Sign up to access the complete lesson and track your progress!
Unlock This CourseOcean evaporation is one of the most important processes in Earth's water cycle. Every day, millions of tonnes of water transform from liquid to gas and rise into the atmosphere from our oceans. This invisible process powers weather systems, creates clouds and eventually brings rain back to land. Understanding what controls ocean evaporation helps us predict weather patterns and understand climate change.
The oceans cover about 71% of Earth's surface and contain 97% of all water on our planet. This makes them the biggest source of water vapour in the atmosphere. Without ocean evaporation, there would be no rain, no rivers and no life as we know it!
Key Definitions:
Ocean evaporation is the engine that drives the water cycle. When water evaporates from the ocean surface, it carries energy with it. This energy is released when the water vapour condenses to form clouds, powering weather systems around the world.
Temperature is the most important factor affecting ocean evaporation. Just like a kettle boils faster when you turn up the heat, warmer ocean water evaporates much more quickly than cold water. This happens because heat gives water molecules more energy to escape from the liquid surface into the air.
When the sun heats the ocean surface, it provides energy to water molecules. The warmer the water gets, the faster these molecules move around. Eventually, some molecules gain enough energy to break free from the liquid and become water vapour. This is why tropical oceans near the equator, where temperatures can reach 30°C, have much higher evaporation rates than polar seas where temperatures hover around 0°C.
High temperatures (25-30°C) create rapid evaporation. These areas produce most of the world's water vapour and drive major weather systems like hurricanes.
Moderate temperatures (10-20°C) create steady evaporation rates. These areas experience seasonal changes in evaporation as temperatures vary.
Cold temperatures (0-5°C) mean very slow evaporation. Much of the surface may be frozen, preventing evaporation entirely.
For every 10°C increase in ocean temperature, the evaporation rate roughly doubles! This is why climate change, which is warming our oceans, is intensifying the water cycle and creating more extreme weather events.
Wind plays a crucial role in ocean evaporation by constantly moving air over the water surface. Think of it like blowing on hot soup to cool it down - the moving air carries away water vapour and makes room for more evaporation to occur.
When air sits still over water, it quickly becomes saturated with water vapour, which slows down further evaporation. But when wind blows across the ocean surface, it carries away this moist air and replaces it with drier air that can absorb more water vapour. The stronger the wind, the faster this process happens.
Areas with consistent strong winds, like the "Roaring Forties" in the Southern Ocean, have much higher evaporation rates than calm areas. Trade winds in tropical regions also create zones of high evaporation that feed into major weather systems.
These steady winds blow across tropical oceans, creating high evaporation zones that fuel hurricanes and monsoons. They're essential for moving water vapour from oceans to continents.
Humidity is like a sponge's ability to absorb water. Dry air can absorb lots of water vapour, whilst humid air is already "full" and can't take much more. This directly affects how much water can evaporate from the ocean surface.
When air above the ocean is very dry (low humidity), it acts like an empty sponge, rapidly absorbing water vapour from the surface. But when the air is already moist (high humidity), it's like a saturated sponge that can barely absorb any more water. This is why deserts near coasts often have high evaporation rates - the dry desert air constantly pulls moisture from the nearby ocean.
Relative humidity is measured as a percentage. Air at 100% humidity is completely saturated and cannot absorb any more water vapour, stopping evaporation entirely. Air at 0% humidity (which rarely occurs naturally) would cause extremely rapid evaporation.
Ocean water isn't pure - it contains about 35 grams of salt per litre. This salt affects evaporation because it makes it slightly harder for water molecules to escape from the liquid surface. However, the effect is relatively small compared to temperature and wind.
Salt molecules in seawater form bonds with water molecules, making it slightly more difficult for water to evaporate. This is why a glass of salty water will evaporate more slowly than a glass of fresh water at the same temperature. In the ocean, areas with higher salinity (like the Mediterranean Sea) have slightly lower evaporation rates than areas with lower salinity (like near river mouths where fresh water dilutes the salt).
Interestingly, when water evaporates from the ocean, it leaves the salt behind. This means that areas with high evaporation gradually become saltier, which then slightly reduces further evaporation - creating a natural balance.
The Mediterranean Sea demonstrates how multiple factors work together. Its warm temperatures and dry surrounding air create high evaporation rates, but this makes the water saltier than average (39 grams per litre vs 35 grams globally). The high salinity slightly reduces evaporation, but the warm temperatures and dry air more than compensate, making it one of the world's most evaporative seas.
Air pressure also influences evaporation, though it's often overlooked. Lower air pressure makes it easier for water molecules to escape from the liquid surface, whilst higher pressure makes it more difficult.
Think of air pressure like a blanket pressing down on the ocean surface. When pressure is high, this "blanket" is heavy and makes it harder for water molecules to escape. When pressure is low, the "blanket" is lighter and water can evaporate more easily. This is why water boils at lower temperatures on high mountains where air pressure is reduced.
In weather systems, low-pressure areas often coincide with increased evaporation, whilst high-pressure systems can suppress it. However, pressure changes are usually accompanied by changes in wind and humidity, making it difficult to separate the effects.
Different parts of the world's oceans show dramatically different evaporation rates based on how these factors combine. Understanding these patterns helps explain global weather and climate.
High temperatures, consistent trade winds and relatively low humidity create the highest evaporation rates on Earth. These areas fuel tropical weather systems.
Warm temperatures but often calm winds and higher humidity create moderate evaporation. These areas are often associated with stable weather patterns.
Cold temperatures and often ice-covered surfaces create minimal evaporation. These areas contribute little to global water vapour but are crucial for ocean circulation.
The Arabian Sea showcases extreme seasonal variation in evaporation. During winter, cool temperatures and calm conditions create low evaporation rates. But during summer, intense heating and strong monsoon winds create some of the highest evaporation rates on Earth, fueling the Indian monsoon that brings rain to billions of people.
As our climate changes, ocean evaporation patterns are shifting. Warmer oceans generally mean more evaporation, but changes in wind patterns and humidity are creating complex regional effects.
Global warming is increasing ocean temperatures, which tends to increase evaporation rates worldwide. However, this creates more water vapour in the atmosphere, which can increase humidity and partially offset the temperature effect. Meanwhile, changing weather patterns are altering wind speeds and directions, creating winners and losers in different regions.
Scientists predict that wet areas will generally become wetter (due to increased evaporation and precipitation) whilst dry areas become drier (as increased evaporation removes more water than increased precipitation adds). This intensification of the water cycle has major implications for weather extremes, agriculture and water resources.