Weather: Are our summers getting more humid?

Wednesday, December 19, 2007

New research suggests that absolute humidity has risen over the last 30-plus years and that this is primarily due to human activity

by HENRY HENGEVELD

I find the hot days of summer harder to take these days. They seem to be "steamier" than they used to be.

This, of course, could be entirely due to a reduced tolerance for heat and discomfort associated with advancing age. On the other hand, it may also be because summers are becoming more humid. In fact, there is new research evidence to suggest that this is indeed the case.

There are two primary ways of measuring and recording humidity in the atmosphere. The first records the total amount of water vapour found within a parcel of air, and thus indicates what fraction of the air mass is water vapour. In meteorological jargon, this is referred to as "absolute humidity" (although atmospheric scientists also use the term "specific humidity").

The second is to measure the amount of moisture in the air relative to the maximum it can hold before it begins to condense into water droplets and form clouds and rain. This is known as "relative humidity" and is recorded as a percentage of maximum moisture content. That is, once relative humidity reaches 100 per cent, the water vapour normally condenses into cloud droplets.

Thus, the lower the relative humidity, the drier the air, and the greater the ability of our skin to evaporate water into the air - and vice versa. Since it is such evaporation processes that help cool our skin, it is relative humidity rather than absolute humidity which affects our human comfort levels during hot weather.

The maximum capacity of air to hold water vapour (the reference point for measuring relative humidity) increases with temperature. Therefore, a rise in absolute humidity could be caused by a rise in temperature alone, even if relative humidity remains constant.

Alternatively, if temperatures remain constant, a rise in absolute humidity would also cause a rise in relative humidity. Sounds confusing, but it's much like the relationship between the size of our stomachs and the amount of turkey we can eat at Thanksgiving dinner in determining how stuffed we are.

In early October of this year, a team of British scientists, led by Katherine Willett from the University of East Anglia, published some new research results on trends in atmospheric humidity in the prestigious science journal Nature. In their study, they undertook a detailed analysis of atmospheric moisture content measured near the Earth's surface at a large number of observing stations around the globe. Their data show that the average absolute humidity of the lower atmosphere has increased significantly since their global records began in 1973. This is shown as the dark blue line in the accompanying Figure 1, page 53.

The research team also compared their observations with a reconstruction of past atmospheric behaviour using climate models. When they tried to simulate the behaviour using only natural climate fluctuations and the influences of past changes in solar radiation and volcanic dust on the climate system, the expected change in average absolute humidity over the 30 year period was very small (dashed line).

However, when they added the effects of rising greenhouse gas and aerosol concentrations, they obtained a relatively good agreement with observations (the light blue line). They conclude that the rise in absolute humidity must be primarily due to past changes in the composition of the Earth's atmosphere - and hence due to human activities. One more indication that our entire weather system - not just temperature - is changing due to human influences!

Since atmospheric water vapour is a key component of our global weather machine, an increase in humidity can affect our weather and climate in a number of ways.

First, water vapour is the most important greenhouse gas in the atmosphere. Thus, while human activities do not significantly affect its atmospheric concentration directly through emissions, its increase in response to rising temperatures significantly amplifies the initial temperature rise - something atmospheric scientists refer to as a positive feedback.

Secondly, water vapour is the source of the condensed water droplets that make up clouds. Hence, dry air masses like those in arid regions have little cloud, while very moist ones have an abundance of clouds. Clouds affect the amount of sunlight reaching the Earth's surface, but also reduce heat loss from the surface at night. Change the water content in the atmosphere, and you change cloud properties.

Third, rising water vapour concentrations also increase the moisture that is available for rainfall. Therefore, while the number of days of precipitation will likely decrease under warmer climates, the intensity of the rain or snow when it does occur will also increase. This has implications for related risks of flooding and soil erosion, and for the amount of snow that falls during winter storms.

Water vapour also contains a lot of stored energy - energy that is returned to the atmosphere when the water vapour condenses into water droplets. This energy, usually released in higher levels of the atmosphere, becomes an important factor in the intensity of hurricanes, thunderstorms and other severe weather events.

Finally, of course, there is the human comfort factor. Higher relative humidity is a key factor in the discomfort associated with hot summer days. Perhaps the good news in the British study is that relative humidity has remained relatively constant over time, at least when averaged around the globe.

Perhaps my perception of an increase in the "steaminess" of our hot summer days is an age thing after all! BF

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