This week we welcome long-time friend of GLOBE, Dr. Peggy LeMone, Chief Scientist for the GLOBE Program from 2003-2009, as our guest blogger. Dr. LeMone is currently working in the field of weather and cloud formation at the National Center for Atmospheric Research (NCAR).

Originally posted at http://spark.ucar.edu/blog/measuring-rainfall on September 23, 2013.

Dr. Peggy LeMone is an NCAR Senior Scientist who studies weather and cloud formation. For more information about her research, visit Peggy’s home page.

A guest post by NCAR scientist Peggy LeMone

The Boulder, Colorado area received huge amounts of rain in mid-September.  You also learned that rainfall amounts vary a lot. Which brings us to the questions – How do you measure rain?  And how accurate are the measurements?  Even though I have done weather research for many years, during this storm I was reminded how hard it is to measure rain accurately.

This is the story of my attempts to measure rain during the storm. It’s also about the many possible sources of error when making rain measurements – from old rain gauges to growing trees and even, possibly, inquisitive raccoons.

By Monday morning (September 16), I had measured over 16 inches, or 405 millimeters (mm), in our backyard rain gauge from the storm which began September 10.  The gauge is the same type the National Weather Service uses. It has a funnel that deposits rain into an inner tube with a smaller diameter (like this one), but bigger. The inner tube’s diameter is just small enough to make the depth of rain ten times what it would be in a gauge without the tube and funnel.  Thus, each inch in the tube is equivalent to 0.1 inches (a tenth of an inch) of rainfall.  This is equivalent to how the GLOBE rain gauge measures rain: the inner tube acts like a 10x magnifying glass for the area of the rain gauge.  This makes it easier to read accurately!

My gauge is old. I inherited it from a weather-observing neighbor who moved away.  The funnel and inner tube doesn’t quite fit, so, I leave the gauge open and then pour the rain into the inner tube using the funnel.

On the morning of September 12th, the gauge was so full and heavy, with over seven inches (178 mm) of rain that I decided to stick a meter stick in the gauge to measure the rain amount, and save pouring into the inner tube for the end of the storm.  The gauge tilts slightly, so I took a measurement on the uptilt side and the downtilt side and calculated an average.   That evening I found that the bottom of the gauge sagged in the middle, leading to an even deeper measurement than the downtilt side.  With these flaws, the lack of the ten-to-one exaggeration of depth, and some measurements being taken in the dark with a flashlight, my data were only approximate. I recorded measurements to within the nearest quarter inch (see the graph below).

Were my measurements accurate? On Friday morning, September 13, I took measurement using a more accurate method to compare with my estimates.  After bailing out five full tubes of rain, I poured the remaining water through the funnel into the tube to a depth of 13.5 inches (343 mm), spilling a little bit during this process.  The result was 0.38 inches (9.5 mm) more than my rough estimate from the night before – a storm total of 14.52 inches (369 mm) up to this time. On the graph, this is marked as 1. (The lower shows the uncorrected values.)

But the rain hadn’t stopped.  I awoke on the morning of September 15^th and heard reports that up to 2 inches (51 mm) of rain fell overnight. I went outside to check our gauge – only to see that it had been knocked over (probably by raccoons).  Fortunately, I have a second rain gauge in my backyard – a plastic gauge that registered about 0.25 inches (6 mm). I added a conservative 0.2 inches (5 mm), since this gauge was under trees (marked as 2 on the graph).

The final number:  16.37 inches (416 mm) of rain, more or less.

The exposure of the rain gauge is undoubtedly the greatest source of error.  According to the National Weather Service and CoCoRAHS (both of which use citizen volunteers to measure rainfall), “exposure” of the rain gauge is important. Rain may be blocked by nearby obstacles causing the number to be lower than it should. Or, rain may be blown into or away from the gauge by wind gusts.  The recommendation is that the gauge be about twice the distance from the height of the nearest obstacles, but still sheltered from the wind.

The gauge was certainly sheltered from the wind.  It is located about 10 feet (~2 meters) south of the house, which is about 15 feet (5 meters) high, and to the west of a fence and small trees as well as the tree in the photograph.   There is a much smaller tree to the southwest.

