The Five Seasons of the Sonoran Desert

Seasons are a way of dividing up the year based on changes in weather and daylight hours. Winter, spring, summer, and fall – the traditional four – are familiar to most people. However, while traveling in southern Arizona recently, I had the opportunity to learn about the unusual fifth season of the Sonoran Desert.

The Sonoran desert, covering a large part of the southwestern US and Northern Mexico, basically divides its summer into two parts. “Fore-summer”, occurring in May and June, is very hot and very dry. “Summer monsoon season” follows it from July to mid-September and brings the region soaking rains.  It is considered the major growing season.

Surprisingly lush by desert standards, the Sonoran Desert is one of the wettest deserts in North America.  This is due to the fact that winter there is considered a second rainy season. While the precipitation that falls between December and January is generally not as intense as during the monsoon months, it tends to be more widespread. Overall, the region averages between 3 and 12 inches of rain a year. Spring and Fall are generally warm and dry.

Climate Change at Rocky Mountain National Park

Rocky Mountain National Park (RMNP) protects 415 square miles of spectacular mountain environments in northern Colorado. It is home to a diversity of ecosystems – alpine, subalpine, and montane – that are each uniquely adapted to the climate zone of their elevation. This is why, as I learned during a recent visit,  climate change is a serious issue for the park.

According to the National Park Service, the average annual temperature in RMNP has increased 3.4°F over the past century. A report from a weather station inside the park (Grand Lake), shows that the number of frost-free days has increased from an average of 65 in the mid-20th century to an average of 100 in this past decade.


In the 20th century, the area including Rocky Mountain National Park experienced a warming trend. The five-year rolling average (thick red line) allows the viewer to look beyond annual variability to focus on long-term trends. (Analysis of PRISM data, original source Daly 2008). Credit: NPS

This warming trend, says the NPS, has caused a number of environmental changes in RMNP. The winter snowpack is melting approximately 2 to 3 weeks earlier, resulting in less water being available for people, plants, and animals during the summer. There has been an explosive increase in the number of mountain pine beetles surviving the now warmer winter months, allowing them to devour more trees. The phenology, or the timing of natural events, can also be thrown out of sync when warm spring weather arrives earlier than normal. Wildflowers that bloom before the arrival of butterflies, for example, can leave the insects with a reduced food source. This puts a kink in how the larger food chain fits together.

In the park’s alpine tundra region, the American Pika is at particular risk. According to scientists, this small furry relative of the rabbit can only live at high elevations in cool, rocky environments. They say it cannot survive in temperatures above 75°F for more than a few hours.  While other species adapted to lower elevations can move upslope as average temperatures rise, the pika has nowhere to go.

American Pika on the rocky terrain of RMNP's alpine tundra region.  Image Credit: The Weather Gamut.

The American Pika, a native of RMNP, is sensitive to even small changes in climate.  Image Credit: The Weather Gamut.

Another impact of climate change is the spreading of non-native plant species that can thrive in the now warmer environment of RMNP. While Cheatgrass, a native of Eurasia, is found throughout the western US, it used to be limited to lower elevations.  Now, it is found as high as 9,500 feet in parts of RMNP. In addition to crowding out native plants and changing the look of the landscape, this invasive species is highly flammable. Its presence increases the danger of wildfires – something the West certainly does not need.

While these are just a few examples of the observed and expected impacts climate change is and will have on RMNP, scientists are continuing to research how additional increases in temperature will affect this national treasure.

Why Air Temperature Decreases with Height

While visiting Colorado recently, I had the opportunity to explore Rocky Mountain National Park, and it was largely a vertical experience. Within its borders are 72 named peaks that reach above 12,000 feet in elevation. Traveling from the Beaver Meadow Visitor Center – elevation 7,840 feet – to the Alpine Visitor Center – elevation 11,796 feet – the drop in temperature was anything but subtle.

The reason for air being cooler at higher elevations is twofold. First, the sun’s rays heat the Earth’s surface, which in turn, radiates that warmth into the atmosphere. As you climb in altitude, there is less surface area of land available to heat the air. Second, as air rises, it expands and cools. This is because air density and pressure aloft are lower than at the surface.

