Relative Humidity

Relative humidity describes the degree of saturation of air by water vapour. This parameter is almost always used to characterise the air humidity in the Arctic. Some of the above cited papers are devoted to studying this parameter, either entirely (Meteorology of the Canadian Arctic 1944; Rae 1951; Putnins 1970; Vowinckc! and Orvig 1970; Sater et al. 1971; Pereyma 1983) or to a great extent (Petterssen et al. 1956; Zavyalova 1971). In winter, the relative humidity in the Arctic should be calculated with respect to the saturation vapour pressure over ice, which is lower than it is over water. This permits the air to be slightly supersaturated with respect to ice, while it is not saturated with respect to water. In practise, this means that for winter we receive significantly higher values of relative humidity than when they are calculated in respect to water. From the Figure 6.3 it can be seen that the differences are quite large and exceed 30%. It is well known that the annual course of relative humidity is usually opposite to the course of air temperature. However, in the Arctic such a situation occurs only when the relative humidity in the cold half-year is computed with respect to ice (see Figure 6.3). Supcrsaluration occurs frequently near the surface in winter as long as no condensation takes place.

Figure 6.3. Mean annua! course of relative humidity (U) and air temperature (T) (after Zavyalova 1971). 1 - relative humidity determined using hygrometer, 2 - relative humidity corrcctcd according to the formula proposed by Malmgren, 3 - air temperature.

The mean monthly relative humidity over ice in the Arctic, computed according to a formula proposed by Malmgren (1926), shows more than 100% in the period from November to April, but on particular days supersaturation can also be observed in October when temperature drops below -25°C (Radionov et ai. 1997). Radionov et al. (1997) found that thermal conditions favorable to the supersaturation of water vapour in the air occur continuously over 75% of the period from December through March, with a maximum in February (89%). In January (Figure 6.4), supersaturation occurs over the entire Arctic, excluding the Atlantic region and the southern and central parts of the Baffin Bay and Pacific regions. This phenomenon also does not occur in the southern coastal part of Greenland. The highest relative humidity (>106%) is observed in the central part of the Greenland Ice Sheet. The secondary maximum (>104%) occurs on Ellcsmcre Island and in the Arctic Ocean from the Greenland and Canadian Arctic side. The relative humidity computed with respect to water (Figure 6.5) shows almost the opposite pattern and generally 20-30% lower values. In practise, because measurements of relative humidity in low temperatures are still made using hair higromcters, which measure this element with respect to water, the original results of relative humidity measurements and different climatic analyses are presented with respect to water (e.g. Kancvskiy and Davidovitch 1968; Krenke and Markin 1973a, b; Markin 1975; Wojcik 1976; Pereyma 1983; Przybylak 1992a, b).

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Figure 6.4. Mean relative air humidity in January in the Arctic expressed in terms of percentage of saturation with respect to ice (after Atlas Arktiki 1985).

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Figure 6.4. Mean relative air humidity in January in the Arctic expressed in terms of percentage of saturation with respect to ice (after Atlas Arktiki 1985).

In summer, the accuracy of measurements of air humidity is the highest and, because of mostly positive temperatures in the Arctic, the computed method of the relative humidity with respect to water is correct. The highest relative humidity {> 95%) is noted mainly in the Arctic Ocean and in the northern parts of Arctic seas, excluding the Greenland and the Norwegian seas and Baffin Bay (Figure 6.5). High humidity (above 90%) occurs over a large portion of the Greenland and Norwegian seas and in the other Arctic seas. However, in the Baffin Bay region such high humidity is only observed in its south-western part. The lowest relative humidity is to be found in the southernmost continental parts of the Arctic (about 70-85%) and in the inner part of Greenland, and other large islands.

The relative humidity in Hornsund shows a dependence on cloudiness similar to that of water vapour pressure (Figure 6.1), i.e., significantly higher values (about 15-20%) are observed in all months (except August) on cloudy days than on clear days. The value of the relative humidity in August is computed from only a few days, so its representativeness may be not high.

Figure 6.5. Mean relative air humidity in (A) January and (B) July in the Arctic expressed in terms of percentage of saturation with respect to water (after Atlas Arktiki 1985).

The mean monthly daily courses of the relative humidity in the Arctic in the cold half-year (October-March) are uniform. From April the daily course becomes elearer and is best developed in the late spring and summer months. For example, in Spitsbergen (Homsund), the greatest range in the mean monthly daily courses of relative humidity occurs in August (6%) and in May (5%). The lowest values were observed most often at 14°° and the highest in the "night" or early morning hours (Przybylak 1992a). The daily courses of the relative humidity in the warm half-year, similar to those of air temperature, become clearer when the cloudincss decreases.

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