What is El Niño, and How Does it Affect Hawai'i?

If you ever pay attention to the weather forecast, you’ve probably heard about a phenomenon called El Niño.  However, if you’re like most people, you probably don’t have a clue as to what El Niño refers to or what it means.  You may have the vague notion that it causes changes in the weather, but you might be unsure at to the nature of those changes.  For example, does it make it rain more?  Less?  Does it make it hotter or cooler?  This confusion is understandable, because the effects of El Niño vary depending on where you are!  In this post we’re going to dispel some of the mystery and confusion surrounding El Niño in general and explain how this phenomenon affects Hawai’i.  Looking at El Niño gives us the chance to examine a number of other aspects of the oceanic-atmospheric relationship, and helps us to think about oceans and the atmosphere as a big system.  You may find this information useful, since many climatologists are predicting unusually strong El Niño conditions for 2014.

Explaining ENSO

Map from Wikipedia.

El Niño, which means “the (male) child” in Spanish, is actually part of a larger periodic cycle that affects ocean currents around the equator in the Pacific Ocean.  This cycle is called the El Niño Southern Oscillation, or ENSO for short.  Before explaining ENSO, though, let’s look at how the equatorial Pacific Ocean usually works.  A good place to start is the trade winds.  In Hawai’i we know that the trade winds generally blow from the northeast, and are generally very reliable in the summer.  These winds provide a nice breeze and keep the weather pleasant most of the time.  The trade winds are part of a pattern of atmospheric circulation called the Hadley Cell circulation, which we’ve explained elsewhere.  These winds flow across the Pacific towards the equator, and because of the Coriolis effect (from the rotation of the earth), they also blow towards the west.  Related to this is another pattern called the Walker circulation, which causes the prevailing winds to blow towards the west along the equator.  This is because the air pressure is normally high around Tahiti due to sinking air.  The wind always blows out of high pressure areas.  In contrast, by Australia and Indonesia the air pressure is low, which means that air is rising up through the troposphere.  Wind always blows into low pressure areas.  The wind pushes the water towards the west, and so the warm equatorial water tends to pool in the western Pacific around Southeast Asia and Australia.  These are the normal conditions, as you can see in the diagram.  There are some interesting aspects to this circulation pattern.  For example, it causes the thermocline to be deeper in the western Pacific.  It also makes sea level slightly higher in the western Pacific.  When this pattern is particularly strong we call it “La Niña”, which in Spanish means “the (girl) child”.  La Niña conditions tend to decrease rainfall in the eastern Pacific in places like southern California.

As you can see from the maps in the above paragraph, this means that the water temperature is higher in the western Pacific.  This means that there is a lot of evaporation, which provides moisture to the equatorial regions of South and Southeast Asia.  At the same time, the water is much cooler off the coast of South America.  Since the wind normally blows to the west, this draws water away from South America, which in turn pulls up cold water from the depths below.  This nutrient-rich water creates the richest fishing area on the planet.

Graphic from Windows to the Universe.

Reversing the Walker Circulation

Every now again, however, the westward flow of the Walker Circulation weakens or even shuts down all together.  On average, this happens about every 2-8 years.  When the winds weaken, the warm water that has pooled in the western Pacific starts to move back towards the east (1), since the wind is no longer pushing it.  The difference in air pressure between Tahiti and Indonesia is much less in this case, and in some instances the trade winds will actually blow in the opposite direction.  The term “El Niño” refers to these conditions.  El Niño has significant effects not only on ocean temperatures, but wind and weather throughout the Pacific and beyond.  Areas in the eastern Pacific get abnormally wet, whereas the western Pacific is drier.  For example, the strong El Niño in 1998 caused widespread storms across the western part of the United States.