The exact rate at which the temperature decreases with height – the environmental lapse rate – varies with location and daily conditions. On average, however, for every 1000 feet gained in elevation, the temperature drops by about 3.6°F.

Image Credit:British Geographer

Image Credit: The British Geographer

Opposing Winds Help Shape Great Sand Dunes National Park

When you think of the Rocky Mountains, sand dunes are probably not the first thing that come to mind. While driving across southern Colorado earlier this month, however, giant white sand dunes glimmered in the distance. No, it was not a mirage; it was Great Sand Dunes National Park and Preserve.

Sprawling across the arid San Luis Valley between the San Jose Mountains to the west and the Sangre de Cristos to the east, the dunes cover 30 square miles and rise as high as 750 feet. They are the tallest dunes in North America and were formed, and continually maintained, by a complex interaction of geology and weather.

Over the millennia, according to the National Park Service, rocks from the surrounding mountains eroded and their sandy sediments were transported by stream to the valley floodplain. Prevailing southwesterly winds then carried the sand grains toward a low curve in the Sangre de Cristo Mountains, where they accumulated in a natural pocket.  On occasion, when the wind direction reverses during storms, the sand is pushed back toward the west. This process causes the dunes to grow vertically.

Two mountain streams, the Medano and Sand Creeks, also border the dunes. They capture sand grains from the eastern side of the dunes and carry them back to the valley floor.  This effectively recycles the sand and re-exposes it to the winds.

While the forces of wind and water are continually reshaping the massive dunes, they essentially remain in the same position.

Sand dunes nestled against the foothills of the Sangre de Cristo Mountains, Great Sand Dunes National Park, CO.  Image Credit The Weather Gamut.

Massive sand dunes nestled against the foothills of the Sangre de Cristo Mountains, Great Sand Dunes National Park, CO.  Image Credit: The Weather Gamut.

Swamp Coolers

Coming from the humid east coast, one of the first things you notice upon arrival in the southwestern United States is how dry the air is. Dew points are often in the 40s while the air temperature soars into the 80s and 90s during the summer. This is why swamp coolers are popular in the region.

A swamp cooler is an evaporative cooling device. It takes hot, dry outside air and blows it across water soaked pads. This allows the process of evaporation – the transition of liquid water to water vapor – to cool the air that is pumped into a building. It also adds some moisture to the inside air, making it more comfortable.

While the U.S. Energy Department says swamp coolers cost about one-half as much to install as central air conditioners and use about one-quarter as much energy, they do not work well everywhere. In hot, muggy climates, for example, the high relative humidity would significantly reduce the rate of evaporation. Moreover, adding extra water vapor to the air would not be considered a bonus in an already uncomfortably humid environment.

For this reason, only 3% of homes nationwide utilize swamp coolers, according to a report from the Energy Information Administration. In the arid Rocky Mountain region, however, they are found in more than 26% of all households.

Sign advertising swamp coolers in Salida, CO.  Image Credit: The Weather Gamut

Sign advertising swamp coolers in Salida, CO.   Image Credit: The Weather Gamut.

Why U.V. Intensity Increases with Elevation

One of the most important items on the packing list for my trip to Colorado this past week was sun-block. Averaging 300 days of sunshine per year at a mean altitude of 6,800 feet above sea level, the U.V. index in the Centennial state can range from high to extreme during the summer months.

The U.V. index is a scale that measures the intensity of the sun’s ultraviolet radiation. Readings vary from place to place as local factors affect the amount of U.V. light that reaches the ground. These include, the thickness of the ozone layer, latitude, season, cloud cover, and elevation. Developed by the NWS and EPA in the early 1990’s, it informs the public about the daily health risk of unprotected exposure to the sun.

