If you look at what happens in nature, such as clouds beginning to form when the air rises and grows colder, the idea that condensation begins when the air grows too cold to “hold” the water vapor in it seems to make sense. But, saying cold air can’t hold as much water vapor as warmer air is at best a metaphor for what happens. It’s a metaphor that can lead people astray as they try to understand weather. To begin with, it doesn’t make any physical sense because the air around us is mostly empty space with molecules of nitrogen, oxygen, water vapor, and other gases zipping around at speeds in the neighborhood of 1,000 mph near the ground. There’s plenty of room for water vapor molecules to join the mix. As we saw above, they would displace some of the nitrogen and oxygen molecules. We can think of these displaced molecules spreading out into nearby, drier air. As the air grows colder the average speeds of all of the gasses in the air slow down, but there is still plenty of room for water vapor. If so, you’d see water molecules moving around at various speeds in the water in the glass glass and also in the air above the water as vapor. On the average, the molecules in the glass are moving slower than those in the air. The liquid molecules in the glass are, on the average, slow enough for intermolecular forces to hold them in the glass while they otherwise move freely. The vapor molecules in the air are moving fast enough to overcome intermolecular forces. In both cases the molecules are not all moving at the same speed, but at a wide range of speeds with most of these speeds relatively close to the average, but a few are moving much faster and others much slower. Some of the molecules in the glass will be moving fast enough to escape the water and fly into the air; they evaporate into vapor. In a similar way, some vapor molecules in the air are going slowly enough to be pulled into the water by intermolecular forces when they hit the water; they condense into liquid. If a breeze is not carrying away water molecules that evaporate into the air and the temperature doesn’t change the number of molecules evaporating and those condensing are roughly the same. The air isn’t becoming more humid and water is staying at the same level in the glass. If you cooled the room the average speeds of the water molecules in both the water and the air would slow. You’d see more water molecules in the air moving slowly enough to stay in the water when they hit―they’ve condensed. At the same time fewer molecules in the water are moving fast enough to evaporate. The water level in the glass would increase. If you warmed the room and the water the average speed of the molecules would increase and more would evaporate until a new equilibrium is reached between evaporation and condensation. In the everyday atmosphere, if the air is cooled enough as it rises water molecules slow down enough to attach to tiny particles in the air known as condensation nuclei to begin forming fog or cloud drops. At ground level they form dew drops on grass and objects such as cars. For more on why saying condensation begins when the air can no longer hold the water vapor in it is both wrong and can lead to wrong conclusions, see Alistair B. Fraser’s “Bad Clouds” page on his Bad Meteorology site on the Pennsylvania State University Web site.
Hot air expands, and rises; cooled air contracts – gets denser – and sinks; and the ability of the air to hold water depends on its temperature. A given volume of air at 20°C (68°F) can hold twice the amount of water vapor than at 10°C (50°F). The relationship of how much water a given mass of air actually holds compared to the amount it can hold is its relative humidity. When air holds as much water vapor as it can for a given temperature (100% relative humidity), it is said to be saturated. If saturated air is warmed, it can hold more water (relative humidity drops), which is why warm air is used to dry objects--it absorbs moisture. On the other hand, cooling saturated air (said to be at its dew point) forces water out (condensation). This is why a container of a cold beverage sweats: it cools the air next to it and moisture from the air condenses on the outside of the can. Air warmed by ocean currents picks up a lot of moisture. As the heated air rises, it expands, which is measured at the surface as low air pressure. Expanding air cools, which forces it to lose its moisture as rain or snow. The opposite is true for sinking air. Such air compresses and warms. In a zone of high pressure like this, moisture is absorbed by the air from its surroundings.
 Physical Properties of WaterWater is an enormously efficient heat-sink. Solar heat absorbed by bodies of water during the day, or in the summer, is released at night, or in winter. Sites on islands or coasts benefit from the moderating effect of the ocean and have "maritime" climates (like San Francisco). Sites away from the coast lack this temperature buffering and have extreme "continental" climates (like Wichita).
Like the heated air in a hot-air balloon, heated water expands. Solar heat absorbed at the equator causes water to expand. Such heated water raises the normal level of the sea surface, and such changed ocean topography can be measured. The heat in the water is carried to higher latitudes by ocean currents where it is released into the atmosphere. Water chilled by colder temperatures at high latitudes contracts (thus gets denser), sinks, (lowers the local topography) and returns to the equator via the global ocean water circulation conveyor belt to complete the cycle.
« Back to Meteorology, Climatology page Cold air causes the warm moisture in our breath to condense into tiny droplets of water that appear like a small, misty cloud. Many people think seeing your breath has everything to do with temperature, but the spectacle has just as much to do with the amount of moisture in the atmosphere. Because our bodies contain nearly 70% water, the air in our lungs is almost completely saturated with water vapor (water in gas form) and is the same temperature as our bodies (98.6oF). Cold air cannot hold as much moisture as warm air. So when one exhales a warm, saturated breath on a cold day the cold air rapidly lowers the temperature of our breath, whereupon the combination briefly reaches dew point. At dew point, air can no longer hold water vapor; when air is cooled beyond dew point water vapor turns to liquid form, the physical process known as condensation. It is this liquid form of your breath – minuscule droplets of water – that creates the fleeting, misty cloud we see when breathing in cold weather. Seeing your breath requires just the right combination of temperature and humidity. Though it is pretty common to see your breath in cold weather (usually below 45oF), the next time you have fun making breath clouds, you’ll know it’s because of the exact science of atmospheric moisture and temperature. Published: 11/19/2019. Author: Science Reference Section, Library of Congress
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