This has indirect effects on humans, as it can lead to droughts in India, China, and other places in Asia.  Historically El Niño conditions have contributed to some of the most devastating famines in history (2).  There are economic effects as well.  For example, when El Niño occurs it shuts down the cold water upwelling off the coast of Peru that plays such a large role in the fisheries there.  The fisheries industry is extremely hard hit when this happens, which in turn affects the entire economy of Peru.  At the same time, the Polynesian wayfinders that initially discovered and settled in Hawai’i and other islands in the Pacific had a keen understanding of El Niño.  The periodic reversal of the trade winds allowed them to expand to islands and archipelagoes that would have been much more difficult to reach under normal conditions.

El Niño’s Impacts on Hawai’i

Now that we understand the basics of El Niño, we can begin to look at how this curious phenomenon affects the weather here in Hawai’i.  Both La Niña and El Niño seem to affect our rainfall patterns.  As you know, Hawai’i has two seasons: the dry Kau season, which runs from about May to September or October, and the wetter Ho’oilo season, which lasts from October to May.  When there is a particularly strong La Niña, the Kau season tends to be significantly wetter, whereas the El Niño phase generally correlates with much lower rainfall totals (and even drought) during the Ho’oilo season.  In addition, during La Niña the normal wet season is often abnormally wet.  For example, in early 2006 Hawai’i experienced 40 straight days of rain during the La Niña wet season.  You can see the pattern illustrated in the rainfall maps of Maui and Kahoolawe.  Since this year is an El Niño year, many experts are expecting drier than average conditions starting in around October.  However, as we were writing this blog it was still uncertain as to how strong the 2014 El Niño would turn out to be.

Climatologists and other researchers at the University of Hawai’i are currently carrying out work to deepen our understanding of ENSO’s effects on the archipelago.  For example, one research project is focusing on how microclimatic variables such as solar radiation, relative humidity, temperature and potential evapotrasporation respond to ENSO phase changes in different seasons here.  Other work focuses on quantifying the effects of ENSO on the dry season, since the dry season is often downplayed as ENSO is generally discussed as a wet season phenomenon in the context of Hawai’i.

Although we understand the basics of ENSO, looking into the future there is still a lot of uncertainty surrounding the phenomenon.  One big unknown is how the ENSO cycle will be affected by climate change.  It is likely that one or more of the physical processes that are responsible for determining the characteristics of ENSO will be modified by climate change, but it isn’t yet possible to reliable speculate as to whether ENSO activity will be enhanced or dampened, or if the frequency of events will change.  Thus there are exciting frontiers of climate research that you might someday contribute to as you continue your studies of geography and atmospheric processes.

Notes 

(1)  This reverse flow is called a “Kelvin wave”.

(2)  For an outstanding account of how El Niño, coupled with colonial administrative policies, contributed to famines in 19th century, see Mike Davis’s Late Victorian

A Storm is Coming: Winter Weather Patterns In Hawaii

Pacific Surface Analysis showing approaching cold front

One of the main reasons that millions of tourists flock to Hawai'i each year is because the weather year round is fairly pleasant and predictable, and not as subject to the seasonal shifts that characterize the climate at higher latitudes.  But if you've spent any time in Hawai'i you've likely noticed that there are indeed seasonal shifts.  In the "summer" it's usually a little bit warmer, but the refreshing tradewinds blow a good bit more regularly, which helps to cool us off and brings rain to windward and mauka areas.  In the "winter" the trades aren't as reliable, and we have more frequent kona winds.  The Hawaiians, being excellent geographers, have names for these two seasons.  The warmer season is called Kau and generally lasts from approximately mid to late April until October, whereas the cooler season is called Ho'oilo and lasts from mid to late October until April.  The changes that come with Ho'oilo are the subject of this blog post.

The Big Picture...

One major feature of Ho'oilo is the periodic occurrence of thunderstorms, which in general are relatively rare in Hawai'i due to the tradewind temperature inversion.   However, in the winter months, cold air and low pressure systems sweep down from the north, bringing occasionally severe weather along with the massive swells that the North Shore is so famous for.  But did you know that these storms are a part of the global system of atmospheric circulation?  It all begins with the earth-sun relationship, which you can read about in a previous post.  Since the earth is tilted, the point on the earth's surface that receives the sun's energy directly shifts over the course of the year, which basically means that the latitude that receives the most energy migrates over the course of the year.  This spot, called the subsolar point, is loosely tied to the Inter-Tropical Convergence Zone (ITCZ), an area of convection (rising air) and thunderstorms that helps to drive the entire global atmospheric circulation system!  You've probably learned in geography class about the ITCZ, which is part of the three cell model of circulation (1).

