Weather

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Weather

• Quartermaster Requirement 15

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Quartermaster Requirement 15

• Demonstrate your ability to read a barometer,
  thermometer, anemometer, psychrometer, and
  weather vane.
• Be familiar with the Beaufort scale of winds and
  seas.
• Read and understand a local weather bulletin.
• Know how to obtain current marine and weather
  reports from the National Weather Service in your
  area either by telephone or radio.
• Know weather signs for your local area, including
  cloud types, and prepare a 48-hour forecast from
  them.
• Compare your forecast with the actual weather
  that occurred.
• References
• "Weather" on page 235.
• Weather merit badge pamphlet, No. 33274.

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Wind

• Wind is the flow of air commonly classified by
  spatial scale, speed, the types of forces that
  cause them, the geographic regions in which they
  occur, or their effect.
• global winds, such as the wind belts exist
  between atmospheric circulation cells.
• upper-level winds typically include narrow belts
  of concentrated flow called jet streams.
• synoptic-scale winds that result from pressure
  differences in surface air masses in the middle
  latitudes,
• winds that come about as a consequence of
  geographic features, such as the sea breezes.
• mesoscale winds are those which act on a local
  scale, such as gust fronts.
• microscale winds blow on a scale of only tens to
  hundreds of meters and are essentially
  unpredictable, such as dust devils and
  microbursts.

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Wind Forces

• Forces which drive wind or affect it are
• the pressure gradient force
• the Coriolis force
• buoyancy forces
• and friction forces
• Air tends to flow from a region of high pressure
  to a region of low pressure.
• On a rotating planet, flows will be acted upon by
  the Coriolis force
• The two major driving factors of large scale
  global winds are
• the differential heating between the equator and
  the poles
• the rotation of the planet

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Beaufort Scale
Beaufort Wind Scale
BeaufortForce WindspeedKnots Description Sea Condition
0 0 Calm Sea like a mirror
1 1 - 3 Light Air Ripples but without foam crests
2 4 - 6 Light Breeze Small wavelets. Crests do not break
3 7 - 10 Gentle Breeze Large wavelets. Perhaps scattered white horses
4 11 - 16 Moderate Breeze Small waves. Fairly frequent white horses.
5 17 - 21 Fresh Breeze Moderate waves, many white horses
6 22 - 27 Strong Breeze Large waves begin to form white foam crests, probably spray
7 28 - 33 Near Gale Sea heaps up and white foam blown in streaks along the direction of the wind
8 34 - 40 Gale Moderately high waves, crests begin to break into spindrift
9 41 - 47 Strong Gale High waves. Dense foam along the direction of the wind. Crests of waves begin to roll over. Spray may affect visibility
10 48 - 55 Storm Very high waves with long overhanging crests. The surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shock like. Visibility affected
11 56 - 63 Violent Storm Exceptionally high waves. The sea is completely covered with long white patches of foam lying in the direction of the wind. Visibility affected
12 64 Hurricane The air is filled with foam and spray. Sea completely white with driving spray. Visibility very seriously affected.
 
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Clouds

• A visible aggregate of tiny water droplets or ice
  crystals suspended in the atmosphere
• Can exist in a variety of shapes and sizes.
• Some clouds are accompanied by precipitation
  rain, snow, hail, sleet, even freezing rain.
• Generally classified based on characteristics
  such as altitude, appearance, or origin.

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Cloud Appearance

• Clouds may be thick or thin, have well defined
  edges or be very diffuse, appear hair-like,
  cellular, towering, or in sheets, and be
  associated with fair weather or precipitation.
• Most clouds form due to upward vertical motion of
  air.
• Often associated with weather producing
  phenomena, such as fronts, troughs, and low
  pressure systems.
• Topography can also help move air upwards and
  produce clouds.

