Chapter 8
Weather
Several times a day, ships receive weather reports for coastal areas and predictions for periods of 12, 24, and 36 hours. These predictions are based on reports from ships and weather stations around the world and from satellites. However, ships not in the general area for receiving weather predictions must do their own predicting from data obtained by limited means. You must know something about the use of a few of the weather instruments. To understand their value, you must also know something about the basic elements of the weather, clouds, and cloud formations; and how these cloud formations can be used as an indicator of oncoming weather conditions.
WEATHER INSTRUMENTS
| 8-1. Weather conditions such as air temperature and humidity, wind direction, and velocity are measured by a variety of instruments. This chapter describes how to use and how to interpret readings from some typical instruments. | |
|
THERMOMETERS |
|
| 8-2. A thermometer is an instrument for measuring temperature. It is a glass tube of small bore in which either alcohol or mercury expands and contracts with the rise and fall of the temperature of the surrounding air. | |
| 8-3. Most thermometers are mercury-filled and almost all of them use the Fahrenheit scale, in which the freezing point of water is 32° and boiling point is 212° . However, temperature in meteorology sometimes is expressed according to the Celsius (formerly centigrade) scale in which the freezing point of water is 0° and boiling point is 100° (Figure 8-1). | |
| 8-4. You might be required to convert a Fahrenheit reading to Celsius or vice versa. Since 32° F is equivalent to 0° C, to change a Fahrenheit reading to Celsius you first subtract 32° and then multiply the remainder by 5/9. Say you want to change 41° F to Celsius. Subtracting 32° from 41° gives you 9° . Multiplying 9° by 5/9 gives you 45/9, or 5° C. | |
|
|
Figure 8-1. Celsius and Fahrenheit Scales
| 8-5. To change from Celsius to Fahrenheit, simply reverse the procedure. First multiply the Celsius temperature by 9/5, then add 32° . In the previous example, to change 5° C back to Fahrenheit, you first multiply it by 9/5, which gives you 45/5 or 9° . Adding 32° gives you 41° F. | |
| 8-6. When reading a thermometer, DO NOT touch the lower part of the glass which contains the alcohol or mercury. The heat from your body will cause the liquid to rise in the tube. Your eyes should be on a level with the top of the column to get an accurate reading. You will notice that the top of the column is curved. On a mercury thermometer, the reading is taken at the top of the curve. On an alcohol thermometer, the reading is taken at the bottom of the curve. |
| 8-7. Many do not consider air as having weight. However, under normal conditions, a column of air (1-inch by 1-inch) extending to the top of the atmosphere weighs 14.7 pounds. "Normal conditions" means at sea level with the temperature at 59° F and the air charged with a certain amount of water vapor. We do not refer to this as the weight of air, but as atmospheric pressure, which (in our system of weights and measures) is measured in pounds per square inch. Barometers are instruments for measuring atmospheric pressure. |
| 8-8. The type of barometer used aboard ship is the aneroid (Figure 8-2). The term "aneroid" means without fluid. A barometer of this type contains a metal cylinder from which much of the air has been removed. Outside pressure will make the thin metal ends expand and contract. By means of linkages, this motion is magnified and transmitted to a pointer that shows the pressure on a scale on the face of the instrument. Scales are marked in inches and hundredths of inches. Some instruments also have a millibar scale. Millibars are units of pressure (of the metric system) rather than units of length (Figure 8-3). An aneroid barometer is unaffected by temperature, so readings need not be corrected for changes in temperature. |
Figure 8-2. Aneroid Barometer
Figure 8-3. Inches and Millibars
| 8-9. Barometer readings, along with those of the thermometer and psychrometer, can be used to make short-range forecasts of local weather. A single observation, however, is meaningless, and the actual readings are unimportant. Direction and the rate of change are important. You must note whether the change was rapid or gradual or, if the readings are steady, the length of time the condition has existed. | |
