Atmospheric moisture, as measured by relative humidity, was above average in 2000, slightly below normal in 2001, and lower still in 2002, as shown in Table 1. The annual average of 49% (3.1% above normal) in 2000 coincided with a slightly wetter than average year, whereas both 2001 and 2002 were drier and warmer, particularly 2002. In 2002, humidity was 9.5% below average, at 43%.
Relative humidity is one of a range of measures of moisture in the atmosphere. It is expressed as the percentage of water vapour in the air relative to the amount that the air theoretically could contain at the same temperature; it is thus closely dependent on temperature, and varies through the day (it tends to be higher at night and in the early morning, and lower during the heat of the day). The values discussed here were recorded at 3pm Australian Eastern Standard Time (in the mid-afternoon).
|Table 1. Annual relative humidity statistics in the ACT, 2000–02|
|Annual mean (%)||49||46||43|
|Anomoly (% of long-term mean)||+3.1||-3.2||-9.5|
Monthly average relative humidity values were below the long-term monthly mean in almost two-thirds of all months between January 2000 and June 2003. There was a general downward trend in atmospheric moisture through the period, consistent with an overall decline in rainfall and rising temperatures. Available surface moisture was taken up in evaporation, which increased over the same period, leading to drying out of vegetation and soil.
Figure 1. Monthly relative humidity anomalies at Canberra Airport, January 2000 to June 2003 (%)
The highest relative humidity anomalies, as shown in Figure 1, were recorded in November 2000 (which had the most rain days ever recorded in November at Canberra), and February 2002 (which broke rainfall records for the month of February in the region).
In November 2000 relative humidity averaged 59%, or 40% above normal for the month (almost two standard deviations above the mean). The highest average monthly humidities, but not the largest anomalies, were recorded in July 2001 (61%) and June 2003 (66%); conditions in winter tend to be moister in the ACT in winter, when temperatures are lower than in summer.
Relative humidity was lowest during this period in December 2001 (28%) and during the period October 2002-January 2003; both November 2002 and January 2003 recorded monthly average values of 28%. In October 2002 humidity was at 32% (an anomaly of 15.4%, more than two standard deviations below the long-term mean), and in December it was 30% (an anomaly of 6.9%, or 19% below average). The extended period of below-average humidity from March 2002 to May 2003 is the most noteworthy feature of the graph in Figure 1, particularly as atmospheric moisture values fell rapidly from unusually high levels in February 2002.
Since the beginning of the relative humidity record at Canberra Airport, annual average values have tended to fluctuate with rainfall (Figure 2); wetter years also have above-average relative humidity values, both because there is more moisture in the air and because daytime temperatures are often cooler in wet periods because of the presence of clouds. There are no apparent trends in annual relative humidity values.
Figure 2. Annual average relative humidity at Canberra Airport from 1940 to 2002. The long-term (1961–90) average is indicated by the horizontal line through the centre of the graph (scale in %).
- The lowest annual relative humidity, 37%, was recorded in 1957, which was a very dry year in Canberra (it was also an El Niño year). Other low values occurred in 1944, 1967, 1982 and 1997, all of which were dry years associated with El Niño events.
- The highest annual value of relative humidity was 54% in 1974, the second wettest year on record at Canberra after 1950. There was a La Niña event in 1974–75.
- Relative humidity has declined at Canberra since the early 1990s, by 21% between 1992 and 2002. A similar decline was observed between 1974 and 1982; humidity levels then rose and were generally above average during the 1980s and early 1990s. Fluctuations like this need to be seen in the context of variations in other relevant parameters, such as temperature.
Relative humidity, like other climatic parameters, does not exhibit the same long-term behaviour in every season of the year. In Figure 3 the seasonal average values of relative humidity at Canberra Airport are shown to vary quite differently through the period of the record.
Figure 3. Seasonal average relative humidity at Canberra Airport from 1940 to 2002. Values have been standardised using the long-term mean for the period 1961–90.
- At least two different modes of interaction between summer and winter seasonal relative humidity are apparent. The first, evident between about 1955 and 1960 and in 1965 and 2002, for example, has atmospheric moisture anomalies in both seasons rising and falling together; that is, both summers and winters in these periods are either moister or drier than usual.
- In the second mode, humidity levels either rise in summer (December–February) and fall in winter (June–August), as in 1972 and 1976; or are higher in winter and lower in summer, as in the period 1985–92. At these times a more humid summer is followed by a winter with lower than normal humidity, or vice versa.
- In the period since 1990, winter relative humidity began to decline approximately two years before summer humidity also trended down. The humidity anomalies in winter have also been larger than those in summer, with a range from highest to lowest anomaly of approximately four standard deviations in winter between the early and late 1990s, and a summer range of about three standard deviations.
Analysis, graphs and tables by Dr Janette Lindesay, School of Resources, Environment and Society, The Australian National University.
Relative humidity has been measured for an extended period of time at only one official Bureau of Meteorology station in the ACT, located at Canberra Airport. The record there begins in 1942, with the first complete year of observations in 1943. Other organisations (e.g. ACTEW, CSIRO and ACT Parks and Wildlife) may make independent measurements relevant to the area; their data may be made available independently, or through the Bureau of Meteorology. Most of these records are of considerably shorter duration, but often contain invaluable information for otherwise sparsely-monitored areas.
Information on Bureau of Meteorology weather stations and climate data can be obtained from:
|Organisation||National Climate Centre|
|Phone (BH)||(03) 9669 4082|
|Fax||(03) 9669 4515|