What Is the Average Precipitation in the Marine Biome?

Averaged over the whole ocean, about 2.8 to 2.9 mm of rain falls each day. That adds up to roughly 1,000 to 1,100 mm a year (about 40 to 42 inches), close to what a fairly wet city on land receives. But that single figure hides a huge spread, from stretches of open sea as dry as deserts to belts that are among the rainiest places on Earth.1PubMed. Oceanic deserts

The Number in Daily and Yearly Terms

Those daily rates become yearly totals with one line of arithmetic: multiply millimeters per day by 365. So 2.94 mm a day comes to about 1,070 mm a year. To switch to inches, divide by 25.4, since an inch is 25.4 millimeters, which is why roughly 1,000 mm lands near 40 inches. Rain depth means the layer of water that would pile up on a flat surface if none ran off or evaporated, so 1,000 mm is a depth of about one meter.

The older 2.68 mm figure gives about 980 mm, or 38.5 inches, a year. The gap exists because newer satellites detect light rain and snow that older ones missed, so the newest estimate runs about 9% higher than the oldest.5Journal of Climate. An Update on the Oceanic Precipitation Rate and Its Zonal Distribution in Light of Advanced Observations from Space For homework, about 1,000 mm or 40 inches a year is a fair round answer.

Textbooks differ because each dataset uses different satellites and different years. A difference of a few percent is small next to the regional range, but naming your source and its year makes your answer stronger.

The word “average” deserves care on an ocean this big. It blends desert-dry areas and storm-soaked areas into one figure, so almost no single spot receives exactly that amount.

The satellite estimates also stop at 80°S and 80°N, so the sea near the poles, much of it ice-covered, is left out. Writing “over most of the global ocean” is more honest than claiming the figure covers every sea.

How Ocean Rain Compares With Land

About 403,500 cubic km of rain falls on the oceans each year, versus about 116,500 cubic km on land.6Surveys in Geophysics. The Global Water Cycle Budget: A Chronological Review That means roughly three-quarters of Earth’s precipitation lands on the sea, which makes sense because oceans cover about 71% of the surface. Rain over the ocean also tends to fall more intensely than rain over land.7Remote Sensing. Differences in Global Precipitation Regimes between Land and Ocean Areas Based on the GPM IMERG Product

The bigger total is mostly a matter of area. Total volume is area times depth, so the ocean’s enormous size makes its total huge even if a single square meter gets only a little more than land does. Land averages roughly 800 mm a year, a bit below the ocean. Comparing depth in millimeters is the fair test, and it shows the two in the same general range, with the ocean somewhat higher. Land hides a wide spread too: rainforests and monsoon hills can beat any ocean average, while land deserts get as little as ocean deserts.

Where the Ocean Is Wettest and Driest

Near the equator, where the trade winds meet, a band of thunderstorms circles the globe. It is the most prominent rainfall feature on the planet.8USGS. Precipitation changes in the western tropical Pacific over the past millennium It shifts north in the Northern Hemisphere summer and south in the Southern Hemisphere summer.9USGS. Precipitation changes in the western tropical Pacific over the past millennium The belt’s seasonal movement creates wet and dry seasons. An island under the equatorial rain belt gets a rainy season when the belt moves over it, with frequent afternoon or evening storms. When the belt moves toward the other hemisphere, the same island dries out and gets more sunshine. Places farther from the equator may be under the belt for only a few months, so they get one short wet season rather than rain all year. Farther from the equator, mid-latitude storm tracks bring frequent rain to colder oceans like the North Atlantic and the Southern Ocean.

The driest water lies around 20–30° north and south, where persistent high-pressure systems settle over the ocean.10PubMed. Oceanic deserts This sinking, stable air stops tall rain clouds from forming, so yearly rain in these “ocean deserts” can be as low as on land deserts.11PubMed. Oceanic deserts Dry patches at 20–30° latitude often sit on the eastern side of ocean basins, next to the west coasts of continents. Closest to the poles the air is so cold that it holds little water vapor, and totals fall off again.

Coasts are special. In the tropics, rain peaks at the coastline and drops off fast within about 300 km on either side.12Journal of Climate. How Much is the Precipitation Amount over the Tropical Coastal Region? This coastal strip collects about 34% of all tropical rain, with 52% over open ocean and 14% inland.13Journal of Climate. How Much is the Precipitation Amount over the Tropical Coastal Region? Land-sea breezes can trigger rain even when the air higher up is fairly dry.14Geophysical Research Letters. How important is tropospheric humidity for coastal rainfall in the tropics?

Why So Much Rain Falls on the Sea and Where It Goes

About 449,500 cubic km of water evaporates from the oceans each year, more than the 403,500 that falls back as rain.15Surveys in Geophysics. The Global Water Cycle Budget: A Chronological Review The surplus does not vanish: about 46,000 cubic km of ocean water vapor drifts over land and falls there each year.16Surveys in Geophysics. The Global Water Cycle Budget: A Chronological Review17Hydrology and Earth System Sciences. Intercomparison of freshwater fluxes over ocean and investigations into water budget closure About the same amount then returns to the sea through rivers and runoff.18Surveys in Geophysics. The Global Water Cycle Budget: A Chronological Review The cycle balances, so the sea is not slowly drained.

