Southeast Alaska is the warmest climate zone of Alaska (Searby 1968; Shulski and Wendler 2007) and the only region where the mean monthly temperatures stay above the freezing point year-round. It is a mid-latitude maritime climate (Cfb) according to Köppen’s classification (Köppen 1884, Köppen and Geiger 1940). “C” indicates that it is a temperate climate with no monthly mean below the freezing point, the second letter “f” refers to precipitation and indicates that significant precipitation is observed in all seasons, while the third letter “b” indicates the degree of summer heat. The warmest month of the year is below 22 °C, but there have to be at least 4 months with temperature above 10 ° C for this classification; for Sitka, it was exactly 4 months.
The mean annual temperature of Sitka for the last climate normal (1981–2010) was calculated as 7.4 °C. The coldest month is January with a mean temperature of 2.4 °C and an average low just above freezing with 0.2 °C. August is the warmest month with a mean monthly value of 14.0 °C and a mean daily maximum 16.6 °C (see Fig. 3). The maximum of August is rather typical for a maritime climate, as the ocean lags in warming up in summer when compared to a more inland station, where the highest temperature is normally observed in July. The annual variation between the warmest and coldest months is relatively small with 11.6 °C. There are only 5.1 days, on average, per year when the maximum temperature rises above 21.1 °C (70 °F). On the other side of the spectrum, there are only 10 days annually, when the maximum temperature of the day does not rise above the freezing point.
Using Eurasian data, (Conrad 1944) calculated a so-called “continentality index” C, where C is defined by
$$ C=1.7\partial T/ sin\alpha -20.4 $$
with ∂T (°C) being the difference in the mean temperature between the warmest and coldest months and α is the latitude. On this scale, the maritime climate of Tromsö on the western coast of Norway registers zero while the continental climate of Verkhoyansk (Interior Siberia), where the coldest temperature on the Northern Hemisphere was measured, rates 100. Applying this empirical formula, a value of 4 could be calculated for Sitka. This strongly maritime climate is rather typical for the area. There are two additional first-order meteorological stations along the coast, to the North, Juneau (58° 22″ N, 134° 35″ W) and to the South, Annette (55° 02′ N, 131° 34′ W). For the last climate normal, they recorded mean annual temperatures of 9.5 and 11.1 °C, respectively, with mean annual differences between the coldest and warmest months of 16.3 and 12.9 °C. Calculating the “continentality,” values of 3 and 1 were obtained, even slightly more maritime than Sitka. This compares to Interior of Alaska (Wendler and Shulski 2009), where a value of 70 was found for Fairbanks. Here, the winters are cold, while the summers are relatively warm for the high latitudes. At Ft. Yukon, a few miles North of the Arctic Circle, the highest temperature ever measured in Alaska was recorded in June 1915 at 37.8 °C (100 °F). The highest temperature ever measured at Sitka was a modest 31.1 °C reported on 30 July 1976, while the coldest temperature occurred on 9 January 1953 at −17.8 °C, which compares to the absolute minimum for Alaska of −60.0 °C, observed in Tanana in January 1915, situated also in the continental climate zone of Interior Alaska. Finally, it should be pointed out that different formulations of the continentality index exist, as can be seen from early work (Gorczinski 1920; Johansson 1926; Conrad 1946, 1950).
The precipitation amount is high in southeast Alaska. Little Port Walter (56° 23′ N, 234° 38′ W) holds the record for Alaska with a mean of 5751 mm annually. Here, local uplift enhances the amount. Sitka reports 2205-mm precipitation, much less, but still a very substantial amount when compared to the continental climate of the Interior (e.g., Fairbanks 278 mm) or Arctic Alaska (e.g., Barrow 107 mm). There is a strong annual course in the precipitation, with the maximum observed in fall, when the ocean is still relatively warm while the landmass has been substantially cooled. The fall storms bring a large amount of moisture, and a maximum of precipitation is observed in October (mean value 329 mm) followed by September (298 mm) and November (248 mm). As the thermal contrast between the ocean and landmass decreases, so does the precipitation, and in June, the minimum is reached at 73 mm. More details can be seen from Fig. 4, from which can be also seen that maxima monthly values of the 30-year period can be about twice as large as the normal. There are, on average, 235 days with precipitation of at least 0.01″ (0.25 mm), and in August, 24.4 days of the month report on average rain.
The annual snowfall is, on average, 820 mm; more than half of this occurs in the two winter months of January and February. As there is no month in which the mean temperature is below the freezing point, snow cover, when established, does not last long.
As could be expected from the high amount of precipitation, the amount of cloudiness is high with a mean annual value of 68 %. The maximum occurs in late summer/early autumn (August 77 %, September 74 %, October 73 %), while the minimum occurs in January with a still substantial value of 57 %. Monthly values are presented in Table 1. The atmospheric pressure has a mean annual variation of 15.4 hPa, with the maximum is in July (1017.7 hPa) and a minimum in November (1002.3 hPa). It is, as expected, very close to the annual variation of Annette, a first-order station in the same climate region, both in absolute values and annual course. The mean monthly values are presented in Table 1.
Looking at the annual course of the wind speed, the maximum is observed in November and December, during the months of minimum atmospheric pressure with 4.3 m/s, an expected result, while the minimum occurred in August (2.7 m/s), the month with the highest temperature. The mean annual wind speed could be calculated as 3.7 m/s (see Table 1).
When analyzing a station record for changes in its climate, one has to be careful that it is not affected by other factors, e.g., (1) the heat island effect of a growing city, (2) station relocation, and (3) measurements routines.
Concerning heat island effects of a growing city with time in Alaska (e.g., Magee et al. 1999), this specific element is of little importance for Sitka, as the population has not grown substantially and is at present roughly 9000 inhabitants.
The original observing location of Sitka Magnetic was terminated in 1989, and the observational site was moved some 2 km to the southwest at Sitka Airport. However, for the time period from 1976 to 1989, both stations were operating, which allows a careful comparison. In Fig. 5, such a comparison has been carried out for the above time period and temperature.
One has to be careful insofar that the temperature for the 40-year period from 1827 to 1867 was measured as 5.1°Reaumur (6.2 °C), as the incorrect assumption that these early measurements were carried out in the now commonly used scale of degrees centigrade would add 1.1 °C to the observed warming to the nearly two centuries of observations. Further, the difference in the calendar (12 days) between the Russian and ours, as well as the conversion from degree Reaumur into Fahrenheit, was already carried out by Patterson (1879b).
It can be seen that the temperatures correlate excellently (r = 0.99), but that the Sitka Magnetic was, on average, 0.9 °C colder than Sitka Airport.
When calculating the mean daily temperature, the maximum plus the minimum temperatures, divided by 2, is used in modern times. The Russian observations were carried out for hourly 17 h from 6:00 to 21:00 h (1828–1831), later 19 h from 4:00–21:00 h (1832–1848) and than 24 h (1849–1867). The average of these observations was taken to calculate the daily mean. By doing this, some cold night hours were omitted, especially, in the very early observations of only 17 h, and hence, too high average daily temperatures were calculated. We found that the calculated mean annual temperature was 0.50 °C too high for the 17-h observations, and 0.32 °C too high for the 19-hourly observations. The difference between the (max + min) / 2 and the mean of the 24 h was only 0.04 °C, the latter being warmer, and here, no corrections were carried out. As expected, there is a fairly strong annual course in these differences, as the nights are long in winter and short in summer, where, therefore, the maximum effect is to be expected, which can be seen from Fig. 6.