Overview
This dataset provides in-situ surface observations archived by the Meteorological Service of Canada (MSC). It containes data from the MSC operational observation system as well as from their parteners. Therefore, not all stations are QC or maintained by MSC. The network of stations contains stations with only automatic instruments, and human observing (or manual) stations. Humidity variables are obtained from hourly stations that typically measure more weather variables (e.g., wind or snow on ground).
Provider's contact information
Environment and Climate Change Canada
donneesclimatiquesenligne-climatedataonline@ec.gc.ca
Licensing
Open Government Licence - Canada.
Variable name and units:
Hourly Relative Humidity (%)
Hourly dew point temperature (ᵒC)
Hourly Humidex (unitless)
Spatial coverage and resolution:
Canada, point locations.
Temporal coverage and resolution:
Time period varies per station with data for in the North starting in 1940’s or 1950’s until present.
The data is available at hourly time steps.
The data will continue to be updated regularly.
Information about observations (number, homogeneity)
The number of active stations changed over time. The following figure from Mekis et al. (2018) is presenting the locations of the surface weather stations across Canada with a Needs Index map in the background as of September 2016.
Figure 1. Surface weather stations across Canada, as of September 2016, with a Needs Index map in the background. For further details on station network evolution see Mekis et al. 2018. [Source: Mekis et al., 2018].
Stations over Northern Canda with dew point measurements are the ECCC atomatic stations and stations from the Aviation Monitoring Station network (which include automated and staffed weather stations) operated by NAV CANADA and the Department of National Defence (DND). The ECCC Volunteer Climate Stations provide just temperature and precipitation measurements.
Methodology
Raw and quality-controlled station observations are archived and managed by the MSC’s Archive Operations and Climate Services. Observations from different instruments are stored in different formats in the national archive. The methodology, the instruments and the location of instruments have changed in time. Many initlaly manual stations were replaced with automatic stations. The following is summarising some information regarding the manual measurements that should be considered when using historic station data.
Relative humidity at manual stations was measured either by a Dewcel remote temperature sensing unit or by psychrometers (e,g. Vaisala HMP 35CF, Vaisala HMP 14C), and derived from the dry bulb temperature and dew point temperature with the aid of psychrometric tables. Currently MSC is using Vaisala HMP 155.
Hourly humidity values are obtained from hourly climate stations. For climate stations operating on a 24-hour basis, before June 1, 1957, the climatological day ended at 1230Z of the following day; from June 1, 1957, to June 30, 1961, the climatological day ended at 1200Z of the following day; since July 1, 1961, the climatological day ends at 0600Z of the following day. In cases where knowing time-of-observation is critical, the best approach is to check the historical inspection reports for the climate station.
MSC is also computing Humidex values using hourly measurements of temperature and humidity. The standard Humidex formula used presently by ECCC is:
Humidex = (air temperature) + h
Where:
h = (0.5555) * (e - 10.0);
e = vapour pressure in hPa (mbar), given by:
e = 6.11 * exp [5417.7530 * ((1/273.16) - (1/dewpoint))]
In this equation, dew point temperature is expressed in Kelvins (temperature in K = temperature in °C + 273.16) and 5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the universal gas constant.
Earlier records have Humidex (and vapour pressure) values computed with another formula. Consequently, the 1971 Normals used the old formula, and the 1981 Normals used this new formula.
Information about the technical and scientific quality
This dataset represents Environment and Climate Change Canada’s official station observations. Data are subject to change on an on-going basis as MSC is constantly quality controlling (QCing)from ECCC stations. Not all data has the same level of QA/QC (i.e. aviation dat is not QA/QC by MSC but by NAV CANADA).
Careful attention should be accorded if the data from stations is used in climate analyses because abrupt discontinuities to humidity records can be caused by a number of non-climatic factors. Users should be aware about the following factors:
- Canadian conversion algorithms, in the form of psychrometric tables, were refined in the 1960s and the metric system was adopted in 1977. In addition, from the early 1970s psychrometers were replaced by the Dewcel at principal meteorological stations and within the Automated Weather Observation System (AWOS), leading to a decreasing shift in relative humidity at a number of stations. These steps are especially prevalent for stations experiencing cold temperatures during the cold period (van Wijngaarden & Vincent, 2005).
