Sunday, June 30, 2013

Things to Know About Albedo...

Which side of the T&C logo is hotter: the white side or the black?  Does
it matter?  For the answer see the end of this blog post.  
Have you ever heard that it is cooler to wear lighter clothes on a hot, sunny day?  How could this be?  How could the color of your clothes make a difference in temperature?  This couldn't possibly be true, could it?  Well, it is true, and the difference in temperature is explained by albedoAlbedo refers to the proportion of the sun's energy (insolation) that is reflected by a surface.  Albedo is generally expressed as a percentage, and so if it is high (say, 90%), then more of the sun's energy is being reflected by a surface.  The 90% figure means that 90% of the sun's energy is reflected off the surface.  If albedo is low (10% for example) then more of the sun's energy is absorbed.  In this case, only 10% of the sun's energy is reflected whereas the balance is absorbed.  If a surface absorbs more energy, it heats up more.  We can change the albedo of a surface simply by changing its color; light colored surfaces tend to have higher albedo than darker surfaces.  One very good example of this is clothing; on a sunny day here in Hawai'i white shirts can be as much as 30 degrees cooler than dark shirts.  In this post we'll be looking at another practical example of the difference that albedo can make.

A White Roof?


One of the neat things about studying physical geography is that we can see practical applications of what we learn everywhere.  A couple of years ago the property managers of the apartment complex that houses one of this blog's authors installed a "white roof", coating the roof of the building with a white polymer that significantly increased the roof's albedo.  In other words, the white roof reflected a great deal more energy than the old tar roof.  Immediately after the white roof was installed we noticed a big change in the temperature of the apartment....it was a lot cooler!  Even on the hottest days of the year at the peak of the afternoon sun we no longer needed to run the air conditioner to stay comfortable.  The new roof made a big difference and has saved us money on our electric bill, which in turn leads to decreased greenhouse gas emissions.  In short, everyone wins.  

More recently the management company had the entire building repainted, which was long overdue.  The old color was pale pink, a very light color.  The new color is dark gray.  Based on our experience with the white roof, I was perplexed as to why the management company would go with a dark color, because I reasoned that the darker color would absorb more of the sun's energy, thus making the interior of the building slightly warmer.  I was also curious how much a difference the darker color made to the surface temperature of the building.  So we here at TWITB decided to do an informal investigation: we would measure the temperature using an ExTech infrared thermometer, a handy little tool which uses a laser to measure surface temperatures.    We measured the temperature at approximately 3:20pm, which is approximately the hottest time during the day in Honolulu (why this is the case will be covered in a future post).  Since the painters didn't quite finish the job on the first day there was still some of the old pink surface (see the photo to the left), and so we could compare the results.  We measured the pink surface, the grey surface, and the dark gray doors.  We measured them in the sun and in the shade as well.  Which one do you think we found to be the hottest?

We were really amazed with what we found.  First the pink paint in the shade was 98 degrees whereas in the sun it was 110.  The concrete bricks that had been painted gray were 103 degrees in the shade and 130 in the sun.  The door was most surprising; in the shade it measured 111 degrees, whereas in the sun it measured 148 degrees.  So as you can see the paint really makes a big difference!  Whether this affects the interior temperature is a more complex problem to solve; this depends on the specific heat and conductive properties of the bricks and the door.  

So as you can see the color of a surface definitely does make a difference in surface temperature.  Let's look at some more examples.  In the picture below you will see some cars in a parking lot.  We've placed letters over a few surfaces.  This picture was taken at 3:30pm on a day when the official airport temperature was measured at 88 degrees.  "A" is exposed blacktop.  "B" is a white car in full sun.  "C" is a white parking space marker painted on the exposed blacktop.  "D" is shaded leaves.  "E" is a black car in full exposure to the sun, and "F" is a green car in full exposure.  "G" is blacktop in the shade.  For reference, we also measured a grass patch (not pictured) in the sun; its temperature was 110 degrees.  Match the letters from the picture with the numbers below the picture that correspond to the temperatures of each surface. 


