Humidity is a measure of how much water is held in the air as a gas, in other words, how much water vapour is in the air. The specific humidity tells us the amount of water vapour in grams for each kilogram of moist air. The relative humidity tells us how close (as a percentage) the air is to being saturated - holding as much water vapour as it can at that temperature. The dew point temperature is the temperature to which the air would have to be cooled to become saturated – the closer the dew point temperature is to the air temperature, the higher the relative humidity.
Humidity is a measure of water vapour, which is essential to life on earth.
It is a key part of the water cycle – enabling water to move from the oceans and land surfaces, around the globe, up into the atmosphere and fall out as rain. Increasing amounts of water vapour are linked to increasingly heavy rainfall events.
It is a key part of the energy budget, a greenhouse gas in fact. Water vapour high in the atmosphere traps some of the outgoing long-wave radiation. Without water vapour our atmosphere would be too cold for us to live on Earth. As the amount of other greenhouse gases are increasing, especially long-lived ones such as carbon dioxide, our planet is warming up.
Humidity is also very important for human comfort. A hot and humid day, where the amount of water vapour in the air is high and the air is close to saturation, can be very uncomfortable. The main way our bodies keep cool is through evaporation of sweat from our skin. This becomes more difficult when the air is getting close to being saturated and therefore unable to hold any more water vapour. High humidity can also place animals and plants under significant stress.
Since records began in the early 1970s the amount of water vapour, as measured by the specific humidity, has increased at a rate of 0.09 [0.07 to 0.11] g kg-1 10yr-1 over land, 0.08 [0.06 to 0.09] g kg-1 10yr-1 over ocean and 0.08 [0.06 to 0.10] g kg-1 10yr-1 for land and ocean combined. That’s equivalent to approximately 94,000 Olympic-size swimming pools (50x25x2m) worth of extra water in the bottom 1m of the atmosphere, or almost a Lake Windermere (Lake District, UK).
While there is now more water vapour in the air than in the 1970s, the air over land has become less saturated. The relative humidity over land has decreased sharply since around 2000. The rate of decrease since 1973 is -0.16 [-0.29 to -0.03] %rh 10yr-1. Over the ocean, the relative humidity data has larger uncertainty than over land. There appears to be a small decrease over time at -0.09 [-0.17 to -0.02] %rh 10yr-1 but there is low confidence in this result.
It might seem strange that the relative humidity has decreased over land even though there is more water vapour in the air, but this has happened largely because the temperature has also increased. The increase in water vapour over land and particularly the oceans (which are the main source of water vapour over land) has not been enough to keep pace with the rising temperature over land. Hence, the level of saturation has decreased over land and the relative humidity has fallen.
Humidity observations have been routinely made at weather stations, on ships and on some buoys for many decades. We now have sufficient amounts of humidity observations available digitally to monitor changes in surface humidity from around 1970. Many more observations exist but these have not yet been digitised.
Humidity is traditionally measured using a pair of thermometers. One is a standard dry-bulb thermometer to measure the air temperature. The other is a wet-bulb thermometer, also known as a psychrometer or hygrometer, which has a material wick around the thermometer bulb that is continuously wet because it dips into a reservoir of water. Evaporation cools the wet-bulb thermometer. The difference between the two thermometers tells us how much evaporation has taken place which in turn depends on the amount of water vapour in the atmosphere and the temperature. Since the 1980s, electronic sensors have become more common. These can measure the dewpoint temperature using a chilled mirror to condense water vapour, or relative humidity using electronic resistance which is affected by the atmospheric humidity.
We have used digital archives of humidity observations to build global monitoring products. We have quality controlled these data and made adjustments for biases and non-climate artefacts caused by changes in the observing system such as instrumentation, station location, measurement practices and ship heights. These data are then averaged to monthly mean 5° grids. There are very few stations in unpopulated areas such as deserts and the high-latitudes. Similarly, there are very few ocean observations outside of the main shipping routes. The Southern Hemisphere generally has poorer observation coverage than the Northern Hemisphere. Hence, our global averages are, in fact, only ‘near-global estimates’.
Water vapour is an essential part of the water cycle. Water is evaporated from the surface land and ocean – the oceans, which cover around 70% of our planet, are the main source of water vapour. This water vapour is then moved around by the wind and transported upwards by convection which is where air, warmed by the sun, rises up through the atmosphere. Higher up in the atmosphere, where the temperature is generally cooler, the water vapour condenses into water droplets and clouds are created. When the water droplets become large enough they fall as rain, hail or snow.
As the atmosphere has warmed there is more energy available to evaporate water from the surface and store it as a vapour in the air. So, the rise in specific humidity is directly linked to the rise in air and ocean temperature. Like temperature, it tends to be particularly high during years of strong El Niños. Importantly, the increase we have seen in specific humidity is unlikely to have occurred in the absence of an increase in greenhouse gas emissions. The Intergovernmental Panel on Climate Change (IPCC) Assessment Report 5 (AR5) reported “medium confidence that there is an anthropogenic contribution to observed increases in atmospheric specific humidity since 1973“.
The rate of increase of water vapour is slightly larger over land than over oceans while the rate of increase of temperature is far higher over land than it is over ocean. These differences have led to the decrease in relative humidity over land. The slower ocean warming means that there has not been enough water evaporated over the oceans, which are the main source of water for evaporation, to keep pace with faster rising temperatures over land.
HadISDH global surface humidity monitoring product is a gridded monthly average product created from hourly observations of temperature and dew point temperature made from weather stations and on ships. The data have been quality controlled and adjusted to remove biases and features caused by periodic changes to the station locations, instruments, practices, and ship heights. The data and various diagnostics (e.g., global average time series and long-term trends) can be downloaded from www.metoffice.gov.uk/hadobs/hadisdh.