| 8-10. Each day there is a normal rise and fall of the pressure, with the highs occurring about 1000 and 2200 and the lows coming about 0400 and 1600. The average change during these periods is about 0.05 inches, or 0.01 inches per hour. These daily changes, called diurnal changes, must not be overlooked when considering the amount of change in readings. For example, assume the pressure drops from 30.15 inches to 30.07 inches. At first glance, the amount of drop would seem to be considerable, but, if the normal diurnal drop of 0.05 inches is subtracted from this, the 0.03 inches remaining is insignificant for a 6-hour period. If the same drop of 0.08 inches had occurred between 0400 and 1000, when the barometer normally rises about 0.05 inches, the drop would have to be considered significant. | |
| 8-11. A set of averages is useful in forecasting weather and for middle latitudes. A reading of 29.50 inches is considered low, 30.00 inches is high. | |
| 8-12. The violence and speed of an approaching storm is indicated by the rate and amount of fall of the barometer. When local weather conditions remain unchanged and the barometric pressure drops, a distant storm is raging. If the average fall per hour is from 0.02 to 0.06 inches, the distance from the center of the storm is roughly 250 to 150 miles. If the fall is 0.06 to 0.08 inches or 0.12 to 0.15 inches, the distance is about 150 to 100 miles or 80 to 50 miles, respectively. | |
| 8-13. Some general rules that will help when using the barometer are listed below. | |
|
Table 8-1. Weather Forecasting Table
|
WIND |
BAROMETER READING AT |
CHARACTER OF WEATHER |
|
SW to NW |
30.10 to 30.20 and steady. (1019.3 to 1022.7 millibars) |
Fair with slight temperature changes for 1 or 2 days. |
|
SW to NW |
30.10 to 30.20 and rising rapidly. (1019.3 to 1022.7 millibars) |
Fair followed within 2 days by rain. |
|
SW to NW |
30.20 and above and stationary. (1022.7 millibars) |
Continued fair with no decided temperature change. |
|
SW to NW |
30.20 and above and falling slowly. (1022.7 millibars) |
Slowly rising temperature and fair for 2 days. |
|
S to SE |
30.10 to 30.20 and falling slowly. (1019.3 to 1022.7 millibars) |
Rain within 24 hours. |
|
S to SE |
30.10 to 30.20 and falling rapidly. (1019.3 to 1022.7 millibars) |
Wind increasing in force; rain within 12 to 24 hours. |
|
SE to NE |
30.10 to 30.20 and falling slowly. (1019.3 to 1022.7 millibars) |
Rain in 12 to 18 hours. |
|
SE to NE |
30.10 to 30.20 and falling rapidly. (1019.3 to 1022.7 millibars) |
Increasing wind and rain within12 hours. |
|
E to NE |
30.10 and above and falling slowly. (1019.3 millibars) |
In summer with light winds, rain may not fall for several days. In winter, rain in 24 hours. |
|
E to NE |
30.10 and above and falling rapidly. (1019.3 millibars) |
In summer, rain probably in 12 hours. In winter, rain or snow with increasing winds will often set in when the barometer begins to fall and the wind sets in from the NE. |
|
SE to NE |
30.00 or below and falling slowly. (1015.9 millibars) |
Rain will continue 1 or 2 days. |
|
SE to NE |
30.00 or below and falling rapidly. (1015.9 millibars) |
Rain with high winds, followed within 36 hours by clearing and, in winter, colder temperatures. |
|
S to SW |
30.00 or below and rising slowly. (1015.9 millibars) |
Clearing within a few hours and fair for several days. |
|
S to E |
29,80 or below and falling rapidly. (1009.1 millibars) |
Sever storm imminent, followed within 24 hours by clearing and, in winter, colder temperatures. |
|
E to N |
29.80 or below and falling rapidly. (1009.1 millibars) |
Severe NE gale and heavy rain; in winter, heavy snow followed by a cold wave. |
|
Going to W |
29.80 or below and rising rapidly. (1009.1 millibars) |
Clearing and colder. |
| 8-14. Another factor that plays an important part in our weather is humidity. Humidity is the amount of water vapor (water in a gaseous state) in the air. Any given volume of atmosphere at a given temperature can hold only a certain amount of water vapor. If more and more water vapor is added to the air, the saturation point eventually will be reached and some of the water vapor will condense, or become liquid. The condensation takes the form of fog cloud, dew, rain, or other precipitation. Relative humidity is the ratio of the amount of water vapor in the air to the total amount that the air can hold at the saturation point, or 100 percent humidity. | |
| 8-15. The warmer the air is the more water vapor it will hold. Therefore, cooling a volume of air will reduce its capacity to hold water vapor. If the cooling is continued, the dew point (the temperature at which moisture suspended in the atmosphere will begin to form dew will be reached, and the water vapor will condense and form clouds, dew, or fog. Readings taken from a psychrometer are used to compute relative humidity and dew point. | |