1.  164 degrees.
2.  91 degrees.
3.  143 degrees.
4.  117 degrees.  
5.  114 degrees.
6.  151 degrees.
7.  138 degrees.

You can do similar tests yourself just by touching various surfaces on a hot day.  You'll also notice a big difference between the shade and exposed temperatures for the same surface.  In a future post we'll explain more in depth why color makes a big difference, but for now it's enough to know that darker things are hotter and lighter things are cooler.  As for the T&C logo, check out the pictures below.

White side.
Black side.
Answers: 1. E  2. D  3. F  4. B  5. G  6. A  7.  C  

Friday, June 21, 2013

The Solstice is Upon Us!

Photo from here.
Today is June 21st, the longest day of the year in Hawai'i.  Today the sun came up at 5.50am and will set at 7.16pm, for a total day length of 13 hours, 25 minutes, and 54 seconds.  We call this day the June (or Summer) Solstice.  After today the days will continue to get shorter until December 21st, with a day length of 10 hours, 50 minutes, and 12 seconds.  December 21st is the December (or Winter) Solstice.  In this post we will describe why there is variation in day length over the course of the year.

The Earth-Sun Relationship...


As we all know, the Earth revolves around the sun, a journey that takes approximately 365.22 days.  We also know that the earth is tilted on its axis at an angle of approximately 23.5 degrees.  This is why virtually every globe you ever see is tilted; it is demonstrating the earth's true orientation towards the sun.  But did you know that the tilt is always in the same direction?  This characteristic of the earth's orbit is called axial parallelism, and it is why days are shorter in the winter and longer in the summer.  This in turn is one of the biggest factors in seasonal variability; it is why the continental United States experiences winter, spring, summer, and fall.

Diagram from here.  


Have a look at the model we've provided.  As you can see, no matter what time of year it is, the earth's tilt is in the same direction.  If it is December, then more of the southern hemisphere is exposed to the sun at any given time, and less of the northern is bathed in glory of the sun's warming touch.  It follows then that in December it is summer in the southern hemisphere, and winter in the northern hemisphere.  If you imagine with your mind's eye that the earth is spinning around its axis (remember, one rotation equals one day), you can see that since more of the southern hemisphere is in the sun, the days are longer.  The opposite is true with the northern hemisphere.  Now look at the earth when it is June and the planet's northern hemisphere is at its maximum tilt towards the sun.  Can you see that more of the northern hemisphere is exposed to the sun, whereas less of the hemisphere is?  Thus the days are no longer in the northern hemisphere than in the southern.  Now look at the north pole.  Again, imagine with your mind's eye that the earth is spinning on its axis.  Look at the places close to the north pole.  Is there ever a point during a day (one complete rotation) that these points enter the darkness?  If you answered "no" you are correct!  These areas experience 24 hour days at this point, whereas at the south pole and near it there are 24 hour nights!

So at this point it should make sense to you that there is one day during the year when the northern hemisphere is at its maximum tilt towards the sun (whereas the southern hemisphere is a it maximum point away from the sun), and another point, roughly half a year later where the northern hemisphere is at its maximum tilt away from the sun (whereas at this point the southern hemisphere would be pointed towards the sun).  These two days are called the solstices, and they are the longest and shortest days of the year respectively in the northern hemisphere (and the opposite in the southern hemisphere).  

Now look back at the diagram.  There are two days of the year, one in March and the other in September, where the earth is pointed neither towards nor away from the sun; rather the tilt of the earth is perpendicular to a line drawn from the sun to the earth.  On these days every part of the earth receives 12 hours of daylight and 12 hours of night.  These days are called equinoxes.

So What Are the Tropics?


