered in GRLevel2 Analyst. Notice how the volume of heavy rain (yellow and red) collapses, and, in approximately 18 minutes dissappears.  

Text of the NWS ILN Local Storm Report on the Hilliard, OH microburst:

625 PM EDT THU JUL 11 2013

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....

0400 PM     TSTM WND GST     4 SSE HILLIARD          39.99N 83.12W
07/10/2013  E95.00 MPH       FRANKLIN           OH   NWS STORM SURVEY