All the obstacles suggest that some rain could have been blocked from reaching the gauge, which would imply that the rainfall total is too small.  On the other hand, some rain might have been running down the branch in the picture. (In fact, because of the large amount, I thought this might be the main effect before doing some research on exposure)

It is also recommended that the gauge be level, which it wasn’t.  I’m not too worried about this, since it was nearly vertical.

The conclusion?  There was a lot of rain.  It could have been an inch (25 mm) more or less than my measurement. Acknowledging this is called reporting error. It doesn’t mean that the measurements are wrong, it just gives an idea of how accurate they are. My total was not the largest; there were at least two other measurements near 18 inches (457 mm).

Now that I’ve described all that can go wrong measuring rainfall, let me add that, putting a rain gauge in the right place, and taking an accurate rainfall measurement is fairly easy. If you have a perfect cylinder, such as a GLOBE rain gauge, simply stick a ruler in and read the depth (make sure to correct for any offset of the “zero” line and correct for this offset; and see if the ruler pushes the water level up very much).

If you don’t have a rain gauge but have a bucket (or glass) with sides that aren’t straight up and down, you’ll need to do a little math to figure it out. Here’s what you’ll need to do:

1. Measure the diameter of the bucket at the level of the rain.  Subtract out twice the thickness of the walls.
2. Measure the diameter of the bucket at the bottom in the same way.
3. Calculate the average of the two diameters.
4. Divide by two to find the average radius.
5. Find the average volume of rain = Depth x radius x radius x 3.14.
6. Find the area at the top of the bucket (this is the area over which the rain is collected).
   1. Measure the diameter
   2. Divide the diameter by 2 to get the radius
   3. Area = radius x radius x 3.14 (remember that Area = pi x radius²)
7. Divide the rainfall volume by this area to get the rainfall.

It would be an interesting activity to put several buckets (or rain gauges) in different places in a field, your back yard, or your schoolyard to see how much the measurements vary within the area. Soup cans, though not perfect, would work pretty well for the activity, especially if they’re the same size.  I might try this during the next rainstorm.  (I hope not too soon!)

Does your school collect precipitation data? Have you had an extreme weather event that you were able to record? Let us know by adding a comment!

Early sailors traveling the world’s oceans were all too familiar with an area of the tropical seas characterized by lack of winds and violent thunderstorms.  They called this zone “the doldrums” and dreaded being “stuck in the doldrums.” In his Rhyme of the Ancient Mariner, English poet Samuel Taylor Coleridge offered the following description of the Pacific doldrums:

All in a hot and copper sky,
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the Moon.

Day after day, day after day,
We stuck, no breath no motion;
As idle as a painted ship
Upon a painted ocean.

Today, we have a better understanding of this phenomenon and now know this area as the Intertropical Convergence Zone, or ITCZ.  It shapes atmospheric circulation patterns throughout the world and is considered to be the most prominent rainfall feature on the planet; critical in determining who gets fresh water and who doesn’t in the world’s equatorial regions.  The ITCZ is defined by the coming together, or convergence, of the northern and southern hemisphere trade winds and a decrease in the pressure gradient.  Specifically, in the north, trade winds move in a southwesterward direction originating from the northeast, with somewhat of the opposite effect in the southern hemisphere (where trade winds blow from the southeast to the northwest).

A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution.
From: Figure 7.7 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

The intense tropical sun pours heat into the atmosphere forcing the air to rise through convection and results in precipitation.  Rain clouds up to 9,144 m (30,000 ft) thick can produce up to 4 m (or 13ft) of rain per year in some places.  The ITCZ is not a stationary phenomenon nor are its movements symmetrical above and below the equator.  Many factors, including seasons and land masses, influence its overall movement.

