Near-tropical thunderous rain downpours have succeeded the balmy high temperatures that summer has brought to Glasgow of late. Deep black skies. Thunderbolts. Lightning. (♫ Very, very frightening! Galileo Galileo… ♫) Unusual conditions even for a very wet Scotland.
This Met Office map shows the stormy situation in the skies of Glasgow on 23rd July 2013.
The Hydrologic Cycle
It rains. So what’s new?
Everyone’s heard of the water (H2O) cycle, the continuous movement of water on, above and below the surface of the Earth. Although the balance remains fairly constant over time, individual water molecules can come and go, in and out of the atmosphere.
The water moves from one reservoir to another, from a river to the ocean, or from the ocean up to the atmosphere, by physical processes, such as:
Over the hydrologic cycle, the water goes through different phases: liquid, solid (ice) and gas (water vapour).
The hydrologic cycle involves the exchange of energy, which leads to temperature changes. When water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate.
Basically, when it’s very hot, the water can evaporate into the air. This water vapour collects into the clouds, where it undergoes a range of changes in pressure and temperature.
And then it rains. It just does.
If you’re in Glasgow, it pours…
The phenomenon of the atmospheric river was first identified about 20 years ago. An atmospheric river is an intense band of moisture (water vapour) flowing through the air.
The term was coined by Newell and Zhu (1992) to reflect the narrowness of the moisture corridor involved. Those powerful weather systems cause heavy rainfall, and have been responsible for extreme flooding events in places like California, and recently the United Kingdom and Western Europe.
Atmospheric rivers are up to 300 km wide and sometimes stretch in length for over 1,000 to 2,000 kilometres, and they glide invisibly in the troposphere, between 1 and 2.5 kilometres above the surface of the ocean. When the rivers make landfall and encounter steep terrain, the air is forced upwards where it cools and releases the water vapour into rain.
If the river remains on course for 24 hours, it delivers a continuous flow of heavy rain over the same area. And while the winds don’t change, there is an increasing potential for flooding.
Typically, three to five of these narrow conveyor belts of moisture are present in each hemisphere at any given time. A single one of those atmospheric rivers is capable of carrying a greater flux of water than the largest river on Earth, the Amazon River.
Such a weather system was blamed for the violent flooding that hit the ancient town of Cockermouth (Cumbria) in 2009, claiming one life.
Researchers estimate the incredible volume of water carried by this one atmospheric river at a maximum of almost 300,000 tonnes of moisture per second.
By comparison, the River Thames transports about 65 tonnes of water through London over the same period of time.
The most closely-scrutinised atmospheric river is known as the ‘Pineapple Express‘, because it originates from the Hawaii region. It flows towards the coast of California and has been linked to extremely damaging storms along the West of the United States.
Research into the subject by Lavers & Villarini (June 2013) suggests that warmer weather conditions could create more atmospheric rivers – and make their effects even more severe.
Five different scenarios have been examined to simulate the possible weather conditions over time, and it was discovered that a warming climate would allow the Earth’s atmosphere to keep hold of even more moisture, thereby making the formation of atmospheric rivers even more likely, as well as increasing the possibility of more extreme natural events.
Over the last 30 years, an average of 9 to 11 of the strongest atmospheric river events have been hitting Britain every year. All five models suggest that atmospheric river linked events are set to double in the period 2074-2099, most of which would be expected to make landfall in the United Kingdom.
And meanwhile… What’s the weather like in Glasgow?
Cloudy. Yet, sultry and sunny. 22º Celsius. Seagulls are coasting along the warm updrafts in the patchy blue sky. Swallows yield their shrill cry of war to insects.
With ever more rain on the forecast later… 😉