Chemistry of Everyday Things

CONTENTS OF CURRICULUM UNIT 11.05.06

  1. Unit Guide
  1. Introduction
  2. Rationale
  3. Background
  4. Strategies/Activities
  5. Works Cited
  6. Appendix

The Chemistry of Weather

Deborah A. Johnson

Published September 2011

Tools for this Unit:

Background

Weather is the state of the air at a particular place and time. Weather can be warm or cold, dry or wet, windy or cloudy. To understand how these phenomena occur, we must first start with understanding matter. Matter is anything that takes up space and has mass. Matter exists on Earth in the forms of solids, liquids and gases. What defines the difference between a solid, liquid, and a gas is the way in which the particles that make up the matter are arranged and behave. All matter has particles that are in constant motion and this is based on the kinetic-molecular theory. In a solid, particles are arranged closely packed and these particles vibrate and move around a fixed point. Solids have a definite shape and volume. Examples of solids on Earth are glaciers, ice, snow caps, and icebergs.

As heat is applied to the solid, the particles move farther apart and are in constant motion which makes the liquid to be fluid. A fluid is a substance that can flow; thus, liquids take the shape of its container. Liquids take up the space of its container and have a definite volume. One example of a liquid on Earth can be found in salt and fresh water bodies, such as oceans, lakes, rivers, and ponds. Particles in a liquid move slower and are closer together than those in a gas.

Gases consist of particles that are fast-moving that are far apart. Gases are fluid, as liquids are fluid. Gases do not have a definite volume and take up the space of a container. The particles move rapidly and bounce off one another, like hard spheres. The kinetic molecular theory applies only to ideal gases. "An ideal gas is an imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory. The fact is that on Earth there are real gases which are gases that do not behave according to the kinetic-molecular theory; thus, the particles collide and may stick together instead of bouncing off one another, as they would do in an ideal gas situation". 1 An example of gases that exist on Earth as it relates to weather is the atmosphere.

The atmosphere consists of four layers: the troposphere, the stratosphere, the mesosphere, and the thermosphere. The thermosphere is divided into the ionosphere, and the exosphere. These layers are classified by the changes in temperature. Weather occurs in the troposphere. Tropo- means turning or changing. As the altitude in the troposphere increases, the temperature decreases; thus, snow peaks on the tops of mountains. Earth's atmosphere consists of nitrogen at 78 percent, oxygen at 21 percent, and other gases at 1 percent. The other gases include carbon dioxide; water vapor is also present but not included in these percentages because its amount varies. There are other components, as well, such as particles of liquids and solids.

There are four measurable quantities that express a gas: volume, temperature, number of molecules, and pressure. There are gas laws that express these four properties of gases. As it relates to weather, there are four laws that will be examined: Boyle's law, Charles law, Gay-Lussac's law, and Dalton's law. "The gas laws are simple mathematical relationships between the volume, temperature, pressure, and the amount of a gas. Pressure is the result of molecules making collisions against a container's wall." 2

"Boyle's law states that the volume (V) of a fixed mass of gas varies inversely with the pressure at constant temperature (T)." 3 It is expressed as V = k/P or PV = k, where k is a constant.

This means that if the temperature stays the same and you decrease the pressure on the gas, the volume will increase. We see this on Earth when you climb a mountain. As you go up, the air particles are more spread out; thus, the pressure is reduced. That is why people feel light-headed at higher elevations because there are fewer air molecules per square centimeter.

With meteorology, which is the study of weather, weather balloons are used to forecast weather. Why do balloons rise? This is due to the next law, Charles's law. "Charles's law states that the volume of a fixed mass of a gas at constant pressure varies directly with the Kelvin temperature. Kelvin temperature is a scale that starts at -273.15°C." 4 It is expressed as V = kT or V/T = k, where T = Kelvin temperature. This means if the pressure stays the same and you increase the temperature, then the volume will increase. We see this on Earth with hot air balloons. As heat is applied, the balloon inflates. This goes back to the property of gases that the more heat makes particles move faster and can also change states of matter. This causes ice to melt and water to evaporate which explains part of the water cycle. Remember, particles in solids only vibrate; particles in liquids move quicker and the particles are not as close; and then particles in gases move faster and are spread more apart.