Southern shift of ITCZ in January.
From Figure 7.9 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

Northern shift of ITCZ in July.
From Figure 7.9 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

With this knowledge in mind, we first encountered some of the effects of the ITCZ last year, as we approached the Caribbean coast of Panama aboard our sailing research vessel (RV) Llyr in July 2012. The map above shows the ITCZ located very near to Panama, the narrow strip of land that connects North, Central and South America.   At a latitude of about 9°North, we met up with the storms of the ITCZ during the night.  We could see the arrival of a band of storms on our ship’s radar and plotted a course to avoid them.  The storms had other plans, and we spent the night in their midst, at times feeling like they were chasing us as we tried to take evasive action while they kept building right overhead. Lightning lit the sea around us in an eerie glow and we could see, through the rain, bolts striking not far from the ship.  The next morning, tired but safe, we sailed into the harbor in Bocas del Toro, Panama, having had our introduction to the ITCZ.

Image of the RV Llyr. From Berkshire Sweet Gold

We came to Panama as part of a multi-year research expedition aboard RV Llyr, studying coral reefs, sustainable fisheries and changes taking place in the ocean.  As farmers, we have studied weather for many years, understanding oceans and atmospheric circulation as integrated systems that help produce weather at our forest farm in New England. As social scientists and human ecologists, our interest lies in researching the myriad links between biological and cultural diversity as key elements in sustainable development.  In the coming weeks, we will transit the famous Panama Canal aboard our 53′ steel ketch, and once again pass through “the doldrums” as we make passage for the Marquesas in French Polynesia.  During the 30+ day passage, we’ll be participating in global plankton studies and weather surveys. During our passages through the Pacific Islands, specifically French Polynesia, the Cook Islands, Tonga, and finally Fiji, we’ll perform reef surveys on scuba and hopefully meet with local schools to share the findings and experiences of our expedition.  We are a family of five, with three boys on board, and additional crew members and scientists joining us on expedition.  We look forward to sharing our journey.

Suggested activity: Do you live in a region affected by the ITCZ?  We’d love to hear about your experience as these storms pass through.  Send us a story or an image you have captured about the ITCZ either through a comment here, our website, or our Facebook page.  Be sure to collect temperature and precipitation data to document how your location is affected by the ITCZ, and think about what influence these two atmospheric variables may have on other GLOBE protocols.

Asian Tiger Mosquito. From The Center for Invasive Species Research, University of California, Riverside

Beginning in Albania in 1979, this breed of mosquito was introduced into Europe through the transport of goods from its native region of Southeast Asia.  Since then, the population has increased dramatically and has spread to more than 15 countries along Europe’s southern edge.  Additionally, these regions have seen increasingly milder winters and warmer summers, which lend themselves to prime conditions for mosquito larvae to survive.

The Asian Tiger mosquito is known for transmitting various diseases, such as West Nile, yellow fever, dengue, St. Louis and Japanese encephalitis, and chikungyuna.  And while it is native to Southeast Asia, the species has become well adapted to life in a more temperature climate.  It has been found, in fact, that the eggs of the Asian Tiger mosquito living in temperature climates are more cold resistant than their counterparts in tropical climates.  In addition to Southeast Asia and Europe, there are Asian Tiger Mosquitos living in the Americas, the Caribbean, Africa and the Middle East.

Since 2005, the Asian Tiger Mosquito has been blamed for outbreaks of some of these vector-borne diseases in France, Italy and Croatia.  It is feared that as the climate in these regions continues to change, that the frequency of vector-borne diseases will increase.  To support this suspicion, the European Centre for Disease Control used widely-used computer models to simulate weather records for the years of 2030-2050.  They found similar trends of warming continuing, allowing the mosquito to spread to northern Europe.

Suggested Activity: Get involved in mosquito climate research now.  Start by getting involved in the Great Global Investigation of Climate and taking air temperature, soil temperature and precipitation measurements. You can then take these data and connect to the number of reported cases of one of the vector-borne diseases. And make sure to let us know about your research.  You can tell us about it through the GLOBE website or our Facebook Page.

-Jessica Mackaro

The Maldives are a great location for such an experiment because during the months of November through March, the country experiences its dry season with respect to the monsoon, and pollutant heavy air can be seen traveling from thousands of kilometers away from countries like India and Pakistan.  Furthermore, the island nation has a low elevation and is extremely sensitive to changes in sea level rise.