At high elevations, the atmosphere thins and is less able to absorb U.V. radiation. With every 1000-foot increase in height, according to the National Institutes of Health, U.V. levels increase by about 4%.  So, in Denver, “the Mile High City”, U.V. radiation is about 20% stronger than a location at sea level at the same latitude. Heading up into the Rocky Mountains, where peaks can reach above 14,000 feet, the U.V. intensity soars even higher.

Credit: EPA

Credit: EPA

Air Quality Concerns in Great Smoky Mountains National Park

Straddling the border of North Carolina and Tennessee, Great Smoky Mountains National Park protects 800 square miles of the southern Appalachian Mountains. It is the largest federally protected upland landmass east of the Mississippi River.  Air pollution, however, does not recognize these human-drawn borders. While traveling in the Smokies recently, I learned more about the air quality issues facing this country’s most visited national park.

According to the NPS, most of the air pollution impacting the park originates outside its boundaries. Emissions of sulfur dioxide and nitrogen oxides from power plants, factories, and vehicles are the main sources. Carried by wind to the southern Appalachians, the height of the mountains and the prevailing weather patterns of the region tend to trap the pollution in and around the park.

Originally named for its naturally occurring smoke-like blue haze, the park in recent years has been shrouded by unnatural white smog. Produced by tiny sulfate particles – released into the air by the burning of fossil fuels – the smog scatters light and reduces visibility. It has degraded views from the park’s scenic mountain overlooks and dulled its signature blue haze. Since 1948, according the NPS, human-made pollution has decreased average visibility in the region by 40% in winter and 80% in summer.

Ground level ozone, formed when nitrogen oxides react with heat and U.V. light, is known to have negative impacts on human health.  In the Smokies, it is also injuring trees and plants. Damaging leaves, it reduces photosynthesis and limits a plant’s ability to produce and store food.  As a result, they are more susceptible to disease, insects, and extreme weather events.

Acid rain is another problem for the park that is rooted in air pollution. It develops when sulfur dioxide and nitrogen oxides react with water and oxygen in the atmosphere to form solutions of sulfuric acid and nitric acid. This type of precipitation alters the chemistry of forest soils and streams. It jeopardizes the health of entire ecosystems, as a large array of species – from fish to trees – cannot adapt to the more acidic conditions. The average pH of rainfall in the Smokies, according to the NPS, is 4.5. That is 5–10 times more acidic than the pH range of normal rainfall.

While air quality issues in the park – like much of the rest of the country – have improved in recent years, it still remains a serious problem. Addressing the matter, the NPS says: “The Park Service is working with state regulatory agencies, the Environmental Protection Agency, and industrial and utility interests to develop a comprehensive plan to prevent future damage through such measures as offset programs, the use of improved technology, and determination of emission caps and government standards for various pollutants. To remedy air pollution problems at the park, additional reductions of nitrogen oxides and sulfur dioxide are necessary.”

The Blue Haze of the Great Smoky Mountains

Traveling in North Carolina and Tennessee recently, I had the opportunity to visit Great Smoky Mountains National Park. While renowned for its “wondrous biodiversity”, the park’s name is derived from a localized atmospheric phenomenon.

The Cherokee, who originally inhabited the area, called the mountains, “Shaconage”, meaning “place of the blue smoke”. It refers to the smoke-like bluish haze that hovers over the park’s rugged peaks and valleys, especially after a rainstorm. According to the NPS, it is a natural by-product of plant transpiration.

While all trees and plants exhale water vapor, the conifer trees in the park also emit terpenes – a naturally occurring organic compound. Released in large quantities, the mix of terpenes and moisture react with natural low level ozone molecules to form tiny particles that scatter blue light. As a result, the mountains appear to be bathed in a gauzy blue mist.

In recent years, according to the NPS, human-made air pollution has been obscuring the Smokies’ signature blue haze.

View of blue haze in Great Smoky Mountains National Park.  Image Credit: NPS

View of the bluish- haze in Great Smoky Mountains National Park. Image Credit: NPS

Alaska’s Glaciers and Climate Change

Glaciers are dynamic.  Over time, they advance or retreat depending on climatic conditions.  They form, and spread, when more snow accumulates in the winter than melts in the summer. Since the end of the Little Ice Age in the mid-1800s, most glaciers have been either stable or in slow retreat.  In the last half century, however, that rate of retreat has increased.

