Weather Requirements
Weather-Old Salt
A. Know five cloud formations and the weather associated with them. Sketch two types.
Latin Root / Translation / Examplecumulus
stratus
cirrus
nimbus / heap
layer
curl of hair
rain / fair weather cumulus
altostratus
cirrus
cumulonimbus
High-Level Clouds
Cloud types include:
Mid-Level Clouds
Cloud types include:
Low-Level Clouds
Cloud types include:
Clouds with Vertical Development
Cloud types include:
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.
- Describe several ways to judge wind direction and velocity by trees, flags, waves, sails, etc.
FORCE EQUIVALENT SPEED DESCRIPTION SPECIFICATIONS FOR USE ON LAND
10 m above ground
miles/hour knots
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
extendslight 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
oninland 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
walkingagainst 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
Occurschimney-pots and
slates removed).
10*** 55-63 48-55 Storm Seldom experienced inland;
Treesuprooted; considerable
Structuraldamage occurs.
11*** 64-72 56-63 Violent Storm Very rarely experienced;
accompanied by wide-spread
damage.
12**** 73-83 64-71 Hurricane Severe damage
*=Small Craft **=Gale ***=Storm ****=Hurricane
- 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.
Sky Cover / Wind / Fronts / SelectedWeather Symbols
clear / Calm / cold front / warm front / Rain
1/8 / 1-2 knots (1-2 mph) / stationary front / occluded front / Rain Shower
scattered / 3-7 knots (3-8 mph) / trough
/ Thunderstorm
3/8 / 8-12 knots (9-14 mph) / radar intensities
/ Drizzle
4/8 / 13-17 knots (15-20 mph) / tornado (T) #300
/ or Snow
5/8 / 18-22 knots (21-25 mph) / severe thunderstorm (S) #287 / Snow Shower
broken / 23-27 knots (26-31 mph) / Freezing Rain
7/8 / 48-52 knots (55-60 mph) / Freezing Drizzle
overcast / 73-77 knots (84-89 mph) / Fog
obscured / 103-107 knots (119-123 mph) / Haze
missing / Shaft in direction wind is coming from / 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.