A map of the Maldives. From Worldatlas.com

Through the research, Ramanathan and his colleagues discovered that these pollutants are primarily composed of black carbon soot that comes from the burning of fossil fuels and biomass.  With the longevity of the research, they were able to understand that there is a strong heating effect of these pollutants.   But black carbon soot affects more than air temperature – it destroys millions of tons of crops annually and causes human health concerns.  The good news is that this type of emission is easy to reduce due to the face that its lifespan in the atmosphere is short.

Sources of black carbon emission. From AGU.org

If these types of pollutants are reduced quickly, the long-term negative effects of climate change can be reduced by nearly 50% in the next 20-30 years.  With Ramanathan’s research, The Climate and Clean Air Coalition (CCAC) was established.  The CCAC is focusing on the reduction of short lived pollutants by nearly one third to protect and improve human health and agriculture.

And while the relationship between black carbon soot and warming is better understood, and has recently been presented by the International Global Atmospheric Chemistry Project, the affect the black carbon has on clouds and the type that form is still unknown.  Further research is necessary to understand the feedback between black carbon affected clouds and climate change.

Suggested activity: If you’re a GLOBE school in an area that sees seasonal fluctuations in air quality, you can perform your own research study to see the affect that air pollution has on your local temperature, cloud type and cloud cover.  Start by taking air temperature, cloud clover, cloud type and aerosol measurements and enter them into the GLOBE database.  Then as your database grows, start to examine the relationships that exist between the variables.  Then, be sure to tell us about it.  You can share your future research plans with us through a comment, email or on our Facebook Page.  For more information on Ramanathan’s research, watch this video.

-Jessica Mackaro

Scientists are well aware of the potential implications that climate has on these trees, what they aren’t aware of is the affect that the reduction in forest will have on the world’s ecosystems.   Trees act like giant lungs, taking in carbon dioxide and releasing oxygen.  Studies have shown that trees take in more than 50% of human-generated carbon dioxide and store it.   Therefore, if these big trees continue to die, there’s more carbon dioxide left in the atmosphere, which can lead to additional atmospheric warming.  Furthermore, if the trees are dead, they cannot provide the key nutrients, such as nitrogen or seeding, to the surrounding soil to allow the forest to re-establish itself after fire or windstorm.

Forest die-off can also affect things like surface moisture and climate classification.  Heat and drought affect each tree species differently, which can result in a long-term shift in the dominant species found in a location.  For example, a forest may become grassland.  This will also affect soil moisture, as there will be no tree canopy to intercept rainfall or prevent the exposure to harsh sun and wind.

But it goes further than that.  Trees provide homes to many different types of animal life, from mammals to birds and reptiles.  As the trees die, these animals are forced to look for a new habitat.   It is feared that as trees die, so will different species that rely on these old trees.

The GLOBE Program has protocols that can aide in the examination of how these forests are changing. Looking at land cover classification while taking air temperature and precipitation measurements can start the foundation for an exploration between climate change and land cover change.  The month of January features a repeat of the Climate and Land Cover Intensive Observing Period (IOP).  With that IOP, teachers and students are encouraged to classify their land cover as well as take photographs.  By keeping these records over the years, GLOBE schools can contribute to studies following forest mortality.

Suggested activity: Participate in the January Climate and Land Cover IOP by establishing or visiting your land cover site.  Submit your photographs and land cover classification to the GLOBE website.

-Jessica Mackaro

Map of sea ice extent on 21 August 2012 as compared to the 1979-2000 median; Credit: NSIDC

The Georgia Institute of Technology looked into the connection between Arctic sea ice extent and Northern Hemisphere snow cover.  What they found may be critical to seasonal snow forecasts and temperature anomalies across North America and Europe: the lower the sea ice extent, the snowier the winters can be.

Why does this happen?  Scientists believe that the enhanced melting of sea ice causes changes in the atmospheric circulation, increases the amplitude of the jet stream and increases the amount of moisture in the atmosphere.   Let’s take a closer look at these three impacts.

1. Changes in the atmospheric circulation.  Because of melting sea ice, the westerly winds are able to decrease.  This sets up a “blocking pattern”, which is the positioning of high and low pressure systems, so that cold air masses can move easily from high latitudes to middle and lower latitudes in Europe and North America.
2. Increasing the jet stream amplitude.  The jet stream is a rapidly moving zone of winds high in the atmosphere.  This stream is formed by the contrasting warm temperatures to the south and cold temperatures to the north.  The amplitude of the jet stream refers to how far north and south it reaches.  So a high amplitude jet stream means there is more than 1610 km between the bottom of the trough to the peak of the next ridge.

