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Or A Few Useful Things You Should Know Before Heading Out on a Daysail in the Harbor Or Fascinating Trivia You Can Us

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Or A Few Useful Things You Should Know Before Heading Out on a Daysail in the Harbor Or Fascinating Trivia You Can Us

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    1. 1 Or A Few Useful Things You Should Know Before Heading Out on a Daysail in the Harbor Or Fascinating Trivia You Can Use to Impress People Jonathan Taylor Manhattan Sailing Club 2005 Winter Educational Seminars

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    3. 3 Tonight’s Topics The Raw Ingredients – Earth, Wind (the Atmosphere), and Fire (the Sun) Global Scale Weather Patterns Medium Scale Weather Patterns How to Read a Weather Map Local Scale Weather Patterns

    4. 4 What is the Atmosphere? The atmosphere is a mixture of molecules (mostly nitrogen, oxygen, and water vapor) in a gas state and fine particles of dust and other crud It would like to float off into space but is held to the planet by gravity 90% of the contents of the atmosphere is in the bottom layer which is called the troposphere The troposphere is about 9 miles thick at the equator and 5 miles thick at the poles Most weather takes place in the troposphere

    5. 5 Important Properties of the Atmosphere Every blob of air has:

    6. 6 Structure of the Atmosphere

    7. 7 Relationship of Height, Pressure, and Temperature Rule 1: Pressure decreases with increasing height Rule 2: Temperature decreases with decreasing pressure

    8. 8 Temperature Effects on Air Fact: Warmer air is less dense than cooler air. (If you heat a blob of air, the air molecules get further apart, so the blob expands and becomes less dense) Fact: Warmer air rises. (A blob of less dense air is lighter than the air around it, so it rises Also: Cooler air sinks.

    9. 9 Humidity Air can contain only so much water vapor before the water vapor begins to condense into droplets The amount of water vapor actually present compared to the maximum amount that the air could contain is called the relative humidity Fact: Warmer air can hold more water vapor before condensing than cooler air can

    10. 10 Humidity The temperature at which the air can hold no more water in vapor form is called the dew point. Relative humidity would be 100% When air is cooled to its dew point visible water droplets begin condensing out of the air forming clouds (or fog, which is just a cloud at the surface) and possibly precipitation

    11. 11 What Makes Weather? (Why doesn’t the atmosphere become homogenous and just sit there?) The sun heats the Earth’s surface unevenly: Equatorial surfaces receive more radiation than polar surfaces Regions of maximum radiation move north and south with the seasons Land heats and cools faster than water Different types of land surface heat and cool faster than others Clouds can act as a blanket to keep heat from radiating off into space Lower level atmosphere takes on the heat and humidity characteristics of the underlying Earth surface – warm vs. cold, humid vs. dry The atmosphere is constantly trying to reach equilibrium (thermodynamics is beyond the scope of this course!) Oh yes, and butterflies!

    12. 12 Uneven Heating Puts the Atmosphere in Motion A warmer area on the Earth warms the air above it The warmer air rises, removing some air from the surface The surface air pressure drops as air is removed To fill the “void”, air begins to flow in from areas of higher pressure – WIND! The rising air eventually cools and descends

    13. 13 What Causes Clouds and Rain? Clouds and precipitation are caused by rising air As air rises, the pressure decreases As pressure decreases, the temperature decreases Temperature reaches dew point, water droplets condense out of the air Fair weather is caused by sinking air As air sinks, the pressure increases As pressure increases, the temperature increases Moisture evaporates, air dries out

    14. 14 Coriolis Force Coriolis Force - An “apparent” force caused by the rotation of the Earth Fact: The Coriolis force deflects wind to the right in the northern hemisphere, to the left in the southern hemisphere Fact: The faster the wind, the greater the deflection Fact: The closer to the poles, the greater the deflection

    15. 15 Global Scale Weather Patterns Imaginary global airflow if there were no Coriolis Force:

    16. 16 Global Scale Weather Patterns Global airflow with the Coriolis Force:

    17. 17 Global Scale Weather Patterns The Jet Stream A “river of air” 6-7 miles up - averages 50 knots; can exceed 200 knots! The position of the Jet Stream undulates north and south as low altitude temperature boundaries change (wave troughs indicate southward push of cold surface air, ridges indicate northward push of warm surface air) The Jet Stream is weak and mostly over southern Canada in the summer Through friction on the air below, the Jet Stream and the waves embedded in it steer the weather systems below… …and the Polar Front is generally below!

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    19. 19 Position of Jet Stream on Thanksgiving Morning 2004

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    21. 21 Medium (“Synoptic”) Scale Weather Patterns High pressure systems Formed by a column of descending air Little horizontal pressure difference until you get to the edge (i.e. little wind in the interior, most wind is at the edges) Coriolis force keeps air from rushing away from high pressure, so system takes on a life of its own Sinking air – so fair weather

    22. 22 Low Pressure Systems Cooler surface air pushes warmer surface air upward; warmer air pushes over cooler air Rising air removes mass from surface, pressure falls Air rushes towards the low, deflected towards the right by Coriolus, can’t fill the void, system takes on a life of its own If air mixes enough, will form complete circulation Jet Stream trough has the required energy to cause complete circulation – lows form near Jet Stream troughs Rising air – so clouds and precipitation

    23. 23 Air Masses Global weather also creates air masses – giant blobs of air with uniform temperature, humidity, and pressure (at a given height) When they form, air masses take on the characteristics of the surface underlying them Once formed, they move over the earth steered by the jet stream and surface winds

    24. 24 Fronts Boundaries between air masses are called fronts

    25. 25 How to Read a Medium-Scale Weather Map (Such as one you’d find in the paper or on the Internet)

    26. 26 Surface Weather on Thanksgiving Morning 2004

    27. 27 Pressure Systems

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    31. 31 Air Masses and Fronts

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    33. 33 So What’s the Weather in NYC?

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    35. 35 Going on a Trip? About a week before you arrive, start looking at the daily weather maps for the area and get a sense for what weather systems are present and what they’re doing day-by-day That way, when you arrive you will know what the weather will be like for several days out

    36. 36 Local Scale Weather (Not on the weather map!)

    37. 37 Local Scale Weather (Not on the weather map!)

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