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Weather Almanac for April 2009
THE MORNING DEW
When I walked out of my home this morning, headed for my art group gathering, I found my car roof and windshield coated with moisture. The clear skies overnight had allowed the temperature to fall, and from the moisture on the car, I knew the temperature of the windshield and car body had fallen below the dew point temperature. It was easy for me to clean the dew-covered windshield, not like the scrap, scrap, scrap when such mornings a few weeks previous brought frost to the Robson Valley. I had a quick answer to the dewfall, having lived in sea-side Victoria, British Columbia for several years: a squeegee wedged next to my seat.In Victoria, the more humid night air of the coast often produced heavy dew deposition when the temperature dropped following sunset, and the squeegee provided a quick remedy to the dew problem, one much preferred to wet gloves or hands.
Dew is more than a nuisance to driving, however; for in some regions it provides a much welcome component to the local water balance and a major source of moisture for plants and animals. I find dew a source of beauty, particularly for the camera’s eye, when dew clings to plants and small animals. I am always enthralled by the little jewels of water clinging to leaves and other objects. Dew remains one of the unconquered challenges for my non-photographic artwork.
I titled this piece “The Morning Dew” but dew is not just a morning phenomenon. It can occur at anytime when the temperature of the surface layer over some object such as a leaf or vehicle drops below the dew point. Though we do not often think about it in this manner, all water condensation on a surface is technically dew, including the moisture that forms on our welcomed cold drink on a hot, humid day in summer.
When I decided to write this piece on dew, I initially thought I would be hard-pressed to write more than a page, the formation of dew seems today to be a simple and straight forward process to explain. But, in my research, I found that in the late-18th century, the question of how dew formed engendered much debate, and hypotheses arose from such scientific luminaries as Isaac Newton, Benjamin Franklin and Charles Darwin. One hypothesis held that dew seeped out of the soil and rose from the surface into the air. Another suggested it “fell” from the skies as an imperceptible rain. The true nature of dew formation was eventually satisfactorily explained in 1812 by Dr William Wells, an American living in London, England. As we shall see, Wells showed that dew did not “rise” nor “fall,” contrary to the many poets who have used the phrase but simply appears, like a fairy-book creature, when the conditions are right.
The Formation of Dew
A trained or experienced weather observer can deduce a number of clues as to the weather prior to the dew’s appearance on a given morning. Most importantly, dew tells us that the near-surface temperature has fallen during the night below the dew-point temperature, that temperature at which saturation of the air with water vapor would occur. Secondly, the formation of dew suggests that the probable cause of that temperature drop was a clear nocturnal sky that allowed the surface heat to escape and the temperature to drop. Most likely, winds were also light perhaps calm. There would also be a high probability that a surface temperature inversion was present.
The formation of dew is linked to the temperature and the humidity of the air surrounding the dew-covered surface. I refer the reader to my previous article Sometimes It’s Relative: Humidity for more details on humidity, but here is a summary.
Humidity, according to meteorologists, can be either absolute or relative, giving us the terms absolute humidity and relative humidity. In our high-paced media world, the adjective preceding humidity is often dropped, and the speaker or writer just says “humidity,” expecting us to understand which of these two they are speaking. Weathercasters, including those formally trained in meteorology, are often the worst offenders (mia culpa, as well) because other professionals usually understand which is meant by the context.
Absolute humidity is defined as the mass of water vapor contained in a given volume of air, that is, the density of water vapor, generally expressed as grams per cubic metre. It is like saying there are X grams of cashews in a jar of a specific volume of mixed nuts (too darn few!). Absolute humidity is used by meteorologists because it varies only moderately through an air mass over time scales of a day or more, and most of that variation occurs in the surface layer of the air. Moisture may be added to an air mass by evaporation, subtracted by condensation, or altered by mixing along the air mass edges.
Relative humidity, in comparison, is a dimensionless ratio, usually expressed as a percentage, between water’s vapor pressure in the air and the saturation vapor pressure for the air temperature. Relative humidity is usually what the media mean when they say “humidity.” Relative humidity can be a confusing measure because its value varies with the air temperature. For example, the morning may have a relative humidity of 78 percent which by afternoon drops to 53 percent as the air temperature rises. The absolute humidity, however, may remain unchanged through that period.
