The men who "go down to the sea in ships" fight a continuous close action with the elements that make up the weather. To seafarers the state of the weather is more important than it is to most people ashore. Accurate weather forecasting may not be as vital now as it was in the days of sailing but situations still arise in which the safety of a ship and the lives of her crew depend on the action you take to avoid the full fury of a storm. Even when actual safety is not considered, possible damage to the ship, her gear, and the like, should be minimized by security measures taken well in advance of a approaching storm.
THE ATMOSPHERE
The atmosphere (air) is a mixture of independent gases. Near the surface of the Earth the percentages by volume are approximately 78% nitrogen, 21 % oxygen, 1 % argon, with traces of other gases such as carbon dioxide, hydrogen, neon, and helium. Water vapor, which is found in relatively small but widely varying amounts; 1% of the total atmosphere may be taken as the average. The quantity of water vapor present is much greater in equatorial regions than in polar regions, and greater over the ocean than over land. The atmosphere has definite weight, called atmospheric pressure, and it is measured by an instrument called a barometer.
Large-scale changes in temperature, pressure, and water vapor content of the air cause the changes in weather. Warm air is lighter in weight and can hold more water vapor than cold air. Moist air with a temperature of 50°F is lighter than drier air of the same temperature because water vapor is lighter than air. Cold or heavy air has a tendency to flow toward and supplant warm or lighter air, and as the air begins to move, other forces come into play, making the movement of air masses and weather complex. Temperature, humidity, and atmospheric pressure are all factors in considering the weather. You probably don't need to be told that 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 medium.
Most thermometers are mercury-filled and practically all of them use the Fahrenheit (F) scale, in which the freezing point of water is 32° and its boiling point is 212°. Temperature in meteorology, sometimes is expressed according to the Celsius (C) (formerly Centigrade) scale, in which the freezing point of water is 0° and its boiling point is 100°.
You might be to convert a Fahrenheit reading to Celsius, or vice versa. If 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 9°. Multiply by 5/9, and you get 45/9, or 5°C. To change from Celsius to Fahrenheit just reverse the procedure. First multiply Celsius temperature by 9/5, then add 32.
A thermometer must be read properly to obtain an accurate result. First, if you handle it, be sure that you do not touch the lower part of the glass containing the alcohol or mercury, because the heat from your body can affect the height of the mercury or column. Make sure that the top column is level with your eyes; otherwise you will be reading a higher or lower graduation than the one actual one.