| 8-16. There are two types of instruments used aboard ship to determine relative humidity and the dew point. These two instruments look different, and a different method is used to get a reading, but both instruments will give you the same results. | |
| 8-17. A hygrometer consists of two thermometers mounted vertically in a ventilated case or box (Figure 8-4). One thermometer, known as the dry bulb, has a mercury bulb exposed directly to the air. The other thermometer, known as the wet bulb, has a bulb covered with muslin. The muslin is stretched tightly around the bulb and kept moist by a wick immersed in a small cup filled with water. The wick consists of a few threads of lamp cotton long enough to allow 2 or 3 inches of it to be coiled in the cup. The muslin is kept thoroughly moist, but not dripping, at all times. | |
| 8-18. A sling psychrometer also consists of two thermometers (Figure 8-5). They are mounted together on a single strip of material and fitted with a swivel link and handle. | |
| 8-19. One thermometer is mounted a little lower than the other and has its bulb covered with muslin. When the muslin covering is thoroughly moistened and the thermometer well ventilated, evaporation will cool the bulb of the thermometer causing it to show a lower reading than the other thermometer. With the sling psychrometer, twirling the thermometers by using the handle and swivel link causes ventilation. The uncovered thermometer shows the dry-bulb temperature reading and the muslin-covered thermometer shows the wetbulb temperature reading. |
Figure 8-4. Hygrometer
Figure 8-5. Sling Psychrometer
| 8-20. The dry-bulb thermometer records the temperature of the free air. The wet-bulb thermometer records what is known as the temperature of evaporation, which is always less than the temperature of free air. | |
| Note: The difference between the temperature readings of the dry-bulb and the wet-bulb shows how close the air is to a state of saturation. | |
| 8-21. When the wet- and dry-bulb temperatures are known, the relative humidity of the atmosphere may be found by referring to Table 8-2 for determining relative humidity. The table may be readily understood by reviewing the following example. | |
| 8-22. Assume the temperature of the air (dry-bulb) is 60° and the temperature of evaporation (wet-bulb) is 56° ; the difference is 4° . Look in the column headed "Temperatures of the air;" find 60° and follow the same horizontal line across to the column headed "4° ." Here the figure "78" will be found. This means that the air is 78 percent saturated with water vapor. The amount of water vapor present in the atmosphere is 78 percent of the total amount it could carry at the given temperature (60° ). The total amount or saturation is represented by 100 percent. Any increase in the amount of vapor beyond this point would show in the form of mist or rain. The relative humidity over the ocean's surface is generally about 90 percent; it is even higher in the doldrums. Due to this increased moisture, the relative humidity at sea is normally higher than that cited in the above example. | |
| 8-23. The dew point spread is the number of degrees between the actual temperature (dry-bulb) and the dew point. Use Table 8-3, to find the temperature at which dew will begin to form. Example: The dry-bulb temperature is 60° and the wet-bulb reads 56° , the spread between the dry-bulb and wet-bulb reading is 4° . Using the table, read down for the value of 4° and across to the columns for 60° and you find a value of 7. This 7° tells you that there is a 7° dew point spread. This 7° spread is subtracted from the dry-bulb temperature of 60° , and that tells you that 53° is the dew point temperature. |
Table 8-2. Determining Relative Humidity
|
TEMPERATURE OF THE AIR, DRY-BULB (THERMOMETER) |
DIFFERENCE BETWEEN DRY-BULB AND WET-BULB READINGS (PERCENT) |
|||||||||
|
1° |
2° |
3° |
4° |
5° |
6° |
7° |
8° |
9° |
10° |
|
|
24 |
87 |
75 |
62 |
50 |
38 |
26 |
||||
|
26 |
88 |
76 |
65 |
53 |
42 |
30 |
||||
|
28 |
89 |
78 |
67 |
56 |
45 |
34 |
24 |
|||
|
30 |
90 |
79 |
68 |
58 |
48 |
38 |
28 |
|||
|
32 |
90 |
80 |
70 |
61 |
51 |
41 |
32 |
23 |
||
|
34 |
90 |
81 |
72 |
63 |
53 |
44 |
35 |
27 |
||
|
36 |
91 |
82 |
73 |
64 |
55 |
47 |
38 |
30 |
22 |
|
|
38 |
92 |
83 |
75 |
66 |
57 |
59 |
42 |
34 |
26 |
|
|
40 |
92 |
84 |
76 |
68 |
59 |
52 |
44 |
37 |
30 |
22 |
|
42 |
92 |
84 |
77 |
69 |
61 |
54 |
47 |
40 |
33 |
26 |
|
44 |
92 |
85 |
78 |
70 |
63 |
56 |
49 |
43 |
36 |
29 |
|
46 |
93 |
85 |
79 |
72 |
65 |
58 |
51 |
45 |
38 |
32 |
|
48 |
93 |
86 |
79 |
73 |
66 |