We've all heard the term "tropics", as in tropical storm, tropical paradise, and tropical fish.  But what does this really mean?  The "tropics" describes a very specific area on the earth's surface: all latitudes where the sun passes directly overhead at some point during the year.  Let's go back to our diagram of the earth-sun relationship.  Since the earth is rounded, there is a point on the earth's surface that is closer to the sun than all other points.  If you were standing on that point, the sun would be directly overhead.  Now since the earth is tilted, the spot on the earth where the sun is directly overhead changes over the course of the year.  The spot where the sun is currently overhead is called the subsolar point, and the latitude where the sun is directly overhead is called the solar declination.  Since the earth is tilted at an angle of 23.5 degrees, the subsolar point is found between 23.5 north latitude (the Tropic of Cancer) and 23.5 south latitude (the Tropic of Capricorn).  The area between these two lines of latitude is the tropics, and as we all know, Hawai'i is in the tropics.  This means that the sun will be directly overhead at solar noon in Hawai'i on two days during the year, one in May, and one in July.  Any place outside the tropics never ever experiences the sun directly overhead!  This is one more aspect of Hawai'i's geography that makes it special.  



Lahaina Noon?


Honolulu SkyGate at Lahaina Noon.  Photo from here.  
If you've ever been to Maui (or if you live there!) you may have visited Lahaina.  Lahaina is a major tourist attraction and is known as an old capital of the Hawaiian kingdom.  But Lahaina also gives its name to the day when the sun is directly overhead at solar noon in Hawai'i.  We call this "Lahaina Noon".  This name was immortalized in a contest sponsored by the Bishop Museum.  "Lahaina Noon" was chosen for these two days when the sun is at its greatest intensity because La Haina means "cruel sun" in Hawaiian.  On this day at solar noon you can witness something that no one on the continent ever sees (at least on clear days): a complete lack of shadows.  Since the Hawaiian Islands run to the northwest, Lahaina Noon happens on different days at different places.  In the table below you can see the dates for this year (2013); the Bishop Museum normally provides the dates on their website as well.

Information from the Bishop Museum.


Kau Ka La I Ka Lolo


Map from NOAA.  
The old Hawaiians were very attuned to the movement of celestial bodies (the sun, stars, and the moon) and referred to the day when the sun was directly overhead as kau ka la i ka lolo.  This expression can be translated as "the sun rests on the brains".  We understand now that these days had special significance to the old Hawaiians as well.  Based on research by University of Hawai'i Department of Anthropology graduate Dr. Kekuewa Kikiloi we have learned that many of the archaeological remains on Mokumanamana Island (generally known as Necker Island, one of the Northwest Hawaiian Islands) are tied to rituals associated with the passing of the sun directly overhead.  Mokumanamana happens to be right on the Tropic of Cancer, and so the sun is directly overhead here.  The Old Hawaiians were keenly aware of this fact, and so the island had a priestly significance to them; high-ranking priests would make periodic journeys to Mokumanamana to calibrate their calendars and for other ceremonies.

Necker island landsat image from Papahanaumokuakea National Monument website.  


Exercises:


Will the islands receive more energy from the sun on there respective Lahaina days or on the June Solstice? 

What compass direction will the Sun be oriented at solar noon on the days following the first Lahaina noon day? 

Why are there two Lahaina noon days for all of the Islands South of Mokumanamana (Necker) island?

Have the islands located north of Mokumanamana ever experienced the Lahaina noon sun?
  

Saturday, June 8, 2013

A Trip up Ka'ala 2: Climate and Altitude in Hawai'i

In our last post we described a recent trip we took up to the top of Mt. Ka'ala, the highest mountain on the island of Oahu, topping out at just over 4000 feet.  We described how the temperature changes with the altitude, decreasing the higher you get.  We discussed the rate at which this happens, which is described as the adiabatic lapse rate.  We also discussed humidity and how atmospheric moisture content affects this cooling rate.  All of this helped us understand the basic pattern of rainfall on the Hawaiian islands.  In this post one of the geography crew will describe how climate, which is determined by temperature and moisture, changes with altitude here in Hawai'i.  He'll also tell us about how these different zones are associated with vegetation.

Climate Zones


Have you ever wondered why clouds
Koppen chart from here.  
form high on the mountain slopes and not at the mountain base, or why there is a dramatic change in plant species as you move upslope?  A lot of this has to do with the distinct climate characteristics found in the islands.  Four of the five major climate zones described in the K√∂ppen climate classification system can be found in the Hawaiian Islands and within these zones there exists a host of distinct microclimatic sub-zones determined by temperature and precipitation characteristics.  The differences in microclimates and the associated vegetation that can be found within these microclimates can be attributed to the vertical profile of the atmosphere.  As you increase in elevation you decrease in temperature at a specific lapse rate depending on your environment and this temperature profile dictates the phase change of water in the atmosphere.

