Liquid water in the atmosphere
Review of condensation and evaporation
Recall that vapor pressure is a function of the amount of water vapor in the air. Saturation vapor pressure refers to the vapor pressure such that the rate of condensation over a liquid surface would equal the rate of evaporation. The ratio between a ctual vapor pressure and saturation vapor pressure is called the saturation ratio and defines relative humidity.
Saturation vapor pressure is a function of temperature but vapor pressure is not. Saturation vapor pressure increases as temperature increases. Therefore, as temperature increases, and the amount of water vapor in the air remains constant (a function of the vapor pressure), the relative humidity will decrease. So relative humidity is not a measure of how much water vapor is in the air, but it's a measure of how much water vapor is in the air based on how much water vapor the air can hold at its curr ent temperature.
Another method of representing relative humidity is mixing ratio. Mixing ratio, represented by the letter w, is the grams of water vapor in the air per kilograms of dry air. Specific humidity represented by the ltter q, is grams of water vapor per total grams of air.
Dewpoint refers to the temperature that the air would need to be cooled to such that the vapor pressure equals the saturation vapor pressure.
Supersaturation
Can the relative humidity of the air be above 100 %. Yes If the vapor pressure exceeds the saturation vapor pressure, the air is supersaturated. Condensation will occur if there are condensation nuclei or if there is a hydrophilic surface. Hydrophilic means water loving - a surface that water will easily adhere to.
To explain this principle, we will consider what occurs in cloud development. The essential ingredient for cloud development on Earth is vertical motion. Air at a given temperature, dewpoint, and vapor pressure often are subjected to forces that will cause the air to rise. As it rises, its temperature cools thus its satura tion vapor pressure cools, however, the vapor pressure remains nearly constant. The saturation ratio and thus the relative humidity then increase. Once the temperature has been cooled to below the dewpoint temperature, the relative humidity will be grea ter than 100 % and supersaturation will exist. The rate of condensation onto a surface will then exceed the rate of evaporation.
Tiny particles called condensation nuclei can be found worldwide in the atmosphere. Volcanic ash, dust, and pollen are examples of objects that can behave as condensation nuclei. What essentially happens is water condenses onto the surface of the cloud condensation nuclei. If this condition exists for a sufficiently long period of time, a cloud droplet is formed. A cloud is thus a collection of cloud droplets. In certain circumstances, cloud droplet condensation in a cloud will grow to a level where there will be numerous collisions between the cloud drops causing some droplets to grow into raindrops.
To understand the difference between cloud drops and raindrops and why cloud drops don't fall to the ground as raindrops, we need to consider the concept of air resistance. Any falling particle in the atmosphere is subjected to air resistance. The am ount of resistance an object encounters is a function of its cross-sectional area, its density, and its shape. Air resistance acts opposite to the direction of motion. When air resistance exactly balances the force of gravity pulling downward, an object is suspended and is remaining at the same level. This is the case with cloud droplets and is why they do not fall to the ground as precipitation. As the droplets grows bigger however, the effect of gravity becomes significantly stronger than the air resistance and the object falls to the ground as precipitation.
Precipitation formed according to the above process is called warm rain. We will discuss cold rain in the coming weeks which refers to precipitation that forms as ice crystals or snow in the area of a cloud that is above the freezing lev el and melts into rain as it falls downward.
Why are clouds not always present in the sky? If you go high enough in the atmosphere, the temperature will equal the dewpoint temperature where condensation would begin to occur. The necessary ingredient is vertical motion. There needs to be a mech anism for air to be lifted so that it can cool to its dewpoint temperature and the begin stages of cloud formation can take place. You can have a very humid day where the air would only need to be lifted a short distance for clouds to form, but if there is no mechanism to lift the air, clouds will not form. We will be talking about different situations that lead to vertical motion in the coming weeks.
The level in the atmosphere where the temperature is equal to the dewpoint temperature and cloud formation will be possible if air is being lifted is called the Lifting Condensation Level (LCL).
Fog
Fog can essentially be referred to as a ground cloud. We will talk about two types of fogs today. Advection fog and radiational fog.
As its name implies, advection fog refers to fog that has been advected into a region. This is a typical situation along a coastline or a lakeside. Fog will form over the water and then drift or be blown onto land. Recall that advection refers to qu antities that are transported by the wind.
Radiational fog is when surface air is cooled to its dewpoint temperature. This condition will typically occur on cool, clear nights. Once the sun sets, on a clear night with no clouds, heat will quickly be radiated to space. On cloudy nights, the a tmosphere does not cool as quickly because the clouds can actually serve as a blanket and can trap some of the heat in.
If the air cools below the dewpoint temperature, supersaturation exists and water will begin collecting onto hydrophilic condensation nuclei that are present in the air. This once again is the beginning of cloud development. This is what occurs in ra diational fog formation: a cloud essentially forms right at the ground.
Dew
Sometimes the concentration of condensation nuclei in the air will not be sufficient enough for a cloud to form, however, supersaturation may exist. Hydrophilic surfaces (such as grass, car windows, leaves) will experience the same thing that a cloud condensation nuclei will experience and will collect water in this situation. This is essentially what dew is. As with radiational fog, dew will typically occur on cool, clear mornings.
More on cloud development
The cloud formation process described above, where water condenses onto tiny objects called condensation nuclei, is called heterogeneous nucleation. When cloud drops form from water molecules collecting together into a liquid droplet without co ndensing onto a condensation nuclei, it is called homogenous nucleation. Homogenous nucleation is possible in theory but does not occur readily in the atmosphere. Relative humidities of well over 400 % would need to exist for homogenous nucleatio n to occur.
There are several different scenarios that can promote the vertical motion necessary for a cloud to develop. One such instance is a frontal boundary called a warm frontal boundary (clouds easily form at cold frontal boundaries as we will learn in comi ng weeks). A warm front describes a situation where warm air advances into cold air. Since cold air is heavier and denser than warm air (as can be explained by the Ideal Gas Law), the warm air will rise up over the top of it. The warm air is being lift ed and once it reaches its dewpoint temperature, condensation will begin to occur.
Another example would be traveling air that encounters a mountain. The air cannot go down or through the mountain and its only alternative is to rise upward. The air cools as it rises and once its relative humidity exceeds 100 %, cloud formation will likely begin.
Convection currents are another mechanism that can transport air upwards. These are typical during the summertime. The ground is heated by the sun much faster than the air above it as the specific heat of the ground is higher than air (meaning that equal amounts of heat will cause the temperature of the ground to be higher than the air). Therefore, the air right above the ground is heated directly by the sun and also by the ground which is being heated as well. Since the pressure of the gas at the ground is essentially remaining constant but the temperature is rising, the Ideal Gas Law tells us that the density will decrease. When the density of a fluid is lower than the fluid above it, that fluid will begin to rise upward. That mechanism can lead to cloud development and often during the summer, shower and thunderstorm activity.
We will expand greatly on that topic on February 6 when we cover the subject of atmospheric stability.
On Friday, 1/30, we will discuss different cloud formations and how they are classified.