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.