 



Image of a high amplitude jet stream; Credit: weather.thefuntimesguide.com

3. Increased atmospheric moisture.  It is easier, energy wise, for water to change state from a liquid to vapor than from a solid to a vapor.  Therefore, with more water existing as liquid than ice, evaporation occurs easier than vaporization would.  This increases the moisture in the atmosphere in the form of water vapor.

So if there is colder weather and more moisture available, then the conditions become more favorable for more frequent snow events.  This report is also timely, because the National Snow and Ice Data Center (NSIDC) is reporting that Arctic sea ice is forecasted to set another record low soon – as early as next week.  The difference about this year’s minimum, compared to the low in 2007, is that it is expected to happen very early in the season.  Typically the lowest Arctic ice extent is measured in late September, right before the equinox when the Arctic begins to enter a sunless winter.  This year’s trend is on track to break the record low and then still have even more time for potential ice melt through the end of the northern hemisphere summer.  Only time will tell how low the sea ice extent will go.

This report is timely for many reasons.  One, as stated above, is that the northern hemisphere is beginning its transition to the autumn and winter seasons.  Second, September brings another occurrence of the Great Global Investigation of Climate, where schools are encouraged to collect temperature and precipitation data.  As a GLOBE school, you can be a part of this research – collect your school’s data, obtain records from the NSIDC and compare the two to see if there is a relationship between Arctic sea ice extent and the winter weather in your community.

-Jessica Mackaro

A look at the High Park fire burning along a creek near Ft. Collins, Colorado in June 2012. Credit: Kerry Webster

Recently, the state of Colorado has seen a number of wildfires flare up.  What causes these fires is always a point of concern.  Was it a thunderstorm,  a campfire that continued to smolder, the butt of a discarded cigarette from a passing vehicle or the spark of a chainsaw of a worker trying to remove a beetle kill tree to prevent a fire?  Currently, there are eleven fires burning across the state, mostly in rural mountainous areas, but some have already destroyed homes and threatened more heavily populated areas, such as subdivisions on the outskirts of Ft. Collins, Colorado as well as in the city limits of Colorado Springs, Colorado.

In addition to threatening communities, these fires can create their own weather systems.  Pyrocumulus clouds are often seen with very intense fires, such as firestorms, which are intense fire-driven wind systems.  Firestorms are formed as the fire takes in air from all sides, forming an updraft.  As the intake increases, the fire can grow even stronger as fresh air feeds it.  Sometimes if the updraft is strong enough, it can form a pyrocumulus cloud.  Sometimes these clouds can condense enough moisture that it falls as rain, helping to extinguish the fire.  Other times, the cloud may grow so large that it becomes a cumulonimbus, producing additional lightning and sparking other fires.

Pyrocumulus from the High Park fire in Colorado from June 2012 Credit: Kerry Webster

The High Park Fire, occurring outside of the city of Ft. Collins, Colorado, is the second largest fire in state history.  This specific fire was started by a thunderstorm that occurred in the early morning hours on 9 June 2012.  As of publishing time today, the fire had consumed 87,284 acres. This fire, consuming many beetle kill timber, has continued to burn due to persistent hot and dry weather.  Coupled with high winds, these three conditions make it easy for fire to spread and continue burning.  Why do hot, dry and windy conditions fuel fires?

Hot weather, in general, increases evaporation rates.  Evaporation is an essential part of the water cycle, driven by the sun. The sun heats the ground, which provides enough energy for water to transition from liquid state to vapor state.  If hot weather persists long enough without relief, evaporation can exceed precipitation, which results in dry conditions.  And if conditions stay dry for a long enough amount of time, plants die and become prime fuel for fires.  Add in the beetle kill, and you have an area of land with timber and dry grasses just waiting to ignite.  The final condition, high winds, is a fairly straight-forward condition: with high winds, sparks and embers are able to travel further than they would in calm conditions, allowing fires to grow quickly.