58 |
51 |
45 |
38 |
32 |
|
50 |
93 |
87 |
80 |
74 |
67 |
61 |
55 |
49 |
43 |
37 |
|
52 |
94 |
87 |
81 |
75 |
69 |
63 |
57 |
51 |
46 |
40 |
|
54 |
94 |
88 |
82 |
76 |
70 |
64 |
59 |
53 |
48 |
42 |
|
56 |
94 |
88 |
82 |
77 |
71 |
65 |
60 |
55 |
50 |
44 |
|
58 |
94 |
89 |
83 |
78 |
72 |
67 |
61 |
56 |
51 |
46 |
|
60 |
94 |
89 |
84 |
78 |
73 |
68 |
63 |
58 |
53 |
48 |
|
62 |
95 |
89 |
84 |
79 |
74 |
69 |
64 |
59 |
54 |
50 |
|
64 |
95 |
90 |
85 |
79 |
74 |
70 |
65 |
60 |
56 |
51 |
|
66 |
95 |
90 |
85 |
80 |
75 |
71 |
66 |
61 |
57 |
53 |
|
68 |
95 |
90 |
85 |
81 |
76 |
71 |
67 |
63 |
58 |
54 |
|
70 |
95 |
90 |
86 |
81 |
77 |
72 |
68 |
64 |
60 |
55 |
|
72 |
95 |
91 |
86 |
82 |
77 |
73 |
69 |
65 |
61 |
57 |
|
74 |
95 |
91 |
86 |
82 |
78 |
74 |
70 |
66 |
62 |
58 |
|
76 |
95 |
91 |
87 |
82 |
78 |
74 |
70 |
66 |
63 |
59 |
|
78 |
96 |
91 |
87 |
83 |
79 |
75 |
71 |
67 |
63 |
60 |
|
80 |
96 |
92 |
87 |
83 |
79 |
75 |
72 |
68 |
64 |
61 |
|
82 |
96 |
92 |
88 |
84 |
80 |
76 |
72 |
69 |
65 |
62 |
|
84 |
96 |
92 |
88 |
84 |
80 |
77 |
73 |
69 |
66 |
63 |
|
86 |
96 |
92 |
88 |
84 |
81 |
77 |
73 |
70 |
67 |
63 |
|
88 |
96 |
92 |
88 |
85 |
81 |
77 |
74 |
71 |
67 |
64 |
|
90 |
96 |
92 |
88 |
85 |
81 |
78 |
74 |
71 |
68 |
65 |
Table 8-3. Air Temperature: Dew Point Spread Table
Note: All figures are in degrees Fahrenheit at 30-inch pressure.
|
DIFFERENCE DRY-BULB MINUS WET-BULB |
AIR TEMPERATURE SHOWN BY DRY-BULB THERMOMETER |
||||||||||||
|
35° |
40° |
45° |
50° |
55° |
60° |
65° |
70° |
75° |
80° |
85° |
90° |
95° |
|
|
1 |
2 |
2 |
2 |
2 |
2 |
2 |
2 |
1 |
1 |
1 |
1 |
1 |
1 |
|
2 |
5 |
5 |
4 |
4 |
4 |
3 |
3 |
3 |
3 |
3 |
3 |
3 |
2 |
|
3 |
7 |
7 |
7 |
6 |
5 |
5 |
5 |
4 |
4 |
4 |
4 |
4 |
4 |
|
4 |
10 |
10 |
9 |
8 |
7 |
7 |
6 |
6 |
6 |
6 |
5 |
5 |
5 |
|
5 |
14 |
12 |
11 |
10 |
10 |
9 |
8 |
8 |
7 |
7 |
7 |
7 |
6 |
|
6 |
18 |
15 |
14 |
13 |
12 |
11 |
10 |
9 |
9 |
8 |
8 |
8 |
8 |
|
7 |
22 |
19 |
17 |
16 |
14 |
13 |
12 |
11 |
11 |
10 |
10 |
9 |
9 |
|
8 |
28 |
22 |
20 |
18 |
17 |
15 |
14 |
13 |
12 |
12 |
11 |
11 |
10 |
|
9 |
35 |
27 |
23 |
21 |
19 |
17 |
16 |
15 |
14 |
13 |
13 |
12 |
12 |
|
10 |
33 |
27 |
24 |
22 |
20 |
18 |
17 |
16 |
15 |
14 |
14 |
13 |
|
|
11 |
40 |
32 |
28 |
25 |
22 |
290 |
19 |
18 |
17 |
16 |
15 |
15 |
|
|
12 |
38 |
32 |
28 |
25 |
23 |
21 |
20 |
18 |
17 |
17 |
16 |
||
|
13 |
45 |
37 |
31 |
28 |
25 |
23 |
21 |
20 |
19 |
18 |
17 |
||
|
14 |
42 |
35 |
31 |
28 |
26 |
24 |
22 |
21 |
20 |
19 |
|||
|
15 |
50 |
40 |
35 |
31 |
28 |
26 |
24 |
23 |
21 |
21 |
|||
| 8-24. A wind vane indicates the direction of wind. An anemometer (Figure 8-6) measures the force or speed of the wind. Aboard ship the two instruments usually are mounted together. They automatically transmit wind force and direction to indicators. These indicators are located at such places as the navigation bridge and the harbor master's office. | |
| 8-25. When a ship is moving, the indicators show apparent wind, which is a combination of true wind and ship's speed. Direction is measured in degrees from the bow of the ship and speed is measured in knots. |
Figure 8-6. Anemometer
| 8-26. A watercraft operator aboard a ship moving through still air will experience apparent wind, which is from dead ahead and has an apparent force equal to the ship's speed. | |
| Note: Apparent wind, as measured from a moving ship, is the force and the relative direction from which the wind blows. | |
| 8-27. If the actual or true wind is 0, and the speed of the ship is 10 knots, the apparent wind from dead ahead is 10 knots. If the true wind is from dead ahead at 15 knots, and the ship's speed is 10 knots, the apparent wind is 15 + 10 = 25 knots from dead ahead. If the ship makes a 180o turn, the apparent wind is 15 - 10 = 5 knots from dead astern. | |
| 8-28. Wind vanes and anemometers measure only apparent wind. There is always the problem of converting apparent wind to true wind. There is more than one method of making a wind vector to find the true direction and true speed of the wind. The maneuvering board lends itself well to finding speed and direction of the true wind (Figure 8-7). | |
| Note: You are always in the center of the maneuvering board. |
Figure 8-7. Maneuvering Board With Wind Vector
| Example: Your ship is on a course of 030° , speed 15 knots. The apparent wind is from 062° , speed 20 knots. | |
| Required: Direction and speed of the true wind. | |
| Solution: | |
| 1. Draw the ship's true course and speed line on the plotting sheet from the center of the board in the direction of the ship's course (030° ). The length of the line equals the ship's speed (15 knots) (use the 2:1 scale). | |