As with most everything in life, whatever goes up must come down.  This is true for air that rises in coriolis effect (to be discussed in a future post), which twists the path of the air (to the right in the northern hemisphere, to the left in the southern hemisphere.  This part of the global atmospheric circulation is referred to as the Hadley Cell, and there are two of them, one to the north of the ITCZ and one to the south.  You can see the general pattern in the figure below, which shows the circulation when it is summer in the northern hemisphere.

Three Cell Model diagram from here.

the ITCZ.  Once it reaches the top of the troposphere (the lowest layer of the atmosphere where virtually all weather happens), it diverges and circulates to the north and the south, sinking at approximately 30 degrees north and south of the equator, but the latitude at which the air sinks shifts along with the ITCZ and the subsolar point over the course of the year.  The places where this air sinks are high pressure areas, because the sinking air is exerting force on anything below it.  The ITCZ, conversely, is a low pressure area because the air is rising there.  Because of the rotation of the earth, the sinking air is subject to the

How this Affects Hawai'i...

As you can see, a major area of sinking air is usually located to the northeast of Hawaii.  Here in Hawaii we call this high pressure area the "Hawaiian High", but in general it referred to as the Northern Pacific Subtropical Anticyclone.  Anticyclones are areas of sinking air where the wind circulates outward from the high in a clockwise direction.  Note from the graphic the direction that the wind blows coming out of the high.  You should notice that our islands are right in the path of the wind!  This is the source of the tradewinds, which blow about 80% of the time in the Kau season.

July patterns.  Approximately location of Hawai'i denoted with red circle.  Map from here.

When it is winter in the northern hemisphere it is summer in the southern hemisphere, since the subsolar point and ITCZ shift to the south.  Along with this travels the Hadley cells.  Another characteristic of the northern hemisphere winter months is that the Hawaiian High tends to weaken, and so the tradewinds are less consistent.  At the same time, the storm-producing polar front (another part of the global atmospheric circulation), moves to the south.  One major characteristic of the polar front is that it produces low pressure systems that drive cold fronts and produce heavy rainfall and severe weather.  These are the same types of systems that generally bring high snowfall totals to the continent in the winter months.  Hawaii is much further south (and surrounded by the ocean), so with the exceptions of Mauna Kea, Mauna Loa, and Haleakala we don't get any snow.  But a few times a year the cold fronts do sweep down and roll over Kaua'i, Oahu, and the other islands, moving from west to east.

January patterns.  Red circle approximates Hawai'i's location.  Map from here.

When this happens there is a fairly noticeable sequence of atmospheric events that will, if you know what to look for, help you to predict the weather over the next couple of days and amaze your friends.  The first thing that will happen is that the wind will start blowing from the south (Kona).  This happens because the wind blows roughly parallel to an approaching cold front, heading in the direction of the low pressure area that is at the center of the storm system.  The wind will gradually strengthen.  You may also notice a very characteristic cloud progression.  The first clouds you notice will arrive a day or two ahead of the front (depending on how fast the front is moving).  These clouds will be very high (cirrus) clouds and will cover much of the sky.  Then as the front continues to move towards your island, you'll see lower and lower (and thicker, more ominous) clouds appear, until finally the sky is socked in by low cumulus clouds.  The reason that this happens is that the cold air that is approaching is abruptly pushing up the warmer, moist air in front of it.  This causes the air to cool, which leads to cloud formation.

When the front arrives it will bring with it significant rainfall and pretty heavy winds in some cases.  Sometimes the fronts pass quickly, but sometimes they may stick around for a couple of days.  After the front passes, you should notice clear skies, and the direction of the wind will shift; instead of coming from the south it will be coming from the west or northwest.  Then after a day or two if high pressure conditions return to the north of the islands, the trade winds will return.