Close up of a tree on fire as part of the High Park Fire outside of Ft. Collins, CO in June 2012 Credit: Kerry Webster

The GLOBE program has protocols to examine many aspects of fires.  In addition to atmosphere protocols that measure temperature and relative humidity, GLOBE has a fire fuel protocol that helps students measure the different types of fuels for fires.  Students are able to learn about the different types of living and dead organic materials that can become fuels for wild fires.  Through the use of this protocol, students can further understand fire behavior and effects, such as fire spread (how fast a fire moves) and fire intensity (the flame length).

Progression of the High Park Fire, with red being most recent burn. Credit: Kerry Webster

As summer continues in the northern hemisphere, fire awareness becomes even more important.  The State of Colorado has implemented a state wide fire burn ban; therefore no open flames are allowed until the ban is lifted.  But Colorado isn’t the only place experiencing fires.  Many states in the western United States are under fire watches, as the region experiences drastic precipitation deficits.  The need for rain is high, and the summer, typically a relatively dry season for the region, is only beginning.

Special thanks to Kerry Webster, a firefighter with the National Park Service in Boulder, Colorado for providing the photographs in this post.  Kerry has been actively fighting both the High Park Fire outside of Ft. Collins, Colorado as well as the Flagstaff Fire, which ignited on 26 June 2012 due to lightning in rural Boulder County, Colorado.

-Jessica Mackaro

Growing up, I always heard sayings about months and the corresponding weather patterns.  Some included “April showers bring May flowers” and then of course the one dealing with the month of March, “in like a lion, out like a lamb”.  With this March beginning with a tornado outbreak throughout the Ohio River Valley (131 reported tornadoes on March 2nd), some may wonder, will it actually go out like a lamb?  Is there any truth behind this saying, or is it just a phrase society has been hooked on?

NOAA's Storm Prediction Center reports for the March 2, 2012 tornado outbreak. Image courtesy of SPC http://spc.noaa.gov/climo/reports/120302_rpts_filtered.gif

When we examine March in terms of weather events, there is a large amount of variability.  In the northern hemisphere, March is a transitional period between the seasons with winter exiting and spring entering. The transition between the two seasons is what causes March to have its variability in terms of weather phenomena.  Growing up in northern Illinois in the central United States, I remember having Spring Breaks with snow falling and others with temperatures warm enough to do outdoor activities.  That is a substantial difference in terms of weather from year to year.  The transition period can be observed when the seasons change between winter and spring; it does not matter your location.  However, it can be more prevalent in regions with a more drastic change in the seasons opposed to regions that have the same weather variations throughout the year.

With the transition causing so many differences from year to year, it is hard to say, “in like a lion, out like a lamb” is always accurate.  Although it can be true some years where the beginning and end of March are considerably different.  In Illinois, the end of March is generally very pleasant.  Temperatures are getting warmer, the snow is melting, and there isn’t much variability in temperatures over a few days.  That is because spring is beginning to settle in and winter has exited.  Don’t forget, spring is still an active period for weather patterns, as it marks the beginning of the severe weather season in most locations.

Using your GLOBE data, how many years do you have with March that comes in like a lion and out like a lamb?  How many years are there that have no considerable change?  Do you believe March usually comes in like a lion and out like a lamb?  Try it out for yourself.  And, what other weather phrases have you heard?  Add a comment or send an email at science@globe.gov to let us know.

- Ashley Kaepplinger

The recent volcanic eruption in Iceland marked by the spectacular “curtain-of-fire” and near-complete shut-down of air travel in Europe in mid-April will probably earn a place in the history books (see pictures of the Icelandic volcano at the Washington Post.)

The Icelandic Volcano. Credit: Washington Post

Two of the most commonly cited volcanic eruptions in the climate literature are Krakatua (1883; Indonesia) and Mt. Pinatubo (1991; Philippines). The most massive explosions of Krakatua took place in August, 1883, and rank among the most violent volcanic events in recorded history. In the year following the eruption, average global temperatures reportedly fell by as much as 1.2 °C (2.2 °F). Weather patterns continued to be chaotic for years, and temperatures did not return to normal until 1888. The eruption injected an unusually large amount of sulfur dioxide gas high into the stratosphere, which was subsequently transported by high-level winds all over the planet. This led to a global increase in sulfurous acid concentration in high-level cirrus clouds and the clouds became brighter. The increase in cloud reflectivity (or albedo) meant that more incoming light from the sun than usual was reflected back to space, and as a result, the entire planet became cooler, until the suspended sulfur fell to the ground as acid precipitation.