| 2. Label the center of the plotting sheet "e" for the ship's true course, and "r" at the end of the ship's speed line. (This line "er" is one side of the wind vector.) | |
| 3. Using parallel rules, measure the direction from which the apparent wind is blowing (062° ). | |
| 4. Move this line to the tip of the ship's course and speed line "r". | |
| 5. Draw a line from "r" in the direction the apparent wind is blowing. The length of this line is equal to the speed of the apparent wind (20 knots). | |
| 6. Label the end of this line "w." (This line "rw" is the second side of the wind vector.) | |
| 7. Draw a line from the center of the maneuvering board "e" to point "w." (This line "ew" is the third side of the wind vector.) | |
| 8. Measure from "e" to "w" to find the true direction of the wind. | |
| 9. Measure the distance from "e" to "w" to find the true speed of the wind. | |
| Solution: Direction of the true wind is from 109.5° ; speed of the true wind is 10.8 knots. | |
| Note: Measure wind from the direction it is blowing. |
CLOUDS
| 8-29. Clouds consist of condensation of water vapor and are a direct expression of the physical changes taking place in the atmosphere. They play an important part in weather forecasting. |
| 8-30. The cloud classifications adopted by the World Meteorological Organization are used universally. There are ten cloud types, each of which may have several variations (Figure 8-8). The ten types are grouped, according to the height of their bases above the surface of the earth, into three families. |
|
Figure 8-8. Basic Cloud Formations and Indications
CLOUD FORMS| 8-31. The high-cloud family contains clouds with a mean level of 20,000 feet. These include: | |
|
|
| The middle-cloud family contains clouds that have bases lying between 6,500 feet and 20,000 feet. They are: | |
|
|
| 8-32. The low-cloud family contains clouds with bases lying from 6,500 feet at the upper level down to near the earth's surface. The vertical extent of the cumulus and cumulonimbus is often so great that the tips may reach into the middle- and high-cloud family levels. The clouds in this family include: | |
|
|
| 8-33. Although you will never see all types of clouds at the same time, quite often you may see two or three layers of clouds of different types. |
BASIC ELEMENTS OF WEATHER
| 8-34. Weather is the state of the earth's atmosphere with respect to temperature, humidity, precipitation, visibility, cloudiness, and other factors. All weather may be traced to the effect of the sun on the earth. Most changes in weather involve large-scale horizontal motion of air. Weather is of vital importance to the mariner. The wind and state of the sea affect dead reckoning and reduced visibility limits piloting. The state of the atmosphere affects electronic navigation and radio communication. If the skies are overcast, celestial observations are not available; and under certain conditions refraction and dip are disturbed. When wind was the primary motive power, knowledge of the areas of favorable winds was of great importance. Modern vessels are still affected considerably by wind and sea. |
| 8-35. Uneven heating of the earth's surface cause differences in atmospheric pressure which, in turn, causes winds. As air is warmed, it expands and becomes less dense. When it cools, it contracts and becomes dense. This results in higher atmospheric pressure. Equatorial regions of the earth receive considerably more heat than the polar areas. This excess of heat at the equator is the basis of a definite world pattern. The prevailing winds of the regions of the world are described below (see Figure 8-9). | |
|
Figure 8-9. The General Pattern of World Winds
| 8-36. If the earth did not rotate, the trades would blow due north and south. But the Coriolis force, in a manner of speaking, draws these winds off their course to a westerly direction, causing the northerly winds to become the northeast trades, and the southerly winds, the southeast trades. | |
|
|
| 8-37. Air moving from the high-latitude sides of the same high-pressure belts toward the poles produces the prevailing westerlies. Here again the rotation of the earth causes the wind to deviate from the north-south direction. The deviation is opposite from that of the trades simply because these winds blow toward the poles while the trades blow toward the equator. | |
| 8-38. In the Southern Hemisphere, westerlies are persistent throughout the year and blow from nearly due west because the area is largely uninterrupted ocean. However, in the Northern Hemisphere, large land masses present frictional blocks and break up the continuity of the highs and lows. Therefore, northern westerlies vary considerably in strength and direction. | |