The entire north Pacific at the time this post was written.  The symbols point in the direction the wind is blowing.  From National Weather Service.

That pretty much sums up winter cold fronts in Hawai'i.  These don't happen in the summer time because the polar front, which is the source of the disturbances, moves northward in the summer time.  So the next time the wind starts to blow from the south, keep your eyes on the sky, and you may be able to apply what you've learned here and in class.  And when you do, you can remember the kilo lani, or "sky watchers", who were special kahunas in Old Hawai'i that had a tremendous amount of knowledge about their natural environment, including the atmospheric conditions and signs that helped them to predict the weather.

Exercises

Notes 

(1)  To be discussed in a future post

How to Outline a Textbook Chapter...

Photo by John Delay

School is starting up, and that means there's a fresh new crop of young, budding geographers eager to begin learning about way the world works.  But besides learning about the way the world works, students should also be working to develop study skills, which will help us not only to do well in class and retain the material that has been covered, but also to organize information and be more effective problem solvers in life in general.  One important skill that all students should master is how to outline a textbook chapter.

Outlining a textbook chapter helps you to distill out the most important concepts and material while organizing it in a way that makes it easy to review.  Many students are reluctant to outline chapters because it takes some time, but I promise that in the long run it really pays off, because you won't have to read the chapter again when it is time for an exam, and it will help you to remember the most important and useful points of the chapter.  For a standard textbook chapter, it generally takes me between 2-3 hours, but this includes a careful reading of the chapter.  It may take you a little longer, or you migth do it a little more quickly than me, but by the end you will have a great understanding of the chapter and you will also know what points are less clear to you so you can ask questions in class.

The steps to outlining a chapter are pretty simple.  Some guides say to read the chapter first, but I always do my outlines while I am reading through the chapter.  I think this is a much more efficient and effective method.  Some things to remember:

1.  Make a separate heading for each section in the chapter, and pay attention to the nested headings (sub-headings) within the chapter, and follow this pattern of organization in your outline.  This helps you keep track of the relationship of the concepts to one another and their relative importance.

2.  Look for the main idea in each section and subsection and include that in your outline.  Then add in the facts and details that seem most relevant to you.  Sometimes this takes some getting used to, but it is useful to omit trivial points.  Always pay attention to the words in bold.  I usually define these under separate sub-headings.

3.  Repeat these steps for each chapter in the paragraph.

Soon you'll have a great, detailed chapter outline that will help you remember what you've read, and you will be able to go over it in a fraction of the time it takes to read the entire chapter.  And if you keep your outlines you'll probably find they are useful in other classes, or if you ever have to prepare a literature review or take comprehensive exams.

Below I've included a sample outline I made of the first chapter of McKnight's Physical Geography, the textbook we use for 101 at Leeward.  Use this as an example.  Your outlining style may be a little different from mine, but this will give you the basic idea.

Good luck, and have a great semester!

EXAMPLE CHAPTER OUTLINE

This outline took me approximately 2.5 hours for a 30 page chapter.

McKnight Chapter 1: Introduction To the Earth

I.  Introduction
A.  What do geographers study?
1.  Tangible things....rainfall, mountains, trees
2.  Less tangible things...language, migration, voting patterns
B.  What is this book about
1.  Fundamental processes in the natural world
C. This chapter sets the stage for the study of physical geography
1.  Important stuff in the chapter
a.  using science to explain natural environment
b.  the "spheres of the earth"
c.  Earth's place in the Solar System
d.  Latitude and Longitude
e.  What causes the seasons
f.  Time zones....how do they work?

