Weather 101 Weather for the Modern Mariner This is the first in a series of articles that I hope will assist you in better understanding the macro and micro levels of weather and the difficulties involved with forecasting it. The concepts will be introduced in a structured manner beginning with beginning level concepts first including:
As the articles progress so too will the concepts being introduced. If you happen to be coming in later in the series and are perplexed, you should be able to reference information in previous articles to sort out most of your questions. Google searches are also particularly helpful for accessing fundamental information about weather but in the event you have a specific question you are unable to find information on send me the question and I will do my best to answer them. Why is there “weather” in the first place? The root of all “weather” is the sun and its interaction with our planet’s atmosphere and surface layers. The heating of the sun is uneven throughout our planet as there are distinct differences in the sun’s angle of attack to areas of the planet, differences in surface topography and physical make up, and the simply fact that ½ of the planet is in daylight while the ½ half is receiving little or no energy at night. The differences in heating leads to differences in pressure and this leads to what we perceive as wind. From this point forward I want you to think of air the same way you think of water from a fluid dynamics standpoint. Air will always seek to equalize itself and “pool” with the air flowing down from high points and settling in low points. High pressure in the atmosphere simply represents a mountain of air and the Low pressure systems represent valleys. This seems simple enough but now we will introduce the effect known a “coriolis” and it is important to the mariner because it is what allows what would otherwise be a grouping of intense but generally benign thunderstorms to become an organized spinning area of low pressure known as a tropical cyclone or “Hurricane” or “typhoon”. Coriolis defined: “A force per unit mass that arises solely from the earth's rotation, acting as a deflecting force. It is dependent on the latitude and the speed of the moving object. In the Northern Hemisphere, air is deflected to the right of its path, while in the Southern Hemisphere, air is deflected to the left of its path. It is greatest at the poles, North and South, and almost nonexistent at the equator.” With regard to tropical cyclones and the coriolis effect- The tropical cyclone is set apart from a traditional cyclone by having “closed circulation”. If you study weather images you will note that traditional weather vortexs (Low pressure centers) are generally “comma shaped”. [fig. 1] while tropical systems are overall circular in form with convection centered around a rotating core or “eye” [fig 2].
The Earth and its balancing act: The air near the equator is the warmest on the planet as it receives the most energy from the sun while the polar areas are by far the coolest receiving almost no solar energy. These differences in pressure manifest themselves into wind as these mountains and valleys of air seek to balance themselves. High pressure centers or “anti-cyclones” have clockwise rotation in the Northern hemisphere and are generally noted as having subsiding (sinking) air, light to calm winds and fair weather. While high pressure generally means nice weather it can also bring dense fog and this remains one of the greatest threats to mariners operating without radar. Low pressure or a cyclone with winds rotating counter clockwise in the northern hemisphere generally brings clouds, wind and inclement weather. It is the very differences between how high the mountain of high pressure is versus how low the valley of low pressure is and how close together they are relative to one another that will ultimately determine the amount of wind you will see at the surface. This slope or “pressure gradient” should be read like a topographical map to a geographer. Surface pressure is measured in two ways: mb of pressure and inches of mercury. “Inches” on the Bar and a scale measured in “millibars”. No matter how you describe pressure it all means the same thing. The average sea level pressure worldwide is approximately 1013.25mb (29.92 inches of mercury). Equal lines of pressure on a map are called “isobars”. The tighter the isobars are packed together the steeper the slope or “gradient” and the windier the areas in the tightly packed lines of pressure will be. For example: On the map above, pretty much all of Idaho (1028mb HIGH) will be having fair weather and very light winds while folks in the upper mid west will be experiencing breezy to windy conditions with areas of light to moderate precipitation as indicated in the green areas and tightly packed isobars. The center of the low reads 1004mb or a gradient of 24 mb over the gradient as a whole but a full 20mb difference between western South Dakota (1024mb) and the edge of the low in eastern Wisconsin, a distance of just 1000 miles. Cold fronts- Cold fronts are masses of cold air that push warm and sometimes humid air upwards into the atmosphere where it cools and condenses often forming hazardous weather. A cold front on a weather map is denoted by lines barbed with triangles. A warm front is just the opposite. Warm (lines on a map with parabolas on them) over run and generally lay on top of the previously existing cold air. Stationary fronts are a mixture of each from a symbol standpoint (triangles and semicircles). Inclement weather can occur along any of these boundaries where the air masses differ in temperature and “Rh” or Relative Humidity. Jet Stream- The jet streams are high level wind currents that circumnavigate the earth often at high rates of speed. Wind velocities in these rivers of air which separate air masses often are in the 50-90mph range but reach speeds in excess of 250mph! This can have dramatic affects on thunderstorms and can restrict the movements of artic air masses. The jet stream operates between approximately 25,000 and 35,000 feet above the Earth’s surface. Now that we understand the macro cogs in the wheel of the weather machine we can spend the next issue on micro climate functions such local topography, wind currents affected by obstructions and local events like the Santa Ana winds, the Chinook and a Sundowner. There is some interesting activity down in the tropics right now and I may issue a supplementary issue should the forecast maps be correct in bringing a tropical system into Southern California late next week. (unlikely, but possible) 09/15/06 |