In June 1991, the best-documented explosive volcanic event to date and the second largest volcanic eruption of the twentieth century took place on the island of Luzon in the Philippines, a mere 90 kilometers northwest of the capital city Manila. Up to 800 people were killed and 100,000 became homeless following the Mount Pinatubo eruption, which climaxed with nine hours of eruption on June 15, 1991. On June 15, millions of tons of sulfur dioxide were discharged into the atmosphere, resulting in a decrease in the temperature worldwide over the next few years.

Pinatubo eruption provided scientists with a basis for constructing or modeling the change in Earth’s radiation balance (scientists like to call this change “radiative forcing”) due to explosive volcanoes. It is now well established that volcanic eruptions cause the stratosphere to warm and the annual mean surface and tropospheric temperature decreases during a period of two to three years following a major volcanic eruption. If you are interested in more technical details on how volcanoes affect climate, you can read a very good paper written by Alan Robock. Given that the Icelandic eruption is along a Mid-Ocean ridge and volcanic Hot spot, do you think the gases and aerosols will be of different composition than the Krakatoa and Pinatubo eruptions, which are associated with plate subduction along convergent plate boundaries? If there is a difference, what effect might that have on weather and climate over the next few years?

So the disruption of the air travel by the Iceland’s Eyjafjallajökull Volcanic eruptions is just the beginning; other weather and climatic effects will follow.  In the days and months ahead, we are likely to experience darkened sky and spectacular sunsets in different parts of the world.

The following two pictures of the same place were taken on different days. Can you explain why the upper picture is clear but the lower one is not?

Two pictures taken from the same place at Texas Tech University, Lubbock, Texas, USA show: (upper) the bluish sky on a clear spring day in 1998, and (lower) the hazy sky on a dusty day (6 April 2001) at about 5:30 PM local time (pictures downloaded from http://www.atmo.ttu.edu/dust.html)

As we all probably learned from our science classes, the air within our atmosphere is naturally composed of gases, including nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and several others. The atmosphere also frequently contains water vapor (H2O), which is water in its gaseous form. In addition to gases, the atmosphere contains very small particles in solid or liquid form, called “aerosols”. Liquid aerosol particles are in the form of viscous (or oily) droplets rather than water droplets.  Individually, aerosol particles are practically invisible to the human eye, because most of them are 10 microns or less in size (1 micron or micrometre = 1 metre divided by 1000000). By comparison, the human hair has a diameter of between 17 and 181 microns. Certain types of atmospheric aerosols (typically on the order of 0.2 microns in size) can serve as a nucleus upon which water vapor condenses to form clouds. Such aerosols are referred to as cloud condensation nuclei.  However, when there are high concentrations of aerosols in the air, especially near the surface of the earth where people can breathe them, we say that the air is polluted. In fact, the agents of air pollution (or pollutants) can occur either as unhealthy gases mixed up with the air or as aerosols floating in the air. People that monitor the air quality in different places often report the aerosol content of the air to the public in terms of the concentration of particles by mass per unit volume of air (typically expressed in units of micrograms per cubic meter). For air-quality purposes, aerosols are often referred to as PM10, which means all particulate matter (PM) in the atmosphere whose aerodynamic diameter (apparent diameter while floating in the air) is 10 microns or less. A subgroup of the PM10 often identified in air-quality monitoring is called PM2.5, which means all particulate matter whose aerodynamic diameter is 2.5 microns or less. There are several different types of aerosols depending on the materials or chemicals they are made of and where the aerosols come from. People and animals inhale aerosols in the air they breathe. The tinier the particles are, the easier they can enter the lungs and cause serious harm to our health. Therefore, for a given aerosol type, those in the PM2.5 size group are more harmful than the larger size group.

