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 Köppen 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

Welcome to "The World in the Box"

E Komo Mai!

Welcome to "The World in the Box", a blog written and maintained by teaching faculty from several campuses in the University of Hawai'i system.  You may have been directed to this blog as part of an assignment; if so we hope you find the information here useful.  If you've randomly ended up here as the result of a google search, welcome to you as well, and we hope you will enjoy the content of our blog.  We welcome questions and comments, but please remember to remain respectful of others.  Our goal with this blog is to create a space where we can interact with students and others interested in the geography of Hawai'i in an informal way; we hope this blog will not only enable us to discuss and elaborate on material from the courses we teach but also serve as a forum to talk about other aspects of Hawai'i.

This brings us to the title of the blog.  You may be wondering what "the world inside the box" means.  It's really quite simple.  We frequently see maps of the United States, and Hawai'i is generally shown in a little box tucked away into a corner of the map.  This gives the sense of remoteness and isolation.  But for us as geographers Hawai'i is a fascinating place, full of diversity and wonder.  From the geographer's perspective, few places on Earth provide the same opportunity to learn so much, from physical geography topics like climate and volcanology to biogeography and human geography topics like the culture and history of the Hawaiian people.

For example, did you know that Hawai'i has some of the steepest gradients in all of the world?  This means that you can walk from an alpine shrub ecosystem, through tropical rainforest, and end up in a desert over the space of just a few kilometers!  Did you know that Hawai'i is home to a carnivorous caterpillar that ambushes flies?  Or that our islands have more species of flightless flies than any other place on the planet?  While flies aren't the most glamorous of species, the tremendous diversity of species indicates just how special these islands are.  You no doubt are aware that these islands are volcanic in origin, and we can see island-forming processes in action on the Big Island.  People come from all over the world to study about volcanoes in Hawaii, and the Hawaiian names for the two basic types of lava, a'a and pahoehoe are used by geographers and volcanologists all over the world.  Hawai'i is also home to native people with a fascinating history and culture, and the Old Hawaiians were excellent geographers.  

We will be discussing all these subjects and more with this blog.  So again we welcome you to "The World in the Box".  We hope this blog will help you to appreciate the incredible beauty and wonder of these islands.  We also hope it will help you to understand concepts from your classes.

A Hui Hou!