Download Weather Requirements

Weather Requirements
Weather-Old Salt
A. Know five cloud formations and the weather associated with them. Sketch two types.
Latin Root
cumulus
stratus
cirrus
nimbus
Translation
heap
layer
curl of hair
rain
Example
fair weather cumulus
altostratus
cirrus
cumulonimbus
High-Level Clouds
Cloud types include:
Cirrus Clouds
The most common form of highlevel clouds are thin and often
wispy cirrus clouds. Typically
found at heights greater than
20,000 feet (6,000 meters), cirrus
clouds are composed of ice
crystals that originate from the
freezing of supercooled water
droplets. Cirrus generally occur in
fair weather and point in the
direction of air movement at their
elevation.
Cirrostratus Clouds
Cirrostratus are sheet-like, high-level clouds
composed of ice crystals. Though cirrostratus
can cover the entire sky and be up to several
thousand feet thick, they are relatively
transparent, as the sun or the moon can easily
be seen through them. These high-level clouds
typically form when a broad layer of air is
lifted by large-scale convergence. Sometimes
the only indication of their presence is given
by an observed halo around the sun or moon.
Halos result from the refraction of light by the
cloud's ice crystals. Cirrostratus clouds,
however, tend to thicken as a warm front
approaches, signifying an increased production
of ice crystals. As a result, the halo gradually
disappears and the sun (or moon) becomes less
visible.
Mid-Level Clouds
Cloud types include:
Altocumulus Clouds
Altocumulus may appear as parallel bands (top
photograph) or rounded masses (bottom
photograph). Typically a portion of an
altocumulus cloud is shaded, a characteristic
which makes them distinguishable from the
high-level cirrocumulus. Altocumulus clouds
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.
Altostratus Clouds
High-level clouds form above 20,000 feet
(6,000 meters) and since the temperatures
are so cold at such high elevations, these
clouds are primarily composed of ice
crystals. High-level clouds are typically
thin and white in appearance, but can
appear in a magnificent array of colors
when the sun is low on the horizon.
Usually the clouds are like high layers or
stratas.
Low-Level Clouds
Cloud types include:
Nimbostratus Clouds
Nimbostratus are dark, low-level
clouds accompanied by light to
moderately falling precipitation.
Low clouds are primarily composed
of water droplets since their bases
generally lie below 6,500 feet
(2,000 meters). However, when
temperatures are cold enough, these
clouds may also contain ice
particles and snow.
Stratocumulus Clouds
Stratocumulus clouds generally
appear as a low, lumpy layer of
clouds that is sometimes
accompanied by weak intensity
precipitation. Stratocumulus
vary in color from dark gray to
light gray and may appear as
rounded masses, rolls, etc., with
breaks of clear sky in between.
Clouds with Vertical Development
Cloud types include:
Cumulus Clouds
Fair weather cumulus have the
appearance of floating cotton and
have a lifetime of 5-40 minutes.
Known for their flat bases and
distinct outlines, fair weather
cumulus exhibit only slight vertical
growth, with the cloud tops
designating the limit of the rising
air. Given suitable conditions,
however, harmless fair weather
cumulus can later develop into
towering cumulonimbus clouds
associated with powerful
thunderstorms.
Cumulonimbus clouds
Cumulonimbus clouds (Cb) are
much larger and more vertically
developed than fair weather
cumulus. They can exist as
individual towers or form a line
of towers called a squall line.
Fueled by vigorous convective
updrafts (sometimes in excess
50 knots), the tops of
cumulonimbus clouds can easily
reach 39,000 feet (12,000
meters) or higher.
Lower levels of cumulonimbus clouds consist mostly of water droplets while at higher
elevations, where temperatures are well below 0 degrees Celsius, ice crystals dominate.
Under favorable atmospheric conditions, harmless fair weather cumulus clouds can
quickly develop into large cumulonimbus clouds associated with powerful thunderstorms
known as supercells.
Thermometers measure air temperature.
Thermometers work because matter expands
when heated. Most thermometers are closed
glass tubes containing liquids such as
alcohol. When air around the tube heats
the liquid, the liquid expands and moves
up the tube. A scale that shows the
temperature is on, or attached to, the
tube.
A barometer measures air pressure. An
Italian scientist named Torricelli
built the first barometer in 1643.
People still use mercury barometers
based on Torricelli's design to measure
air pressure.
A psychrometer measures relative
humidity, using the cooling effect
of evaporation. Two thermometers
are used in a psychrometer. Notice
that a wet cloth covers the end of
one of the thermometers. Water
evaporates from this cloth, causing
the temperature on that thermometer
to be lower than the other.
An anemometer, like the one shown
in the picture, measures wind
speed. The cups catch the wind,
turning a dial attached to the
instrument. The dial shows the
wind speed. Wind speed can be
estimated without special tools
however.
B. Describe several ways to judge wind direction and velocity by trees, flags, waves, sails,
etc.
FORCE
EQUIVALENT SPEED
10 m above ground
miles/hour knots
DESCRIPTION
SPECIFICATIONS FOR USE ON LAND
0
0-1
0-1
Calm
Calm; smoke rises vertical
1
1-3
1-3
Light air
Direction of wind shown by
smoke drift, but not by wind
vanes.
2
4-7
4-6
Light Breeze
Wind felt on face; leaves
rustle; ordinary vanes moved
by wind.
3
8-12
7-10
Gentle Breeze
Leaves and small twigs in
constant motion; wind
extends light flag.
4
13-18
11-16
Moderate Breeze
Raises dust and loose paper;
small branches are moved.
