The Importance of Understanding Clouds
One of the most interesting features of Earth, as seen from space, is the ever-changing distribution of clouds. They are as natural as anything we encounter in our daily lives. As they ﬂoat above us, we hardly give their presence a second thought. Yet, clouds have an enormous inﬂuence on Earth’s energy balance, climate, and weather.
Clouds are the key regulator of the planet’s average temperature. Some clouds contribute to cooling because they reﬂect some of the Sun’s energy—called solar energy or shortwave radiation—back to space. Other clouds contribute to warming because they act like a blanket and trap some of the energy Earth’s surface and lower atmosphere.
Cloud systems also help spread the Sun’s energy evenly over Earth’s surface. Storms move across the planet and transport energy from warm areas near the equator to cold areas near the poles. Even small changes in the abundance or location of clouds could change the climate more than the anticipated changes caused by greenhouse gases, human-produced aerosols, or other factors associated with global change.
What ingredients are needed to make a cloud?
The two required ingredients are water vapor and aerosols. Water vapor enters the atmosphere through evaporation from open water, the soil, or the leaves of plants. The wind transports water vapor from one region to another through a process called advection. Aerosols come from natural sources such as volcanoes or forest ﬁres, as well as from human activities such as air pollution.
The Right Conditions
Simply having moisture and aerosols present in the atmosphere does not guarantee a cloud will form. To form an analogy to cooking, a bowl of ﬂour, eggs, butter, and sugar will not come together to form a cake unless the ingredients are combined under just the right conditions. What conditions do we need to have present in the atmosphere for clouds to form?
An Initial Lift
Instability in the atmosphere makes the atmosphere conducive to cloud formation, but there still needs to be some mechanism that gives the initial upward thrust to start the process. The most common lifting methods are convection, convergence, lifting along fronts, and lifting caused by topography. Each lifting mechanism can result in clouds that have diﬀerent physical characteristics.
A parcel may be heated by the Sun-warmed surface of Earth, become warmer than the surrounding atmosphere, and start to rise—a process called convection. The rising air parcel becomes diluted as it mixes with the surrounding air, losing some of its buoyancy. However, each successive air parcel following the same path rises a little higher than the previous one. If a parcel rises high enough to cool to its dew point, the moisture within it condenses and becomes visible as a cloud. On a hot summer day, the atmosphere will often start out clear, but the atmosphere is conditionally unstable, and cumulus clouds rapidly build as the day progresses and the sun heats the surface. If conditions are suﬃciently unstable, these cumulus clouds can grow into towering cumulonimbus clouds that produce thunderstorms and tornadoes.
Another process that lifts air from lower to higher altitudes is convergence, which is the “coming together” of surface winds. When air converges on a location, it can’t pile up there forever. The converging air must go somewhere, and from Earth’s surface, it can only go up. Large-scale convergence can lift a layer of air that is hundreds of kilometers across.
As it does with convection, the air cools as it rises. If it cools to its dew point temperature, water vapor in the air condenses into cloud droplets. Uplifting of air that results from convergence is usually much weaker than uplifting from convection. As a result, clouds generated through convergence don’t usually build up into the towering skyscrapers that convective clouds can become.
Questions for Homework
on a separate Word document answer the following questions and email to me at email@example.com
1. How do clouds regulate the earths climate?
2. What ingredients are need for clouds to form?
3. Describe the process of convection?
4. Describe convergence?