Objectives: Students will learn about what conditions need to be present in order for cloud formation, condensation of water vapor, to take place. Students will also learn aspects of the water cycle.
11.A.3a Formulate hypotheses that can be tested by collecting data.
11.A.3b Conduct scientific experiments that control all but one variable.
11.A.3f Interpret and represent results of analysis to produce findings.
12.C.3b Model and describe the chemical and physical characteristics of matter (e.g., atoms, molecules, elements, compounds, mixtures).
12.E.3b Describe interactions between solid earth, oceans, atmosphere and organisms that have resulted in ongoing changes of Earth (e.g., erosion, El Nino).
13.A.3c Explain what is similar and different about observational and experimental investigations.
Energy and Evaporation
Light energy from the sun heats the surface of the planet. Every year, 1.25 x 1024 joules of energy from the sun, or approximately 12.5% of the incoming solar energy, drives the water cycle, is used to evaporate water. First the water must be heated to 100 deg C (4.18 J/g-deg), then it must be converted to water vapor. This requires 2260 J/g.
The wind carries the air and its water vapor across the land. When air moves up and over a mountain or ridge (orographic lifting), it cools as it rises. When the air temperature reaches the dew point, the temperature at which air is saturated with water, condensation can begin.Condensation of water vapor to liquid water releases 2260 joules of energy per gram of water. This heats the atmosphere. Ultimately, the heat is radiated out into space.
Structure of Water Vapor and LiquidIt takes much more energy to evaporate liquid water than to evaporate other liquids of a similar weight. Let's look at boiling points. Methane (CH4, 16 g/mol) boils at -162 deg, ammonia (NH3, 17 g/mol) boils at -33 deg, water (OH2, 18 g/mol) boils at 100 deg, and hydrogen fluoride (HF, 20 g/mol) boils at 20 deg.
Why? Water has uniquely strong intramolecular interactions in the liquid and solid states making the condensed phases MUCH more stable than the gas phase. While in the vapor phase, water molecules are separate, bent H-O-H molecules. In the solid phase, each water molecule is bonded to other water molecules in a rigid lattice. Every oxygen atom of water is connected to 4 hydrogen atoms in a tetrahedral array. Most of the water molecules in liquid water has the same connectivity as ice with 4 hydrogens connected to each oxygen. The difference between solid water and liquid water is that the bonds connecting molecules in liquid water are constantly breaking and reforming. This allows individual water molecule to move through the fluid. Breaking all the bonds between water molecules in order to make gas requires energy.
Clouds and Cloud FormationWater in the gas phase is invisible. Clouds are masses of water crystals or droplets gathered together and suspended in the sky. For condensation to occur water molecules need a surface to stick to and organize themselves into one of the condensed phases.
Aerosol particles are always part of clouds. Air that has no dust particles will not produce clouds unless the humidity is over 100%. There are both natural and man-made particles in the atmosphere. Natural sources of this dust include sea spray salt, volcanic dust and bacteria. Man made sources include photochemical smog from fossil fuels and other industrial processes. These particles from the sources above are referred to as cloud condensation nuclei.
Aerosols also cool the air below them due to the reflection of solar radiation from the sun back to space. This lowers the temperature and promotes condensation.
Several process also cool the air and promote condensation and precipitation. Orographic uplift is another process that cools the air and promotes condensation. Warm, moist air flowing over land rises over mountains. The pressure decreases with altitude and the adiabatic expansion of the air cools it. Convectional lifting is the process where the warm, moist air at the surface rises because warm air has a lower density than cooler air. This air quickly cools with altitude and the water vapor condenses. In the process of convergence lifting, a cold and dry air mass meets with a warm and moist air mass. The warmer air mass is lifted above the cool air mass, causing the warmer air mass to be exposed to colder temperatures producing saturation, which in turn produces clouds.
PrecipitationWhen many cloud condensation nuclei bring together a lot of water, the cloud becomes very heavy. The stored water in the clouds from the nuclei sits in equilibrium. This equilibrium is broken by coalescence of water droplets. These droplets become bigger and bigger. Once they reach a level not sustainable by the cloud's uplift, they fall to the ground as rain.
Similar processes based on different temperatures and pressures allows for the formation of different types of precipitation like hail and snow. These are based on the formation of ice crystals. Snow and ice result in the same process as the formation of larger water droplets, but instead of water coalescing, we have collision of ice particles.
The purpose of this experiment is to learn what conditions are necessary for cloud formation. You will generate water vapor, predict the result of adding various things to the water vapor, and test your predictions. The results of the activity can be compared to how clouds are actually formed in the environment.
What air conditions are needed for vapor to rise?When the sun shines on water in oceans or lakes it heats up the water and supplies the energy needed to turn liquid water into water gas (steam or water vapor). Water vapor contains more energy than liquid water. Sometimes wind blows the warm air and water vapor over a mountain where it quickly moves to a greater height and gets colder. Other times warm air hits a region of cold air and the warm air rises to go over it. Finally, even without wind to drive it, the warm air will rise because it is lighter than surrounding air. The air high above the air is colder than air at the surface. The vapor rises until it reaches a low enough temperature and pressure to condense water vapor into water droplets and form a cloud. When vapor condenses into liquid or solid particles, it loses some of its energy to the air.
What else is needed for clouds to form?Water in the gas phase must have a solid surface to stick to in order to form a droplet of liquid water. There are many particles available in the atmosphere. They can be bits of salt blown off the surface of the ocean, dust blown by the wind from the ground, soot from forest fires or smokestacks, and some chemicals from power plants and factories. These particles are called cloud condensation nuclei (CCNs). Water vapor particles stick to these CCNs, which cause small droplets to coalesce into much larger droplets. The CCNs also lower the temperature of the air by reflecting more of the sun's light back into space.
What causes precipitation?Precipitation is when the water vapor gathered in the cloud falls to the ground in various forms. Clouds have air uplifts that keep the water present in the cloud from falling to the ground. CCNs gather water droplets together to form bigger and heavier droplets. Phase changes due to temperature can occur by changing the structure of the H2O molecules. Temperature and pressure influence this change. When enough CCNs have gathered enough water, droplets too heavy to be supported by the air uplifts form and begin to fall to the ground as precipitation. Different temperature and pressure conditions allow for the formation of different types of precipitation, whether it is rain, snow, or hail. In this experiment the goal is for you to determine which of the available materials (a lit match, ice or soil) act as the CCN to promote cloud formation.
Research Question: Which of these materials (dirt, ice, smoke) is best at condensing water vapor into a cloud?