Aerosols can come from many different sources, some of which are natural and others anthropogenic (i.e. caused by human activities). Some of the main aerosol types and their sources are: (i) chemical pollution aerosols from industries, cars, trucks, and other modes of transportation, (ii) smoke from large and small fires, (iii) dust blown by wind from bare ground surfaces, (iv) sea salt from ocean sprays caused by waves resulting from the action of the wind and other forces that cause sea motion, and (v) volcanic aerosols from eruptions of volcanoes. As you may have guessed, chemical pollution aerosols are almost all caused by people, because of many of the things we do to enjoy life and move around. Smoke aerosols are to a large extent caused by people who set fires to forests, bushes, trash, or anything that produces smoke, although in certain places smoke originate from fires caused by lightning strikes or large accidental events. Dust aerosols are mostly generated by wind, but sometimes people produce dust while moving or conducting certain activities in dusty places. In fact, when we do anything to destroy vegetation anywhere and leave the land bare, we are also helping to provide favorable conditions for dust generation. Sea salt aerosols are mostly natural, and only a very tiny proportion is indirectly produced from human activities that cause waves in the ocean, such as fast moving boats and ship. Volcanic aerosols are entirely natural and often lofted very high in the atmosphere away from where people can inhale them. Ironically, aerosols caused mainly by people, such as chemical pollution and smoke, are mostly in the PM2.5 size range, which are the most harmful to people.

How can we know when there is a high concentration of aerosols in the atmosphere? One simple way is to look up in the sky when the sun is up. If there are no clouds, the sky should look bluish (that is, sky blue) when the air is clean. If the sky is hazy (that is, not bluish) when there are no clouds, then there must be a high concentration of aerosols in the atmosphere. In this case, the color of the sky will depend on the source, type, and amount of the aerosol along our line of sight to the sky. The reason for this is that the Sun’s light is made up of (electromagnetic) waves distributed across a wide range of wavelengths forming a spectrum. Only light whose wavelength is in the visible range (approximately 0.38 to 0.75 microns) of the spectrum can be seen by the human eye, and represent the different colors of the rainbow (violet, indigo, blue, green, yellow, orange, and red: as arranged in ascending order of wavelength). When the Sun’s light is travelling through a clean atmosphere that has no cloud, the air molecules scatter the shorter wavelengths, of which the eyes are most sensitive to the blue light, because air molecules are smaller than visible light wavelengths. This phenomenon was discovered by the 1904 Nobel Laureate in physics, Lord Rayleigh, and is known as Rayleigh scattering. This is why clear sky looks blue during the daytime, except during sunrise or sun set. When the sky is cloudy, the clouds appear white. Since the cloud droplets are much larger than the wavelengths of visible light, the cloud scatters all visible wavelengths almost equally and appears white because the sum of all colors in the rainbow is white. Aerosols in the atmosphere can scatter and/or absorb lights of different wavelengths to various degrees depending on the aerosol type, amount, and size distribution. Therefore, large amounts of aerosols in the atmosphere cause the sky to look hazy in different shades of grey or pale yellowish to brownish colors, depending on the position of the sun relative to the observer.

As a practical exercise, look up the sky one or more times a day for at least a week and take pictures of what you see. Remove the pictures that contain thick clouds. Out of those that have no clouds, or with just a few clouds and much free sky space, try to separate the pictures in which the sky is blue from those in which it is not. Those where the sky is hazy must contain large amounts of aerosols. Try to identify the photo with the largest amount of aerosols. How much aerosols were present on this day with respect to the other days?  Compare this picture carefully with that of the blue sky day and discuss it with your friends, teachers, family, and possibly community, to try to identify the possible source(s) of the aerosols. What could all of you do (if anything) to reduce or stop such aerosol at its source(s)? We know that high concentrations of aerosols in the atmosphere are harmful to people’s health in many ways and it is the responsibility of us all to keep the air clean. In future blogs, we shall discuss many other aspects of aerosols, such as: how far they can travel from their sources, how long they can stay in the atmosphere, how they leave the atmosphere and where they go, how they are measured from the ground, or from aircraft or satellite, how they relate to clouds, and what they can do to the weather and climate.