Weather: The surprising changes in Canada's cloud cover

Sunday, January 4, 2009

Our changing climate is about far more than just rising temperature and altered precipitation patterns

by HENRY HENGEVELD

Canada's cloud cover is changing. There are more high-level cirrus clouds, more cumulus-type convective clouds, and less low-level stratus type clouds – and that affects our temperature and precipitation.

These surprising results emerge from a new study published by Ewa Milewska, an Environment Canada research climatologist, in a recent issue of the Canadian scientific journal Atmosphere-Ocean. 

In her study, Milewska uses data from 84 weather stations across Canada (including 14 in Ontario), each carefully screened for consistency in reporting methods over the study period of 1953-2003, and for the quality of cloud data reported. (See Figure 1) At these weather stations, trained observers record a large range of weather features, including the amount, estimated height and type of cloud visible from horizon to horizon. Milewska breaks these cloud data into three height categories. At the upper end are the high-level clouds, with bases that start at six kilometres above the surface. These are dominated by the wispy cirrus-type clouds, but also include other types such as cirrocumulus (tops of thunderstorm clouds) and cirrostratus (more uniform and flat layers).

At the other end of the range are the low-level clouds which have bases below two kilometres above the surface. These include convective-type cumulus clouds, flatter stratus layers, and a number of other cloud types which range between these two (including fog). In between are the middle clouds, with bases between two and six kilometres. This category is dominated by altostratus and altocumulus clouds.

Earlier studies, including one by Milewska in 2004, have shown that the total cloud cover over Canada has been increasing over the past half-century. Milewska's new study confirms these results. She demonstrates that the majority of the stations analyzed show that the annual number of hours when more than 50 per cent of the skies are covered by all cloud types increased by between 10 and 17 per cent over the
51 years of the study. 

However, as Milewska digs into the individual trends of clouds at the different heights selected, some very significant new insights into cloud behaviour by type become apparent.

For example, there are significant downward trends in the annual number of hours when more than 50 per cent of the skies are covered by low and middle cloud types. While downward trends in central Canada are relatively modest, those in the western provinces and in northern Quebec result in a decrease of between five and 11 per cent over the study period. 

Offsetting this is a strong positive trend in the amount of high-level cirrus clouds across the entire country. Most regions show increases of between 10 and 23 per cent, with cirrus cloud cover over Alberta rising by an astonishing 23-34 per cent. Undoubtedly, some of this apparent increase may simply be due to more of the upper cloud becoming visible to the observer at the surface as the amount of low and middle cloud cover decreases.  However, both the magnitude and the cross-country extent of the rise in the reported cirrus amounts suggest most of the trend is real.

Another interesting feature of the study relates to the trends in amount of total convective versus stratus type clouds. Many of the stations in southern Canada show upward trends in convective cloud types of between 23 and 34 per cent over the 51-year period. By comparison, stratus-type cloud cover decreases by 11-23  per cent across most of Canada. Within these broad trends are other surprises. For example, while the total amount of all convective cloud types increased, strong convective clouds associated with thunderstorms decreased significantly, particularly in Alberta and B.C.

Why are these changes in cloud types important? First, clouds reflect and absorb incoming sunlight. They also absorb and thus trap outgoing heat energy that would otherwise escape to space. However, the net balance of these two influences on surface temperatures very much depends on the type of cloud.

In the daytime, low-level stratus clouds tend to be much more effective at reducing incoming solar energy than in trapping outgoing heat energy, and hence cool the surface. At night, when there is no incoming sunlight, the net effect of these clouds is a warming one. That is why, everything else being equal, cloudy nights are generally warmer than clear ones.

High-level clouds like cirrus, on the other hand, are relatively transparent to incoming sunlight but very effective at trapping outgoing heat energy. Hence, their presence warms the surface. Thus the rise in cirrus cloud cover and corresponding decrease in low clouds over Western Canada may be a key reason for the regional rise in both daily maximum and daily minimum temperatures of between one and two degrees Celsius.

Secondly, the type of cloud we get influences the type of precipitation we receive. Convective clouds are generally associated with showers and thunderstorms, while stratus clouds bring longer periods of precipitation. In general, summer rainfall in Ontario is of the first kind, while that associated with large storm systems often passing through the region in fall and spring are the latter.

While Milewska does not try to investigate in detail these linkages between cloud and temperature and precipitation, she does note, for example, that the consistency between the decrease in Western Canada's strong convective cloud cover and the reported regional decline in mean daily rainfall bears further investigation.

If nothing else, Milewska's study reminds us that our changing climate is about far more than just rising temperature and altered precipitation patterns. Furthermore, it demonstrates, once again, that changes in one aspect of weather (such as temperature or precipitation) are often closely linked to changes in other aspects (such as cloud cover). 

Sounds complicated – and it is. BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada