Humidity In The Desert

Texas sage

9/24/11

     As I mentioned in my last post, so many plants that had been dormant for weeks in the summer heat have started blooming again now that we are having "cooler" weather.  My Texas sage bushes have all burst into bloom this week.  And the Red Bird of Paradise bushes (also known as Pride of Barbados) suddenly have many more and brighter blooms instead of an anemic few.

     But another way I can tell fall is on its way here in the desert southwest is that my skin is becoming very dry and starting to itch because dew points and humidity are rapidly falling.  Until I moved here, I never knew that there was any humidity in the desert.  But when the monsoon season starts here in mid June, dew points begin to rise into the 50's.  Humidity from the moist southerly winds routinely will be in the 30 to 60 percent range.Both are a welcome relief from the parching dryness.

Mexican Bird of Paradise

     In the fall, winter, and spring here humidity runs from 2 to 12 percent, and dew points plunge.  I never knew that minus dew point temperatures  existed.  However, in the winter here, with single digit humidity, minus dew points occur.  I have seen a dew point temperature as low as -34  last winter.  The concept of moisture in the atmosphere, how it is measured, and how it changes is fairly complex.  As I understand it, dew point is the Fahrenheit measure of humidity (the absolute measure of water vapor in the air).  So if the Fahrenheit (or absolute) measure of humidity is in the minus column (below zero), you can imagine how dry that feels.  It can feel cold even if the temperature is 60 degrees F (16 C).


     Just this week dew points have fallen from the 50's to the mid 30's.  And we've had a few days where the humidity has fallen into the single digits.  So bring out the heavy duty moisture creams, and if this fails to take away the itch, grab the benadryl cream.

Desert Plant Adaptations

8/6/11
     After moving to the desert southwest and finding it difficult to deal with the blazing hot sun and triple-digit heat, I wondered how the native plants and trees survived here.  After researching, I found that desert plants and trees have devised unique adaptations to survive the intense heat and sun.

tiny leaves - mesquite tree

I noticed that most native trees, bushes and plants have very small, even tiny, leaves.  As I suspected, tiny leaves have limited surface area reducing water requirements.  I also discovered that some plants reduce moisture loss by foregoing traditional leaves.  Cacti spines are actually modified leaf parts with sharp points.  Cacti and other desert plants also reduce water loss through their stomata (pores) by opening them at night when temperatures are cooler.  Another adaptation to reduce moisture loss is a waxy coating found on such plants as creosote and cacti.  When there is an extreme drought, some trees like the palo verde drop their leaves so there is less plant material that needs water.  When rainfall occurs, leaf growth returns.  I have noticed when we have a rainfall, some bushes, like Texas sage, burst into bloom a few days later.

tiny leaves - palo verde tree

To take advantage of any rain that falls, plants such as agave, hesperaloe and yucca use their funnel-like shapes to collect rainwater and direct it to their main stem and roots.  The spines of cacti act as drip tips for the same reason.  I also learned that cacti develop shallow, but wide-ranging root systems to have a better chance of absorbing rainfall.  An example is saguaro roots, which can extend up to 100 feet in all directions.  Also, cacti and other succulents can store water in their roots, stems and fleshy leaves to use during dry spells.

funnel-shaped desert spoon plant

golden barrel cacti spines as drip tips

     One thing particularly noticeable here in the desert southwest is that many trees and plants are not truly green.  Many, like Texas sage, have gray, silver or pale green foliage.  This is a modification that helps reflect sunshine and reduces heat build up on the leaf's surface.

Texas sage gray-green leaves

bush morning glory silver-green leaves

     Other plants, like brittlebush, have fuzzy hairs on their leaves that act as sun reflectors to keep the plant cool.  Some plants, like the jojoba bush, angle their leaves vertically so that surface area exposed to the sun is minimized.

vertical leaves on jojoba bush

vertical leaves on hopseed bush

     Adaptations to reduce moisture loss, harvest rainwater and reduce sun exposure help desert plants thrive.  Since non-native plants do poorly in the intense heat and sun, most homeowners here choose low-maintenance, water-thrifty native adapted plants for the bulk of their landscaping.