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Cloud Development

• Water existing as a gas is called water vapor.
• When referring to the amount of moisture in the
  air, we are actually referring to the amount of
  water vapor.
• If the air is described as "moist", that means
  the air contains large amounts of water vapor.
• Common sources of moisture for the United States
  are
• the warm moist air masses that flow northward
  from the Gulf of Mexico and western Atlantic
  Ocean
• the moist Pacific air masses brought onshore by
  the westerlies.
• Moisture is a necessary ingredient for the
  production of clouds and precipitation

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Cloud Type

• Clouds are classified into a system that uses
  Latin words to describe the appearance of clouds
  as seen by an observer on the ground.
• The four principal types of clouds are summarized
  in the table to the left.
• Further classification identifies clouds by
  height of cloud base.
• cloud names containing the prefix "cirr-" are
  located at high levels
• cloud names with the prefix "alto- are found at
  middle levels.

Latin Root Translation
cumulus heap
stratus layer
cirrus curl of hair
nimbus rain
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Cloud Altitude

• High clouds
• Have bases above 18,000 feet
• Primarily composed of ice crystals
• Middle clouds
• Have bases between 7,000 and 18,000 feet
• Contain ice crystals or water droplets
• May be associated with some light precipitation
• Low clouds
• Have bases below 7,000 feet
• Most often composed of water droplets, but can
  have ice crystals in colder climates
• Can produce heavy precipitation, thunder and
  lightning

• Fog cloud in contact with the ground
• Multi-level clouds vertically thick spanning
  multiple layers
• Heavy precipitation producers
• Orographic clouds distinct clouds that form via
  interaction between wind and mountainous terrain
  features
• Produced by the strong flow of air interacting
  with mountainous terrain.

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High Clouds - Cirrus

• High altitude wispy clouds.
• Typically found at heights greater than 20,000
  feet.
• Usually quite thin and often have a hair-like or
  filament type of appearance.
• Composed of ice crystals that originate from the
  freezing of super-cooled water droplets
• Curled up ends are very common features.
• Generally occur in fair weather and point in the
  direction of air movement at their elevation.

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High Clouds - Cirrocumulus

• High clouds that have a distinct patchy and/or
  wavelike appearance
• (Above Right) A patchwork cirrocumulus photo,
  composed of many individual cloud elements
• (Below Right) A wavy cirrocumulus photo with its
  banded linear structure.
• These features are common to all types of
  cumuloform clouds.

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High Clouds- Cirrostratus

• High stratiform clouds that usually blanket the
  sky in ill-defined sheets.
• Usually optically thin and the sun and moon can
  usually shine some light through.
• Usually can't detect distinct cells or sharp
  features.

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Middle Clouds - Altocumulus

• Are either in a patchy, scattered distribution or
  can appear in linear bands.
• composed primarily of water droplets and are
  located between 6,500 and 20,000 feet
• Usually form by convection in an unstable layer
  aloft, which may result from the gradual lifting
  of air in advance of a cold front.
• The presence of altocumulus clouds on a warm and
  humid summer morning is commonly followed by
  thunderstorms later in the day.

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Middle Clouds - Altostratus

• Have a more uniform and diffuse coverage
• Often difficult to detect individual elements or
  features.
• Often confused with high-level cirrostratus.
• A halo is not observed around the sun (or moon)
  when viewed through altostratus as it is with
  cirrostratus
• In fact, the sun (or moon) is only vaguely
  visible through altostratus clouds and appears as
  if it were shining through frosted glass

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Low Clouds - Cumulus

• Usually puffy and often have very distinct edges
  and usually a noticeable vertical development.
• Often have a popcorn-like appearance.
• Cells can be isolated or grouped together in
  clusters.
• Given suitable conditions, fair weather cumulus
  can develop into towering cumulonimbus associated
  with powerful thunderstorms.

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Low Clouds - Stratocumulus

• Generally appear as a low, lumpy layer of clouds
  that is sometimes accompanied by weak intensity
  precipitation.
• Can be widely scattered but are usually
  concentrated closer together in clusters or
  layers and have very little vertical development.
  
• Relatively flat clouds usually lack the sharp
  edges and "popcorn" appearance of most normal
  cumulus clouds.
• Vary in color from dark gray to light gray and
  may appear as rounded masses, rolls, etc., with
  breaks of clear sky in between.