II.  Geography and Science
A.  Intro to section
1.  Geography from Greek meaning Earth Description
a.  used to be purely descriptive discipline
B.  Studying the World Geographically
1.  Two basic branches
a.  Physical geography (Environmental)
b.  Cultural Geography (Human)
2.  Fundamental question: "Why what is where and so what?" (4)
3.  Also interested in interrelationships
4.  Global Environmental Change....a broad theme of the book
a.  both human and natural changes
b.  long and short temporal scales
5.  Globalization...another theme running through the book
a.  processes and consequences of an increasingly interconnected world
C.  The Process of Science
1.  Scientific method
a.  Observe phenomena that stimulate a question or problem
b.  Offer an educated guess about the answer (hypothesis)
c.  Design an experiment to test the hypothesis
d.  predict the outcome of the experiment if the hypothesis is supported and if it is not supported
e.  Conduct the experiment and see what happens
f.  Draw a conclusion or formulate a simple generalized rule based ont eh results of the experiment.
2.  Science best though of as a process or even an attitude for gaining knowledge
3.  New observations and new evidence often cause scientists to revise their conclusions and theories or those of others
D.  Numbers and Measurement systems
1.  Two different systems in use
a.  English System (US)...miles, pounds, etc
b.  International System (pretty much everywhere else).

III.  Environmental Spheres and Earth Systems
A.  Earth's Environmental Spheres
1.  Lithosphere....rocks of Earth's crust as well as unconsolidated mineral matter...
2.  Atmosphere...gaseous envelope of air surrounding the Earth
3.  Hydrosphere....comprises water is all its forms....
a.  Cryosphere, or ice and snow, is part of this
4.  Biosphere....all parts where living organisms can exists.
B.  Earth Systems
1.  Definition: a system is a collection of things and processes connected together and operating as a whole (8).
2.  Closed systems....self contained and isolated from outside inclfluences
a.  Earth with respect to matter
b.  Not many other examples
3.  Open Systems....inputs and outputs
a.  most systems are like this.
4.  Equilibrium...when inputs and outputs are in balance over time
a.  If balance changes, equilibrium will be disrupted until a new equilibrium is reached...
5.  Interconnected Systems...most systems are connected with other systems
6.  Feedback Loops....some systems produce outputs that feedback into the system, reinforcing change
a.  Positive feedback loops change the system in one direction
b.  Negative feedback loops inhibit a system from changing
c.  tipping points (thresholds) beyond which the system becomes unstable and changes abruptly until it reaches a new equilibrium.

IV.  Earth and the Solar System
A.  The Solar System
1.  Earth one of 8 planets
2.  lots of other things in the solar system as well
3.  Origins....most think the big bang 13.7 billion years ago
a.  Our solar system 4.5-6 billion years ago from a nebula
4.  Planets
a.  Terrestrial...mercury, venus, earth, mars
i.  smaller, denser, less oblate
b.  Jovian....Saturn, Uranus, Jupiter, Neptune
i.  Larger, more massive, more oblate
B.  The Size and Shape of Earth
1.  The Size of Earth
a.  topographical maps are usually very exaggerated
b.  Relief of the earth isn't very great compared to total size.
2.  The Shape of Earth
a.  Almost, but not quite spherical
b.  Bigger around at equator than through the poles (flattened)
c.  An "oblate spheroid" (12)

V.  The Geographic Grid--Latitude and Longitude
A.  The Geographic Grid
1.  Equator, North Pole, South Pole
2.  Great circles....any plane that passes though the center of the sphere and divides it into two equal halves
a.  this is the largest circle that can be drawn on the sphere
i.  Creates hemispheres
b.  The path between two points on a great circle is always the shortest route (the "great circle route")
3.  Small circles are created by planes crossing through other parts of the sphere
4.  Grid system based on small and great circles.
B.  Latitude: description of location expressed as an angle north or south of the equator
1.  Expressed in degrees, minutes, seconds
2.  Goes from 0-90, N and S
3.  Lines connecting all points of same latitude are called parallels.
a.  these never cross
4.  Descriptive zones of latitudes
a.  low, midlatitude, high, equatorial, tropical, subtropical, polar
5.  Nautical miles...the distance covered by one minute of latitude: 1.15 miles.
C.  Longitude: an angular description of location in the east-west direction.
1.  A line connecting all points of the same longitude is a meridian
2.  Only parallel to one another when they cross the equator
a.  distance between them is not constant.
3.  Establishing the Prime Meridian
a.  problem is that there is no natural baseline for measuring longitude
b.  Prime Meridian through Greenwich England established by international agreement in 1883.
4.  Measuring Longitude
a.  Maximum of 180 degrees
b.  Also uses minutes and seconds
c.  halfway around the world from the PM is the international datae line.
D.  Locating Points on the Geographic Grid
1.  Latitude and longitude together can be used to find an exact location