5*
19-24
17-21
Fresh Breeze
Small trees in leaf begin to
sway; crested wavelets form
on inland waters.
6*
25-31
22-27
Strong Breeze
Large branches in motion;
whistling heard in telegraph
wires; umbrellas used with
difficulty.
7*
32-38
28-33
Near Gale
Whole trees in motion;
inconvenience felt when
walking against the wind.
8**
39-46
34-40
Gale
Breaks twigs off trees;
generally impedes progress.
9***
47-54
41-47
Severe Gale
Slight structural damage
Occurs chimney-pots and
slates removed).
10***
55-63
48-55
Storm
Seldom experienced inland;
Trees uprooted; considerable
Structural damage occurs.
64-72
56-63
Violent Storm
Very rarely experienced;
accompanied by wide-spread
damage.
12**** 73-83
64-71
Hurricane
11***
*=Small Craft
**=Gale
***=Storm
Severe damage
****=Hurricane
C. Be able to read an official weather map and to interpret and understand television and
radio weather reports.
A line connecting points of equal pressure is called an isobar. That means, that
at every point along a given isobar, the values of pressure are the same.
Isobars are represented by solid blue contours. An image of sea-level pressure
reports and isobars has been given below.
The black numbers are station reports of sea-level pressure in millibars. Isobars
are normally drawn at 4 mb intervals, with 1000 mb being the base value. The
small blue numbers are contour labels, which identify the value of an isobar
(for example 1004 mb, 1012 mb, etc.).
The isobars displayed here have been generated from these pressure
observations. For example, pressure reports in South Dakota are 1002.5 mb and
1000.4 mb respectively, and notice that they are contained within the region
bound by the 1000 mb and 1004 mb contours.
An area of relatively lower pressure is centered in western North Dakota, while
the pressure increases outward from this region.
Winds flow roughly parallel to the isobars, as depicted in the schematic below.
This relationship is observed on real surface maps.
In the example below, surface observations and isobars (yellow contours) have
been plotted. Reports of wind direction from Nebraska into Minnesota, show
that the observed wind direction is nearly parallel to the isobars.
The solid black contours represent pressure contours (isobars) in millibars. The isobars
have a contour interval of four millibars. The wind speed is directly related to the
distance between the isobars. The closer the isobars are together, the stronger the pressure
gradient, and the stronger the wind.
The colored regions represent the surface temparature. The contour interval of the
isotherms is 5 degrees Fahrenheit. From the chart above you can sometimes find warm
and cold fronts. Fronts are usually located where temperature changes drastically over a
short distance.
When pressure contours are perpendicular to isotherms it means it is either getting
warmer or colder. By knowing that winds flow counter-clockwise around a low and
clockwise around a high, one can usually see whether there is warming or cooling going
on. Usually when the winds are from the south, and you have isotherms (temperature
lines) perpendicular to the isobars (pressure lines) you have warm air advection (warm air
moving up from the south). The opposite is true if you have winds from the north and
isotherms perpendicular to isobars. In that case you have cold air advection (cold air
coming in from the north) going on.
Map Symbols
Sky Cover
clear
1/8
Wind
Calm
1-2 knots (1-2 mph)
Selected
Weather Symbols
Fronts
cold
front
warm
front
stationary
front
occluded
front
Rain
Rain Shower
trough
scattered
3-7 knots (3-8 mph)
Thunderstorm
radar intensities
3/8
8-12 knots (9-14 mph)
4/8
13-17 knots (15-20 mph)
5/8
18-22 knots (21-25 mph)
broken
23-27 knots (26-31 mph)
Freezing Rain
7/8
48-52 knots (55-60 mph)
Freezing Drizzle
Drizzle
tornado (T) #300
or
Snow
severe thunderstorm (S)
overcast
obscured
missing
73-77 knots (84-89 mph)
103-107 knots (119-123 mph)
Shaft in direction wind is coming from
#287
Snow Shower
Fog
Haze
Smoke
Dust or Sand
Blowing Snow
Keep a weather chart for one week and predict local weather conditions twelve to twenty-four
hours in advance each day.
Weather- Helmsman
G. Weather knowledge as applied to small craft operations
H. Weather warnings
Small Craft Advisory: To alert mariners to sustained (more than two hours) weather or
sea conditions, either present or forecast, that might be hazardous to small boats. The threshold
conditions for the Small Craft Advisory are usually 18 knots of wind (less than 18 knots in some
dangerous waters) or hazardous wave conditions.
Gale Warning: a warning of winds within the range of 39 – 54 mph (34 – 47 knots). Gale
warnings may precede or accompany a hurricane watch.
Storm Warning: A warning of winds within the range of 55 – 73 mph (48 – 63 knots).
Hurricane Watch: An advance statement, not a warning, indicating that a hurricane is
approaching and attention should be given to subsequent advisories. It implies the possibility of
dangerous conditions within 24 to 48 hours. Precautionary action should be taken in case
hurricane warnings are forthcoming.
Hurricane Warning: A warning that indicates that hurricane winds of 74 mph (64 knots) and
higher, or a combination of dangerously high water and rough seas, are expected to impact
a specified coastal area. When a hurricane warning is announced, hurricane conditions are
considered imminent and may begin immediately, or at least within the next 12 to 24 hours.
When a warning is announced, it is of utmost importance that precautionary measures are
taken for protection of life and property.
Hurricane: A violent storm originating over tropical waters, with winds near its center reaching 74
mph and higher. In size, the storm may range from 50 to 1,000 miles in diameter.