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Low Clouds - Stratus

• Usually the lowest of the low clouds.
• Often appear as an overcast deck but can be
  scattered.
• The individual cloud elements have very
  ill-defined edges compared to most low cumuloform
  clouds (e.g. cumulus and stratocumulus).
• Fog can be considered as a low stratus cloud in
  contact with the ground.
• When the fog lifts, it usually becomes true
  stratus.

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Multi-layer clouds - Cumulonimbus

• Can produce lightning, thunder, heavy rains,
  hail, strong winds, and tornadoes.
• Are the tallest of all clouds that can span all
  cloud layers and extend above 60,000 feet.
• Lower levels consist mostly of water droplets
• At higher elevations, where temperatures are well
  below 0 degrees Celsius, ice crystals dominate
• Usually have large anvil-shaped tops that form
  because of the stronger winds at those higher
  levels of the atmosphere.

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Multi-layer clouds - Nimbostratus

• Often included in many texts as low clouds,
• Vertical extent often goes well into the middle
  cloud region
• Often have even taller cumulonimbus clouds
  embedded within them.
• Are very dark, usually overcast, and are
  associated with large areas of continuous
  precipitation.
• On a gray and rainy day, the sky most will most
  likely be filled with nimbostratus clouds.

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Mammatus Clouds

• Mammatus are pouch-like cloud structures and a
  rare example of clouds in sinking air.
• Sometimes very ominous in appearance, mammatus
  clouds are harmless
• do not mean that a tornado is about to form a
  commonly held misconception.
• In fact, mammatus are usually seen after the
  worst of a thunderstorm has passed.

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Orographic clouds - Cap clouds

• Form when air containing water vapor is uplifted
  on the windward slide of the slope and reaches
  saturation producing liquid water cloud droplets
  and a cloud which can "cap" the summit.

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Orographic clouds - Lenticular clouds

• Lens-shaped clouds that can result from strong
  wind flow over rugged terrain.
• Sometimes stack up like pancakes in multiple
  layers.
• The strong flow produces a distinct up and down
  wavelike pattern on the lee side of the mountain
  or large hill and the lenticular clouds tend to
  form at the peaks of these waves.
• They sometimes are very round and the edges are
  so well defined that they resemble flying
  saucers.
• Often placed into the middle cloud category since
  they are most common at those altitudes.

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Precipitation

• When cloud particles become too heavy to remain
  suspended in the air, they fall to the earth as
  precipitation.
• Precipitation occurs in a variety of forms
• Rain and Hail
• Freezing Rain
• Sleet
• Snow

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Weather Forecasting
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Weather Instruments

• Anemometer
• A device for measuring the velocity or the
  pressure of the wind.
• Derived from the Greek word, anemos, meaning
  wind.
• Weather Vane
• also called a wind vane, a movable device for
  showing the direction of the wind.
• Often are in the shape of cockerels or arrows,
  but a multitude of designs have been used

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Weather Instruments

• Barometer
• An instrument used to measure atmospheric
  pressure by using water, air, or mercury or
  springs.
• Pressure tendency can forecast short term changes
  in the weather.
• Numerous measurements of air pressure are used
  within surface weather analysis to help find
  surface troughs, high pressure systems, and
  frontal boundaries.
• Thermometer
• a device that measures temperature or temperature
  gradient, using a variety of different
  principles.
• derived from thermo from the Greek for heat and
  meter from Gree, meaning to measure
• has two important elements,
• the temperature sensor (e.g. the bulb on a
  mercury thermometer) in which some physical
  change occurs with temperature,
• plus some means of converting this physical
  change into a value (e.g. the scale on a mercury
  thermometer)

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Weather Instruments

• Hygrometers are instruments used for measuring
  humidity.
• Psychrometer
• A simple form of a hygrometer
• consists of two thermometers, one of which
  includes a dry bulb and the other of which
  includes a bulb that is kept wet to measure
  wet-bulb temperature.
• Evaporation from the wet bulb lowers the
  temperature, so that the wet-bulb thermometer
  usually shows a lower temperature than that of
  the dry-bulb thermometer, which measures dry-bulb
  temperature.
• Relative humidity is computed from the ambient
  temperature as shown by the dry-bulb thermometer
  and the difference in temperatures as shown by
  the wet-bulb and dry-bulb thermometers.