This summer, I had the opportunity to travel around Alaska. While there, I visited a few of its nearly one hundred thousand glaciers and learned more about how they are responding to climate change.

According to the U.S. Fish and Wildlife Service,  Alaska’s  statewide  glacial  mass  balance – the net gain or loss of ice – has been negative since the middle of the 20th century.  While conditions at individual glaciers vary, the majority are melting. Recognizing that natural variables like the Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO) have always affected Alaska’s glaciers, most scientists agree that human-caused global warming has accelerated glacial retreat across the state.   Earth’s average temperature increased 1.4°F in the last century, but Alaska is warming even faster.  The E.P.A. reports that Alaska’s average temperature has increased 3.4°F in the past fifty years with winters warming by an average of 6.3°F.  These warmer temperatures coupled with shifting precipitation patterns are causing glaciers to both shrink in length and thin in volume.

A striking visual example to this process is Exit Glacier in Kenai Fjords National Park. It is one of forty glaciers in the park that flow out of the Harding Ice Field, but is the only one easily accessible by foot. As such, the trail leading up to it is marked with signs that point out the glacier’s previous extent and progressive retreat over nearly two hundred years. The first sign, 1815, is now over one and a half miles from the current terminus.  According to the National Park Service, this glacier has been receding at a rate of forty three feet per year, on average.  Between September 2011 and October 2012, however, it retreated one hundred thirty three feet.

While shrinking glaciers in Alaska may seem like a remote environmental issue, they have far reaching impacts. A recent study by NASA and the University of Alaska – Fairbanks found that the state’s melting glaciers are one of the largest contributors to rising global sea levels.

Exit Glacier with melt water running off into the outwash plain.  Image Credit: The Weather Gamut

Exit Glacier with melt water running off into the outwash plain.                                                  Image Credit: The Weather Gamut

Sign along the Exit Glacier trail that marks the location of the terminus in 1926. The glacier's current position is visible in the background.  Image Credit: The Weather Gamut

Sign along the Exit Glacier trail that marks the location of the terminus in 1926. The glacier’s current position is visible in the background behind the trees.                                                       Image Credit: The Weather Gamut

The progressive retreat of Exit Glacier in Kenai Fjords National Park, Alaska.

Mapping the progressive retreat of Exit Glacier in Kenai Fjords National Park, Alaska.                     Image Credit: NPS

Denali’s Wood Frogs Freeze for the Winter

While traveling in Alaska recently, I had the opportunity to visit Denali National Park and Preserve.  Its landscape, which includes Mt. McKinley – the highest mountain in North America – and its diverse wildlife were nothing short of impressive.  However, it was the tiny wood frog – the park’s only amphibian – that peaked my curiosity when I learned how it survives the region’s subarctic winters.

Situated at roughly 63°N latitude, winters in Denali are long and extremely cold.  From October to March, temperatures can range from 20°F to as low as -40°F.  These cold conditions drive many creatures to hibernate in dens or migrate south. The wood frog, however, makes it through winter by burrowing into leaf litter and literally freezing solid until spring.

According to wildlife biologists, a wood frog responds to falling temperatures by converting glycogen in its liver into glucose (sugar) and pumping it throughout its body.  Acting like a natural anti-freeze, the glucose lowers the freezing point of water inside the frog and protects its tissues and organs.  As temperatures continue to drop, however, the frog does eventually freeze.

Throughout the winter, the frog is essentially lifeless.  Its heart stops beating and it does not breathe.  Yet, as temperatures rise in spring, the frog thaws and comes back to life. While scientists are not exactly sure how this amazing resurrection works, they have noted that the wood frog’s heart and liver freeze last and thaw first.

Although the wood frog can be found across North America, the Alaskan wood frog is known to endure colder temperatures and freeze for longer periods of time than its southern cousins.  It is also the only frog found north of the Arctic Circle.


Wood Frog

Image Credit: NPS