47 OBSERVING HUMIDITY AND FORECASTING FOG

178. THE MEASUREMENT OF HUMIDITY

Humidity measuring devices are often low on the list of priorities of small craft owners, probably owing to the belief that they are complicated and delicate pieces of equipment. Amongst the simplest and most rugged of hygrometers is the hair hygrometer, which can be obtained with a dial which reads relative humidity directly, requiring no tables or other extraneous equipment. Some are also equipped with thermometers so that temperature and humidity can be read together. Hair hygrometers work, because human and other hair extends in length when the atmosphere becomes wetter. A similar, even more compact type, is the pocket (paper) hygrometer, which also has a dial giving humidity directly, but the element in this case is a paper and metal coil.

However, the wet-and-dry bulb type of hygrometer is less liable to error than the above types, whose readings frequently have to be checked against those of wet-and-dry bulb thermometers to guard against errors caused by changes in the properties of the hair or paper elements and friction in the bearings. The method of operation of the wet-and-dry bulb hydrometer was described in the chapters of Basic Meteorology.

To make an observation, the screen should be placed about 5 ft. above the deck in the open air on the weather side, as free as possible from radiation and any warm draughts. At least 15 minutes should elapse between mounting and reading, but if the water in the wet-bulb reservoir is not at the temperature of the air, a much longer time is required. Pure distilled water should be used, as far as possible, and wick, water and muslin should be scrupulously clean. If salt water spray is thought to have blown over the screen, or at intervals of a week or so, or whenever the muslin or wick appear to be dirty, they should be replaced; the wick should not be allowed to hang down in a bight below the top of the water container; if it does, water will drop off it and the pot will soon be emptied. The water pot should not be placed immediately underneath the wet bulb, but clear to the side furthest removed from the dry-bulb.

In very cold weather, evaporation will take place from the thin layer of ice formed on the surface of the wet-bulb, but at a different rate to evaporation off the surface of the water. It sometimes happens that the wet-bulb reads above the dry when the temperature is falling; the dry-bulb follows the change of temperature with only a small time lag, but the wet-bulb, being coated with muslin, has a greater time lag, so that if the temperature is falling sufficiently quickly, the wet-bulb reads higher than the dry-bulb. When this occurs, the readings should not be recorded or used for finding the relative humidity or dew point.

If the two readings are the same and the weather is not foggy, it is probable that the cistern is empty, or that the dry-bulb is coated with salt or dirt.

Tables 1 and 11 in fig. 42-13 are for finding the relative humidity and dew point for the observed dry-bulb temperature, and the depression of the wet-bulb temperature below that of the dry-bulb. The values below the thick lines in the Tables are valid only when the wet-bulb is covered with ice.

As an example in the use of these Tables, suppose the dry bulb temperature is 66°F., and the wet-bulb temperature is 64°F. The depression of the wet-bulb is, therefore, 2° below the dry-bulb. Entering Table 1 with the dry-bulb temperature 66°F., down the left-hand column, under a depression of 2° along the top line, we find the relative humidity is 89%. Similarly, entering Table 11, we find the dew point is 63°F.

179. FORECASTING FOG AT SEA

Relative Humidity (RH) is taken to be 100% when the atmosphere is saturated, as is the case in fog or when fog is very likely. It is rare for the RH to fall below 40%, even in the height of summer, and its range over 24 hours may normally be between 60% and 95%, with the smallest reading being obtained in the afternoon when the temperature is the highest, and the maximum around dawn when the temperature is the lowest. At sea, the relative humidity is always higher than that to be expected over the land and is rarely far from saturation values. For instance, the annual mean RH on the weather-ships off our western seaboard is about 90%, which is higher than that recorded over the year at Kew. Thus, a high value of RH recorded at sea, especially in yachts and other small craft which are close to the sea-surface, is not necessarily a sure sign that fog is about to clamp down. A better indication is to compare the sea temperature with the dew point. lf the sea temperature falls below the dew point of the air, then fog is almost a certainty.

The following procedure is recommended whenever the temperature of the air is , higher than, or equal to, that of the sea, especially at night, when approaching fog cannot be seen until shortly before entering it. Sea, Dry-Bulb and Wet-Bulb temperatures should be observed at hourly (or half-hourly) intervals and the sea temperature plotted against the dew point, as shown in fig. 42-14. If the curves of sea temperature and dew point converge, fog may be expected by the time at which they coincide.