| 8-39. During the winter, well developed lows over the North Pacific and North Atlantic cause the storm weather with which you may be familiar. | |
| 8-40. In the Northern Hemisphere, the air masses of the converging polar northeasterlies do not mingle readily with the southwesterlies of the temperature zone where they meet. Instead, the cold mass underruns the warm air from the south. The surface between these two air masses is known as the polar front. (Action similar to this occurs in the Southern Hemisphere.) The average position of this irregular front is 60° north latitude. It is important to realize that the polar front shifts. For example, in the Northern Hemisphere, it may extend as far south as Florida and farther north than the 60th parallel. | |
| 8-41. The winds of the polar belt are generally less well known than the trades and the prevailing westerlies. Those who have taken part in any of the polar belt expeditions can attest to their strong and blustery winds. Because of the rotation of the earth, these intense, persistent winds are, respectively, northeasterlies and southeasterlies in the Northern and Southern Hemispheres. |
| 8-42. The cause of land and sea breezes is by the alternating heating and cooling of coastal land and sea areas. The land, particularly in summer, is warmer than the sea by day and cooler than the sea by night. Therefore, there is a variation in atmospheric pressure over adjoining land and sea areas. This causes a system of littoral breezes which blow landward during the day and seaward during the night. These land and sea breezes usually penetrate to a distance of about 30 miles onshore and offshore, and extend to a height of a few hundred feet. | |
| 8-43. In the morning hours as the land warms, the sea breeze begins from 0900 to 1100. In the late afternoon, it dies away. In the evening, the land breeze springs up and blows gently out to sea until morning. In the tropics this process is repeated day after day with great regularity. In higher latitudes the land and sea breezes are often altered by winds of cyclonic origin. In many harbor areas or at the mouths of large river systems, these summer afternoon or evening breezes give rise to sudden squalls. |
| 8-44. Local conditions frequently interrupts the general pattern of belts of pressure and winds. One of the most pronounced and well known of these interruptions is the continental heating and cooling that produces monsoons. | |
| 8-45. When a sailor hears the word "monsoon," he naturally thinks of Asia, but monsoons affect all continents. However, the degree of influence varies from slight deflection of the winds over the smaller continents to absolute dominance in Asia where the pressure pattern is sufficiently distorted between winter and summer to produce opposite wind directions. | |
| 8-46. So vast is the summer low over Asia, that it completely dominates the Indian Ocean as far south as the Cape of Good Hope and east into the South Pacific. Its influence is felt even in the South Atlantic, from where winds blow across the Congo in Africa on into Asia. | |
| 8-47. Winds from the Indian Ocean, Australia, and the western part of the South Pacific cross the equator, heating and picking up water vapor as they go. Low air pressure causes the air to rise and rising air pressure, cools to the dew point. The result is heavy squalls, thunderstorms, and the torrential rainfall of the summer or wet monsoon. | |
| 8-48. Monsoons interrupt the normal pattern of the trades and prevailing westerlies. Just which direction the wind will blow in any area will depend, for the most part, on the direction of the low that influences that area. During the summer monsoon season is when most typhoons occur. |
| 8-49. Lines drawn through points on the earth having the same atmospheric pressure are known as isobars. These lines of equal pressure enclose areas of either high or low pressure. A pressure gradient is the space found between isobars (Figure 8-10). Pressure gradient indicates an increase or decrease in atmospheric pressure per unit distance between isobars. | |
| 8-50. Isobars are spaced closer in the eastern portion of the high-pressure area than in the western section. When isobars are close, the pressure gradient is said to be strong or steep; when they are far apart, it is called weak. Weather in strong or steep pressure gradients is normally subject to sudden changes with varying wind force and direction. In weak gradient areas, the weather changes are gradual and predictable. |