Diagram from here.
Climatic zones on mountain slopes in Hawai‘i  can be characterized with reference to 4 atmospheric layers described by Riehl et al., (1951).  These 4 climate zones with the corresponding atmospheric layers in parentheses are: 1) marine (subcloud), 2) fog (cloud), 3) transitional (inversion), and 4) arid (free atmosphere).  The sub cloud layer extends from sea level to the lifting condensation level ( LCL, 600 - 800 m) at which point clouds begin to develop.   In a simpler explanation the LCL is the point at which a warm moist air mass needs to be lifted so it sufficiently cools to the point at which water makes the phase change from a gas to a liquid (condensation).  The cloud layer exists from the LCL to the base of the Trade Wind Inversion (TWI) which can lie anywhere between 1000 and 4000 m (Cao et al., 2007).   The TWI is a synoptic subsidence of warm air that was originally uplifted at the equator by convective and convergent processes.   The TWI, which is present 80-90% of the year (Cao et al., 2007), has a profound impact on the climate at high elevations  The thickness of the inversion layer is about ~300 m and above this point the stable dry air of the free atmosphere layer can be found.  The height and thickness of each of these atmospheric layers vary in space and time in response to large-scale circulation features and surface heating (Giambelluca and Nullet, 1991).  This layered system exists only in the presence of the trade winds and disappears when cyclonic systems interrupt them.

The vegetation characteristics across elevation gradients in Hawai‘i are dependent on several factors, including, substrate, topography, precipitation, available genotypes and the fragmentation and severe modification of native vegetation, especially at lower elevations (Mederios, 1986).  On hike up the mountain we were able to experience lower two climate zones mentioned above  and the distinct vegetation associated with these zones.  Three basic ecosystem types occur between the leeward coast and the summit of Mt. Ka‘ala. These can be distinguished by rainfall, elevation, and vegetation type. Lowland dry shrubland and grassland occurs at the lowest elevations although introduced trees are also present. Lowland dry and mesic forest, woodland and shrubland occurs further inland. At higher elevations wet forest and woodland can be found. A special type of ecosystem called tropical montane cloud forest (TMCF) occurs in the summit region and harbors many rare natives species. The ecosystems at lower elevations are dominated by introduced vegetation as a result of disturbance. Native vegetation becomes more dominant the farther one moves up the mountain. The vegetation at lower elevations in the Waianae range area of Mt Kaala is dominated by introduced tree species. Near the coast kiawe (Prosopis pallida) is frequent. Moving inland, koa haole (Leuceana leucocephala) becomes a dominant species. Along the first part of the trail to Mt Kaala itself both species can be seen. Larger silk oak trees (Grevillia robusta) can also be seen. Coffee trees (Cofea Arabica) can also be seen. Moving up the steep slope rainfall increases and disturbance decreases. Native tree species like koa (Acacia koa) and ohia (Metrosideros polymorpha) become common. The last leg of the trail moves into the cloud zone and ohia becomes the dominant tree. Olapa or lapalapa Cheirodendendron trigynum or platyphyllum) becomes more frequent. Olomea (perrotetia sanwicense) is also present. These are the dominant trees in the mosaic and bog at the  summit. Also very frequent shrubs include pukiawe (Leptocophylla tameiameia). Rare plants like kolii (tremotlobelia macrostachyus are also present.


References



Cao, G. G., T. W. Giambelluca, D. E. Stevens, and T. A. Schroeder (2007), Inversion Variability in the Hawaiian Trade Wind Regime. J. Climate, 20, 1145–1160, doi: 10.1175/JCLI4033.1

Giambelluca, T.W. and Nullet, D. (1991) Influence of the trade-wind inversion on the climate of a leewared mountain slope in Hawai‘i , Clim Res., 1, 207-216