Warm seas make so much rain because sunlight warms the surface, and water molecules at the top gain enough energy to leave as invisible vapor. Over warm seas this never stops. Moist air near the surface is lighter than drier air, so together with warming from the sea it tends to rise, and as it rises it cools. Cooler air can’t hold as much vapor, so the vapor condenses onto tiny particles as cloud droplets. When droplets bump together and grow heavy, they fall as rain. The tropics get the heaviest rain because warm air holds more water vapor, and the rising air where trade winds meet lifts all that moisture into downpours.

How Scientists Measure Rain Where There Are No Rain Gauges

A rain gauge is a fixed container that catches rain, and over most of the ocean there is no land to put one on. Ships move and rock, so water in a gauge sloshes and can’t be read accurately. Wind over waves can blow rain away from the opening, so it undercounts. Buoys and islands give a few trustworthy spots, but they are tiny compared with the whole ocean.

So satellites carrying radar and microwave sensors estimate rainfall over the whole ocean. Radar sends out a pulse and measures how much bounces back from raindrops, which says how heavy the rain is. Microwave sensors measure radiation coming up from the sea and clouds, which changes when rain is in the way. Computer programs convert these signals into rain rates and combine several satellites to cover the globe.

Scientists check these satellite estimates in other ways, and one of them is sound. Rain hitting the sea makes a loud, distinctive noise underwater,19NASA. Listening to Water: Using Underwater Microphones to Measure Ocean Rainfall so underwater microphones can measure raindrop sizes and rainfall amounts.20NASA. Listening to Water: Using Underwater Microphones to Measure Ocean Rainfall Listening devices on drifting floats hear rain over an area about 5 km wide, similar to a satellite’s view.21Geophysical Research Letters. Evaluating Satellite Precipitation Estimates Over Oceans Using Passive Aquatic Listeners

Salt helps too. Rain dilutes surface seawater, so changes in ocean saltiness reveal how rainfall is changing.22NOAA Climate.gov. Sea Surface Salinity Trends

When Rain Falls at Sea

Rain over the ocean falls more often at night.23Journal of the Atmospheric Sciences. The Diurnal Precipitation Change over the Sea Showers over many oceans peak between midnight and 4 a.m., and storms around midnight.24Journal of Climate. Global Precipitation and Thunderstorm Frequencies. Part II: Diurnal Variations

One likely reason is that during the day the sun warms the upper air and clouds, but at night the cloud tops cool by giving off heat to space. That cooling makes the air in and around the clouds less stable, which favors rising air and rain.

On land the clock is the opposite. The ground heats fast in the sun, so warm air rises in the afternoon and builds late-day thunderstorms. The sea surface warms very little between day and night, so nothing sets off afternoon storms, and sailors often meet squalls in the dark.

How Rain Affects Ocean Life

Rainfall links climate to food webs. Wind lifts dust from deserts and land, and it can travel across oceans before it falls. Rain scrubs that dust out of the air and carries its iron into the surface water, delivering about 60% of the iron reaching the open ocean.25Journal of Geophysical Research: Atmospheres. Aeolian iron input to the ocean through precipitation scavenging: A modeling perspective and its implication for natural iron fertilization in the ocean Iron is a trace nutrient that plankton need in tiny amounts, and many open-ocean regions are short of it. So a rainfall can work like a dose of fertilizer, helping tiny plankton grow in nutrient-poor waters.26Communications Earth & Environment. Atmospheric soluble iron sustains nitrogen fixation in oligotrophic oceans

Plankton are tiny floating plants and other organisms that use sunlight to make food, and they are the base of most marine food webs. They feed small animals, which feed fish, which feed larger animals including seabirds and whales. A change in nutrients that reaches the plankton can ripple up the whole chain.

Some animals need the rain itself. Open-ocean sea snakes don’t drink seawater and rely on freshwater layers from heavy rain, so in dry stretches they can become dehydrated.27PubMed Central. Pelagic sea snakes dehydrate at sea The link also runs the other way: plankton release a gas that helps form clouds, tying life back to rain.28Communications Earth & Environment. Increased oceanic dimethyl sulfide emissions in areas of sea ice retreat inferred from a Greenland ice core

How Ocean Rain Is Changing

Over oceans, already wet regions tend to get wetter and dry ones drier.29International Journal of Climatology. “Dry gets drier, wet gets wetter”: A case study over the arid regions of central Asia Warmer air holds more water vapor, so places with rising, moist air get more moisture to turn into rain. Dry regions with sinking air gain nothing from the extra vapor, and some lose rain as patterns shift. Rain over wet tropical oceans rises about 3% for every 1°C of warming.30Environmental Research Letters. Observed and simulated precipitation responses in wet and dry regions 1850–2100 This is a general pattern, not a rule every spot follows.

El Niño, a periodic warming of the eastern Pacific, already reshapes rain worldwide.31PubMed. Robust twenty-first-century projections of El Niño and related precipitation variability By later this century, El Niño drying in the western Pacific and rain in the central and eastern Pacific are projected to strengthen.32PubMed. Robust twenty-first-century projections of El Niño and related precipitation variability Southern storm tracks are also projected to shift about 2.6° toward the pole.33PubMed Central. The future poleward shift of Southern Hemisphere summer mid-latitude storm tracks stems from ocean coupling