Figure 8-10. Pressure Gradients on a Weather Map
| 8-51. The pressure gradient determines the velocity of the wind. Strong gradients cause strong winds, while weak gradients result in gentle winds. When the pressure is about the same over a large area, the wind, if any, is slight. Wind direction depends mainly on the pressure gradient and the rotation of the earth. Wind direction is named by the direction from which it blows. | |
| 8-52. The direction and force of air flow over a long distance are diverted by the initial velocity generated by the rotation of the earth. This has no effect on our weather condition, except possibly on the movement of storms. Wind tends to blow parallel to isobars. For example, if a person is in the Northern Hemisphere facing away from the surface wind, the low pressure is toward their left and the high pressure is toward their right. If the person were in the Southern Hemisphere the effect would be the opposite. |
| 8-53. Near the equator lies a low-pressure belt where winds are either light and variable or nonexistent. There are frequent thunderstorms and squalls (rain falls in sheets). This belt of baffling winds and rain is called the doldrums and is the breeding place of the most violent of all storms--tropical cyclones. | |
| 8-54. In the Atlantic, tropical cyclones are known as hurricanes; in the Pacific, as typhoons; in Australia, as willy willies; and in the Philippines, as baguios. All are alike in character. The use of the term "hurricane" will apply to all of these systems. | |
| 8-55. Hurricanes are circular or elliptical whirling eddies of air up to 400 miles in diameter. Wind speeds reach as high as 150 or more knots near the center, but decrease toward the edges. In the Northern Hemisphere, the wind blows in a counterclockwise direction (Figure 8-11); in the Southern Hemisphere, it blows in a clockwise direction. Typical of these storms is a calm at the center or eye, which may be 5 to 40 miles in diameter. When the eye of a storm passes over an area, wind that has been violent decreases to a much lower speed and at times become calm and precipitation stops. When the eye has passed, the winds come from the opposite direction. |
Figure 8-11. Hurricane Track in Northern Hemisphere
| 8-56. The actual cause of the formation of hurricanes is unknown. Soon after they form, they begin to move at speeds from 5 to 20 miles per hour. In the Northern Hemisphere, they generally follow a slightly curving path to the north and west until reaching the horse latitudes. The hurricanes then re-curve to the northeast, pick up speed over the surface, become less violent, and finally blow themselves out. In the Southern Hemisphere, the general path of hurricanes is first to the southwest and then they re-curve to the southeast. The life of a hurricane is about 10 days. |
| 8-57. Hurricanes are usually preceded by a day of good visibility. Temperature and pressure are slightly higher than normal. Cirrus clouds appear. Wind changes direction and increases in force; sea swells increase their period. At night, the temperature is generally lower than normal. During the summer and fall in the Gulf of Mexico and the Caribbean Sea, the bearing of the storm is reliably indicated by the direction from which the swells come. In any other area this sign is unreliable. | |
| 8-58. In the tropics, a distinct drop in barometer pressure (diurnal change considered) of 0.10 inches signifies the approach of a hurricane. Anywhere else in the world, such a drop may indicate any type of storm. | |
| 8-59. Upon the approach of a hurricane, cirrostratus clouds replace the common at sunrise and sunset. As the storm draws closer, a bank of clouds appears on the horizon toward the storm. A light and gusty wind blows from your left as you face the storm center. The barometer continues to fall and at times is unsteady. Clouds darken and cover the entire sky. Wind increases and heavy seas develop. |
| 8-60. In the Northern Hemisphere, if you face the direction toward which a hurricane is moving, the portion of the storm on your right is the dangerous semicircle, the portion on your left is the navigable semicircle. In Figure 8-11 the arrows show the direction of the wind. It is apparent that a vessel in the dangerous semicircle would tend to be blown into the path of the storm. A vessel in the navigable semicircle would probably be blown to a position behind the storm. It is also clear that speed of the wind in the right semicircle would be greater as the speed of the storm over the surface would be added to the speed of the wind. In the left semicircle, speed of the storm would be subtracted from speed of the wind; therefore, wind and sea would be less violent. | |