- The change in the observing agency, has also created an artificial positive step in the beginning in the 1990s in 8 MSC hourly stations from the 75 analyzed by Vincent et al. (2007).
- At the microscale, the growth of a nearby tree, landscape changes and changes in urban sprawling (i.e. cities and skyscrapers) may affect the local climate through shade and evapotranspiration.
- Changes to the hydrology of an area such as the introduction or cessation of irrigation practises, the construction or draining of a local reservoir and the man-made water sources (i.e dams, treatment plants, pumping stations, lakes/ponds) can effect real changes in humidity.
Limitations and strengths for application in North Canada
It is a challenge to sustain a cost-effective observing system over Norther Canada because of a large part of the teritory is constituted by remote areas (it is hard for technicians to fly to the site for maintenance, and they often have to wait for the thaw). The special climatic conditions produce a large risk of power and telecommunication outages. Concequently, observations in Northern Canada are sparse and records are often incomplete.
Other challenges are related to changes in the observing network, which involves relocation and closure of sites and changes in instruments and practices. In Canada, psychrometers were replaced by dewcel units in the early 1970s and some as late as the 1980s. This change has created an artificial negative step in relative humidity time series because of a positive bias observed in psychometers records in cold climate: psychrometers yield anomalously high values of relative humidity at low temperatures when the wet bulb is coated by ice, while dewcels housed in a sheltered Stevenson screen are less sensitive to icing than other relative humidity sensors and exhibit lower values of relative humidity at very cold temperatures (Déry and Steiglitz, 2002).
A homogeneity assessment of relative humidity and dew point temperatures was carried out by Vincent et al. (2007). A significant negative step due to the replacement of the psychrometer by the dewcel was observed for relative humidity at 52 MSC stations. For the dewpoint time series, the step for the introduction of the dewcel was observed at nine MSC stations (mostly located in the northeast). The adjustments for the dewpoint do not have as much impact on the annual national trends but have an important impact on the trend of relative humidity.
Very few significant steps were detected in the specific humidity time series, because in cold temperatures, the specific humidity values are very low and do not vary much (Vincent et al., 2007).
References to documents describing the methodology or/and the dataset
The manual specific for aviation observations/reports (MANOBS): http://publications.gc.ca/collections/collection_2019/eccc/En56-238-2-2018-eng.pdf and http://publications.gc.ca/collections/collection_2019/eccc/En56-238-2-2018-fra.pdf
https://climate.weather.gc.ca/doc/Technical_Documentation.pdf
Link to download the data and format of data:
Only hourly data is available on the CDO/MSC/ECCC database (searchable by location; in CSV format): https://climate.weather.gc.ca/historical_data/search_historic_data_e.html
Publications including dataset evaluation or comparison with other data in northern Canada
Déry, S., and M. Steiglitz, 2002: A note on surface humidity measurements in the cold Canadian environment, Boundary Layer Meteorol., 102, 491–497.
Mekis, E., L.A. Vincent, M.W. Shephard, and X. Zhang, 2015: Observed Trends in Severe Weather Conditions Based on Humidex, Wind Chill, and Heavy Rainfall Events in Canada for 1953–2012, Atmosphere-Ocean, 53:4, 383-397, DOI: 10.1080/07055900.2015.1086970.
van Wijngaarden, W. A. and L.A. Vincent, 2005: Examination of discontinuities in hourly surface relative humidity in Canada during 1953-2003. Journal of Geophysical Research-Atmospheres, 110, D22102, doi:10.1029/2005JD005925.
Vincent, L.A., W.A. van Wijngaarden, and R. Hopkinson, 2007: Surface temperature and humidity trends in Canada for 1953-2005. Journal of Climate, 20, 5100–5113. doi:10.1175/JCLI4293.1