Atmospheric rivers (in English atmospheric rivers) are narrow, elongated bands of very moist air in the lower atmosphereThese huge currents are often compared to "I speak in the clouds", as they transmit enormous amount of water vapor over a great distanceTypical atmospheric the river is approximately 250–600 km wide and more than a thousand kilometers longThey are characterized by high water vapor content and strong low-level winds, thanks to which they can transport huge volumes of water from the oceans to the continents.

NASA compares the volume of water vapor carried by these jets to approximately 25 Mississippi River, while the strongest ones can carry up to 15 times more water than the Mississippi.

How they arise and where they occur

These massive streams of moisture are usually associated with low pressure areas and cold frontsThey form over tropical oceans, where warm, moist air rises, sucking in huge amounts of steamStrong winds subsequently they shift towards temperate and polar latitudesWhen this moist air reaches coastal areas and meets cooler air currents, moisture condenses and falls as intense rain or snow.

A well-known example is the moisture flux nicknamed "Pineapple Express", which begins near the Hawaiian Islands and brings warm moisture to the West Coast of the United States. In these areas, it can cause up to 10–12 cm of precipitation in one day.

Atmospheric rivers move in the direction of air flow created by pressure and frontal systems, unlike surface rivers that flow downward. They are most prominently observed over the boundary of extratropical oceans, such as North Pacific or Atlantic Oceanfrom where they intervene the west coasts of America or EuropeSimilar currents also occur above East Asia, Sub-Saharan Africa or South AmericaIt is reported that in each hemisphere there are simultaneously 3–5 atmospheric rivers, which overall carry up to 90 % of all flowing tropical humidity towards the poles.

Dual role in the hydrological cycle

Atmospheric rivers are a natural part of the global hydrological cycleTheir key role is move moisture from the oceans to the continents, thereby replenishing freshwater supplies. In regions such as the west coast of North America, ARs provide up to half of the total annual precipitation balanceThey are a source of domestic water because they replenish reservoirs and snowpack, which is especially important in periods when the soil is at risk of drying out.

However, they are at the same time double-edged swordWhen "river in the sky" encounters mountainous terrain (such as the Cordillera or the Alps), most of the transferred moisture falls quickly to the ground, which can cause flooding. Torrential rains can quickly clog city drains and cause landslides and damage to infrastructure.

For example, during the winter of 2022/2023 Up to nine consecutive atmospheric rivers hit California, which brought months of rainfall in a few weeks. The result was widespread flooding, mudslides, 21 deaths and billions of dollars in damageEurope is no exception, for example, in the autumn of 2024, Spain was hit by extremely intense rains, which led to catastrophic flood loadIn Asia, they are associated with monsoon storms; in India, they were seven of the ten most destructive floods in the monsoon season directly caused by these currents.

Climate change and the future of ARs

Climate change is having an intensifying effect on atmospheric rivers. Rising temperatures are air can absorb approximately 7 % more water vapor for every degree of warming, which intensifies atmospheric rivers.

Current NASA research suggests that in a warming atmosphere, ARs will longer, wider and much wetterIf greenhouse gas emissions continue to increase, the most intense ARs may by the end of the century, to appear twice as often as today, although their total number will decrease slightly.

The IPCC emphasizes with high certainty that warming will lead to greater intensity of extreme precipitation not only in the US but also in Western Europe. The increased frequency of intense precipitation from ARs leads to a greater risk of flooding, especially in mountainous areas. Studies from California also point to an increasingly extreme cycle "feasts or fasts" (feast or famine)This means that after extremely dry years when ARs are absent, a series of consecutive torrential rains can come, causing much greater damage because the soil is already saturated.

Given these developments, which increase the risk of floods and extreme precipitation records, global research (NASA, NOAA, IPCC) highlights the need for better forecasting these phenomena and adaptations – for example, by building flood barriers, improving satellite rainfall monitoring, and effectively managing flash floods. Atmospheric rivers will remain a fundamental link in the hydrological cycle, but their increasingly intense manifestations force us to prepare for frequent extreme weather events. JRi