| 8-61. Face the wind to locate the center of a storm, face the wind. The center lies about 113° to your right. | |
| 8-62. Buys Ballot's law is useful in determining the direction of a storm. The law says that if you face the wind, the low-pressure area (the storm center) lies to your right. Actually it could be up to 130° to your right. |
| 8-63. Whenever winds dangerous to navigation are forecast for an area, Navy and Coast Guard stations and yacht clubs hoist, in some conspicuous place, flags by day and lanterns at night to warn all seamen of the expected conditions. Figure 8-12 shows the types of signals and when to use each one. |
Figure 8-12. Storm Warning Signals
| 8-64. Watercraft operators should be aware of the significance of these signals. Even the small craft warning informs of conditions that are potentially dangerous to boats, including the relatively large ones used by the Navy. | |
| 8-65. Note that the definition of "storm" does not include an upper limit for wind unless the storm originated in the tropics, in which case the upper limit is 63 knots. Tropical storms with winds of greater speeds, of course, are hurricanes. Should no warnings be visible, you can estimate wind speeds by using the information contained in Table 8-4. |
| 8-66. Fog, which may be defined as a cloud on the earth's surface, consists of water droplets or ice particles suspended in the air. It usually forms when the surface of the earth cools the air above to the dew point (Figure 8-13). | |
| 8-67. Radiation fog occurs at night. It only forms when the land cools; which in turn, cools the air above. | |
| 8-68. Advection fog occurs when warm air flows over a cool surface (for example, over a cool ocean current). Advection fog can form only in regions where marked temperature contrasts exist within a short distance of each other and only when the wind blows from the warm area toward the cold area. |
Table 8-4. Table of Beaufort Wind Scale and Correlative Sea Disturbance Scale
Figure 8-13. The Formation of Fog
| 8-69. Steam fog is a type of advection fog. It occurs when cool air blows over a warm surface. Evaporation from the warm surface easily saturates the cool air, causing fog, which rises from the surface like smoke. | |
| 8-70. Frontal fog occurs in the cold air mass of a front. As warm rain falls into the cold air, it evaporates, saturating the cool air and causing the fog. Although the cool air already is saturated, evaporation from the rain continues as long as the temperature of the raindrops is higher than the temperature of the air. Frontal fogs are rarely caused by cold fronts because they usually move so rapidly and have such narrow bands of precipitation. Warm fronts, on the other hand, cause deep and long-lasting fogs that are considered the worst type to encounter. |
| 8-71. A front is the surface between warm and cold air masses. At times, air will lie over a vast cold or warm region long enough for the air to become fairly uniform in temperature and humidity. These air masses, in time, begin to move at varying rates of speed. Frequently, one air mass will meet or overtake another mass that is warmer or colder. These meetings are called fronts. When a cold air mass tends to underrun and displace a warm air mass, the front is called a cold front. When a warm air mass overrides or replaces a cold air mass, it is a warm front. Figure 8-14 shows a simplified cross-sectional diagram of cold and warm fronts. |
Figure 8-14. Characteristics of Warm and Cold Fronts
| 8-72. Sometimes a cold front will overtake a warm front, forming an occluded front or an occlusion. Two different occlusions are noted (depending on the temperature differences). If the overtaking cold air is warmer than the cold air ahead of the warm front, a warm-type occlusion is formed (Figure 8-15). If the overtaking cold air is colder than the other, a cold-type occlusion is formed (Figure 8-16). | |
| 8-73. There are three types of fronts: cold, warm, and occluded. An occluded front is a mingling of the first two fronts. All occluded fronts should be watched closely, because it is along these fronts that the most adverse weather conditions occur. When a ship passes through a front, a noticeable change in the weather may be seen. This is especially true when passing through a cold front, because a cold front is sometimes accompanied by a sudden shift in the wind and a hard squall from a westerly quarter. |
Figure 8-15. Warm-Front Occlusion
Figure 8-16. Cold-Front Occlusion
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