The phenomenon of thermal equilibrium, which explains why our coffee cools in its cup, also occurs on a planetary scale. With notable consequences on the climate: it is in particular the origin of the “sea breeze”, a relatively cold and humid sea wind characteristic of the climate of coastal regions.
By Patrick Augustin, Littoral Opal Coast University
You’ve probably already noticed that when you don’t drink your coffee hot, after a while it gets cold. Conversely, cold yogurt taken out of the refrigerator and left on the table warms up over time.
This phenomenon is called “thermal balance”: the coffee, hotter than the ambient air, will transfer its heat to the surrounding air. Over time, the coffee cools until its temperature equalizes that of the surroundings. In the same way, the yogurt, colder than the ambient air, will absorb heat from the air and heat up, reaching ambient temperature. So, whether it’s coffee cooling or yogurt warming, the thermal balance that occurs in nature causes temperatures to tend toward a uniform temperature, without effort on our part.
This phenomenon also occurs on a larger scale, and it is not without consequences: the thermal balance between the land and the seas or oceans is a key process in the regulation of the global climate. Here’s what we know.
A thermal imbalance between land and sea/oceans
Water has a much higher heat capacity than land. This means that seas/oceans absorb and release heat more slowly than land surfaces. So water heats and cools much more slowly than land.
Under these conditions, a thermal imbalance can form between these two systems. This difference in heat capacity plays a crucial role in atmospheric circulation, wind formation and short- and long-term climate variations.
Indeed, temperature variations on earth are rapid and significant, leading, for example, to hot summers and cold winters in continental areas. The sea/ocean acts as thermal regulators, storing heat during the summer and releasing it during the winter, which helps to soften coastal climates.
The consequence of the existence of a thermal imbalance between the land and the sea/ocean is also perceptible in the diurnal cycle, during which these two systems react differently under the effect of solar radiation. During the day, the land surfaces are warming more quickly than sea surfaces. Thus, near coastal areas, the heated air in contact with the ground rises (under the effect of Archimedes’ buoyancy), accompanied by a reduction in atmospheric pressure.
Above the sea, the heating of the air in contact with the sea/ocean surface is slower, the air is denser and the pressure is greater. This difference in pressure between the air above land and above the sea/ocean causes cold, moist air from the sea to move towards land (figure 1). This mechanism corresponds to a local weather phenomenon called sea breeze.
This relatively colder and moister (denser) sea wind flows horizontally inland to replace the warmer, drier (less dense) air that rises above the land surface. The interface zone between the sea air (located in front of the breeze current) and the air above the land is called the sea breeze front. The passage of the front is often marked by sudden changes in temperature, humidity, wind direction and speed. This thermal and dynamic contrast can also promote, under certain conditions, the formation of clouds.
Indeed, when warm air from land meets sea air at the level of the breeze front, a convergence zone is formed favoring an upward thrust of the two air masses called an updraft. This rise of warm, humid air causes the condensation of water vapor, forming clouds, generally cumulus type. These clouds often form along the sea breeze front, where the two air masses meet.
In certain situations, when the temperature difference between land and sea is particularly marked, warm air can rise quickly, forming not only cumulus clouds, but also cumulonimbus clouds, which can lead to showers or local thunderstorms at the end of the day.
Towards thermal equilibrium
This movement of cool air helps rebalance temperatures by cooling heated land areas, while reducing the thermal difference between the land surface and the sea. By bringing in cooler air, the sea breeze softens temperatures terrestrial, especially during periods of intense heat, and thus restores a certain thermal balance. The sea breeze therefore plays a crucial role in improving thermal comfort. It reduces the feeling of excessive heat for residents, thus improving their well-being.
In some situations, the sea breeze can strengthen as the temperature difference between land and sea increases, which can result in stronger wind near the coast. It can have a significant impact on wind energy production, particularly in coastal regions, especially if onshore winds are weak. Indeed, this meteorological phenomenon can generate regular and predictable winds allowing more efficient and predictable production, thereby reducing the variability of wind energy production. These winds are often strongest in the late morning and afternoon, making them a valuable source of energy for wind turbines located near coasts.
Furthermore, it can also influence the concentration and distribution of atmospheric pollutants since it can disperse pollutants. Indeed, it can participate in the renewal of the air above the land, by replacing stagnant, more or less polluted air with air coming from the sea. However, in certain coastal areas where industrial/automobile pollution is significant , the sea breeze can transport pollution from coastal areas inland. Under certain conditions, it can contribute to the formation of a boundary layer trapping polluted air near the ground, which limits the vertical dispersion of pollutants and can worsen air quality locally.
The thermal balance between these two systems is a fundamental mechanism that regulates the climate. This balance is therefore essential to the stability of ecosystems and human societies. It requires particular attention to maintain natural regulations in the face of increasing pressures linked to global climate change.
This article is published as part of the Science Festival (which takes place from October 4 to 14, 2024), and of which The Conversation France is a partner. This new edition focuses on the theme “ocean of knowledge”. Find all the events in your region on the site Fetedelascience.fr.
Patrick AugustinResearch engineer, Littoral Opal Coast University
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.