VI.  Earth-Sun Relations and the Seasons
A.  Earth Movements
1.  Rotation on the access
a.  Takes 24 hours (one day) in counterclockwise (from N pole) direction
b.  The speed of rotation varies depending on latitude
c.  Rotation has several important effects
i.  Coriolis effect: deflection of winds and ocean currents
ii.  Brings all points through increasing then decreasing gravity of the moon, causing tides
iii.  Diurnal (daily) alternation of daylight and darkness
2.  Revolution around the sun
a.  365 days, 5 hours, 48 minutes, and 46 seconds
b.  Orbit is elliptical and so distance between earth and sun varies
i.  Perihelion is when we are closest to the sun (January 3)
ii.  Aphelion is when we are farthest away (July 4)
3.  Inclination of the Earth's axis
a.  imaginary plane of orbit is called the plane of the ecliptic
b.  Earth is tilted at 23.5 degrees off a line perpendicular to this plane
c.  the tilt is always in the same direction throughout the year.
4.  Polarity of the Earth's Axis
a.  Tilt is always in the same direction (axial parallelism).
b.  Combined effects of rotation, revolution, inclination and polarity result in seasonal patterns.
B.  The Annual March of Seasons
1.  Seasonal variation increases in general as you move away from the equator.
2.  Three things really important
a.  Latitude receiving sun from DIRECTION OVER HEAD (declination of the sun)
b.  Solar Altitude (height o the sun above the horizone)
c.  The lengtu of the day.
3.  June Solstice: About June 21
a.  the point in orbit where the north pole is maximum tilted towards sun
b.  Tropic of Cancer (23.5 N latitude) has sun directly overhead.
c.  Longest day in the northern hemisphere, shortest in southern
d.  24 hours of day north of Arctic circle, 24 hours of night south of Antarctic circle
4.  September Equinox: September 22
a.  All locations on earth experience 12 hours of day, 12 hours of night
5.  December Solstice: Around December 21:
a.  The opposite of the June Soltice...
b.  Sun directly overhead at Tropic of Capricorn (23.5 South)
6.  March Equinox: March 20
a.  Same as the September Equinox
C.  Seasonal Transitions
1.  Latitude Receiving the Vertical Rays of the Sun...
a.  Sun rays only strike vertically between Tropic of Cancer and Tropicc of Capricorn, depending on the time of year
b.  analemma is a diagram showingthe latitude of the vertical rays of the sun.
2.  Day Length
a.  At the equator day length is constant...12 hours
b.  Day length changes more seasonally the further you get from the equator
c.  Overall, the annual variation in day length is the least in the tropics and greatest in the high latitudes
3.  Day length in Arctic and Antarctic
a.  these regions experience 24 hours of daylight and 24 hours of darkness over the course of the year.
D.  Significance of Seasonal patterns
1.  Both day length and the angle at which the Sun's rays strike Earth determine the amount of solar energy received at any particular latitude
2.  The higher the sun is in the sky, the more effective is the warming.
3.  Seasons are basically determined by the amount of sunlight a place gets.

VII.  Telling Time
A.  Standard Time
1.  Telegraph and railroad and other technologies increase connectivity creating a need for standard time....
2.  24 time zones of 15 degrees longitude agreed to in 1884.
B.  International Dateline
1.  180th meridian is the international dateline
a.  opposite from the prime meridian.
C.  Daylight Savings Time
1.  Created to conserve energy during WWI in Germany
a.  US begins the policy in 1918.

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