From the plot in fig. 42-14, for instance, at 22:00 hours it would become evident that there is a probability of running into fog in about an hour’s time, assuming that the sea temperature continues to fall at about the same rate as it has done during the previous 1½ hrs.

From a chart of average sea surface temperatures (Admiralty Pilot Book or Routeing Chart for the appropriate area), it can be seen when a rapid fall in sea temperature may be expected, so that if the dew point is within 5° or so of the sea temperature when approaching the cold-water zone, this will also give a fairly reliable warning of fog.

Sea fog can occur whenever warm air arrives over a cooler sea-surface, and windward coasts should be shunned in likely fog conditions, because the lifting of moisture-laden air over the coast will yield the first fog banks and these are then likely to spread back out to sea. On the coasts of England and Scotland facing the North Sea, sea fogs are prevalent and are called ‘haars’. The season for haars is March to September, so they embrace the sailing months. The months of highest frequency are May and June, when the North Sea is still cold, while the airstreams are often warm and moisture-laden.

To help in the assessment of the likelihood of sea fog, some idea of the lie of the isobars to windward is essential. If the pressure patterns produce a High or ridge over South-West Europe, tropical maritime air can be drawn up over the coastal waters of Britain and, with the light winds, sea fog becomes very likely. Any track which can bring tropical maritime air up on a light wind is a potential fog source.

The force of the wind is important for forecasting sea fog. If the wind is strong, usually the fog will be lifted by turbulence into low stratus cloud. Wind of about force 3 appears to be most favourable for the occurrence of fog. Above force 4, the chance of fog decreases markedly. A small proportion of fogs occur with calms, and a few cases with wind forces 6 or 7. In general, when the wind starts to blow from a more northerly direction in the Northern Hemisphere, the chance of fog forming is reduced, or fog already formed will be quickly dispersed.

The behaviour of smoke from steamers‘ funnels is often a valuable indication. If it hangs about in horizontal streaks, this shows that an inversion of temperature, caused by considerable cooling of the lower layers of air, is already present, and this can lead to shallow fog formation. Ragged ascending funnel smoke and cumulus cloud both indicate instability of the atmosphere and convection and, therefore, no fog.

180. THE EFFECT OF HUMIDITY ON THE HUMAN BODY

At any other time than when bathing, the body will exude perspiration, sufficient to keep the body temperature even. If it is cold, the evaporation of perspiration is low, which means that the wind must be the main agent removing heat by wind-chill. The degree of wind-chill depends on wind speed and temperature and values can by be computed for it, given the weather conditions on any particular day. In a wind of 20 knots and an air temperature of 15.6°C. (6O°F.), the skin will lose between 6 and 7 calories every minute from each square inch of exposed dry skin. Wind chill con- tributes about 75% of the total cooling, while evaporation of perspiration removes another 20%, the odd 5% being due to other minor causes. It must be repeated that these are values for the ‘dry’ body.

As we feel cold when heat flows out of the skin and warm when it flows in, the ideal is to remain in equilibrium, so that when out in the boiling sun, the contribution to body cooling of evaporation by perspiration is high enough to counter the intake of solar radiation. If the sun is too intense and the wind is nil, then heat stroke may ensue, but it requires another meteorological element to conspire with the others before heat stroke is a real danger.

If the air over the skin is saturated-with moisture, then however much perspiration is exuded, no more heat is removed by evaporation than is contributed from the air, by condensation, and one of the important cooling agents of the body becomes ineffective. It is obviously rare to find 100% relative humidity, coupled to nil wind and 38°C. (100°F.) in the shade but, working in confined conditions, say, clearing an engine fault in a small craft cabin over slopping bilges in high summer, might produce the necessary conditions, and heat stroke then becomes a possibility.

In any case, humidity will always contribute to the sensation value of temperature, and an indicator to its effect on rested people indoors is shown in fig. 42-15. To use this diagram, enter with the wet-bulb temperature on the vertical axis and the dry-bulb temperature on the horizontal axis. Then, the sensation of normal persons who are resting indoors will be found. In the relatively-high humidities existing over the sea, small craft cabins will move out of the comfort zones much more rapidly than rooms ashore. It must not be forgotten either that the exertions of work move us out of the comfort zones very quickly as well.

Another risk which must not be disregarded is the extreme cooling which goes with standing-about after rescue from capsize, or falling overboard. The loss of body heat by evaporation, aided by the wind, is often more than the body can supply, even by such heat-producing reflexes as shivering and teeth chattering. Shelter and dry clothes must be sought as rapidly as possible, particularly in the winter months.