The key term in the relative humidity definition is saturation. Saturation is defined as the condition when the partial pressure of water vapor in the atmosphere is at its maximum level for the existing ambient temperature and pressure. At saturation, an equilibrium exists between water vapor and liquid water, and no net evaporation or condensation would occur from a standing water surface when the overlying air is saturated. (You will often hear saturation called the condition where the air contains “all the water vapor that it can hold.” This, however, is not technically correct, but old metaphors die hard.)
The saturation temperature of the ambient air is also called the dew point temperature or simply the dew point. Its value is solely dependent on the absolute humidity of the air in our atmosphere.
You might on occasion hear a weathercaster or meteorologist discuss the dew point of a particular air mass. Since dew point, like the absolute humidity, varies little within an air mass, it is often a good indicator of the air mass type. A high dew point, in the range of 12.8-24.9 oC (55 to 75 oF) indicates a humid air mass; a low dew point, below -1oC (less that 30 oF), a dry air mass. Dew formation is more likely when the region is under the influence of a humid air mass than under a dry one.
With that said, the explanation of dew formation is now quite simple. When the ambient temperature of a surface element such as a leaf or car windshield falls to the dew point, the surrounding surface air layer will become saturated. At saturation, the liquid and vapor states of water are in equilibrium. But when the temperature drops further — below the dew point, the equilibrium is tilted toward the liquid state as more water vapor is converted into the liquid form (called deposition) than escapes the liquid state into the vapor (called evaporation).
Soil moisture can also play a part in the potential for dew formation, particularly after a soaking rain. When tnhe soil is wet from a recent rain, evaporation during the day can raise the absolute humidity in the lowest layer of the atmosphere, the surface layer, such that it is more humid near the ground than higher up in the air mass. This can produce dew with the proper temperature decrease, for several nights even within a dry air mass. If the soil is particularly dry due to lack of rainfall for an extended period, however, dew formation is unlikely unless the air mass itself is very humid.
Finally, when liquid water has been deposited on a surface, the chemical and physical properties of the wet surface, often aided by gravity, may “pull” the liquid water into rounded shapes that form the familiar dewdrops.
Some Dew Climatology
The total amount of dew that forms overnight on land has been estimated to have a theoretical maximum of about 0.8 mm (0.03 inches) per 10-hour night. Under some circumstances, however, deposition of water vapor from the air onto a surface can be much higher.
Estimates of total dew deposition annually range from around 12.5 mm (0.5 inch) in cold and in warm, arid climates to about 75 mm (3 inches) in warm, semihumid-climates. Research in the semi-arid region of India at the Agrometeorological Observatory in Hisar measured dew deposition which averaged between 36 and 80 mm of dew deposition and 130 dew-days for the winter months over a 24-year period. Other climatological research has determined that in the northern Negev Desert about 200 days per year have dew formation.
It is estimated that condensation from humid air masses passing over a large, cold lake such as Lake Superior can deposit several inches (50-75 mm) of dew during June, an amount equivalent to the local average June rainfall.
Dew and Life
Most of us probably do not think of dew as playing any major role in the large scheme of life, but dew can be a critical factor in the lives of many plants and animals, particularly in arid regions of the world. In the most arid regions, dew may provide the only direct water source for small animals, the rest being obtained from their food supply.
Research has also shown that some plants may obtain needed water from dew, absorbing it into their tissues for use in photosynthesis. However, for other plants, dew can serve as a breeding or collection spot for pathogens, such as the fungus Phytophthora infestans which infects potato plants, or as a pathway for harmful pollutants into plant tissue.
Humans have held dew in high esteem for millennia. For example, Greek mythology names Ersa as the goddess of dew. The Jewish faith includes prayers for dew during Passover.
In arid regions, the possibility of extracting water vapor from the air, creating an artificial dew, has been considered as a water supply for human use and for agriculture. One proposed system suggested solar energy could be used to cool a plate on which condensation could form and provide drip-irrigation to plants. A few large-scale dew capturing devices have been tried. One system made by the Indian Institute of Management Ahmedabad in the semi arid, coastal region of Kutch was able to collect more than 200 litres (on average) of dew water per night for about 90 nights during the dew season October-May.
Poets get the last word on dew this day. While many have extolled the beauties of dew, I have chosen the words of Robert Graves to represent them:
The dew-drop carries in its eye
From “Dew-drop and Diamond”
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