The third law that relates to weather is the Gay-Lussac's law. This law states: "the pressure of a fixed mass of gas at a constant volume varies directly with the Kelvin temperature." 5 It is expressed as P = kT or P/T = k, where k is a constant. This means if the temperature increases, the pressure increases, as well. Therefore, as air molecules are heated, they move faster, thus making more collisions and pressure is increased. The language used by meteorologists is high and low atmospheric or air pressure. Air pressure is the phenomenon of a column of air pushing down on a particular area. Air pressure not only pushes downward but it is the weight of air pushing in all directions. The atmosphere closest to Earth has more pressure than the atmosphere at higher altitudes because if you think if a column of air pushing down on an object, there is all the weight of the air nearest to the surface of the Earth. The farther you go up in elevation, the less air is there to add to the weight.

Air has properties which are mass and density, as well as pressure. The density of the atmosphere must be discussed, at this point. Density is equal to mass divided by volume, d = m/V. Since air has density, the denser the air is, the more pressure there is. In other words, denser air has more mass per unit volume than less dense air. Density is affected by three factors: the amount of water vapor, elevation and temperature. Warmer air is less dense than colder air and warmer air holds more water vapor than colder air. Therefore, hot air with more water vapor is less dense and has lower air pressure. But, you may think that if air has more water vapor in it, that the air would be denser. This is where Dalton's law helps to explain this.

Dalton's law deals with the pressure of a gas mixture which is the sum of each individual gas pressure. We know that our atmosphere is a mixture of gases as stated above. Water vapor is less dense than nitrogen or oxygen of the other gases mentioned. "Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases." 6 This law is expressed as: P T = P 1 + P 2 + P 3 + …, where P Tis the total pressure of the mixture, and P 1, P 2, P 3, … are the partial pressures of the component gases. Water vapor has a lighter molecular mass than diatomic nitrogen and diatomic oxygen, which are found in our troposphere. Water vapor has two hydrogen molecules and an oxygen molecule giving it an atomic mass of 18. Diatomic nitrogen has an atomic mass of 28 and diatomic oxygen has an atomic mass of 32. So, although air with water vapor feels heavier and is harder to breathe, it is still less dense than cold, drier air. Water vapor is a greenhouse gas that traps heat on Earth, as well.

Water vapor contributes about 60% to the warming process of the Earth's atmosphere. It is like a sweater surrounding the Earth, trapping the heat radiation from the ground. Energy is stored in water vapor as heat. Water vapor is considered to be a positive feedback to greenhouse warming meaning as more water is evaporated into the air due to higher temperatures, the warmer the air will become, thus making a positive feedback loop. However, along with more evaporation of water into the air leads to more cloud formation which can lead to the clouds reflecting the Sun's radiation away from the Earth. The cloud cover lowers the temperature, thus producing a negative feedback loop and increased water vapor could have a self-correcting effect. There is much debate, currently, about this issue of water vapor and its impact on global warming, which is the gradual increase of the Earth's atmospheric temperature.

Cloud formation is due to a phase change called condensation. Just the opposite of evaporation, condensation occurs when heat is released from a gas. Clouds are formed when water vapor condenses around tiny solid particles in the air. Clouds are classified by their appearance and height. The three basic forms of clouds are stratus, cirrus, and cumulus. Cirrus clouds are high in the sky and are white, thin, and wispy. The cumulus clouds are puffy with a flat base and look dome-like resembling cauliflower. The stratus clouds look like sheets that cover the sky. The height of clouds classifies them further. High clouds are usually white and made up of ice crystals. Since there is more water vapor available at lower altitudes, low and middle altitude clouds tend to be darker and denser. The reason why not all clouds precipitate, although they all contain water, is that the size of droplets are so small that for high clouds, when the droplets fall, even in humid air, they evaporate before they reach the ground. A raindrop that reaches the ground is about a million times larger than a cloud droplet. For a rain droplet to form, these cloud droplets must join together in order to fall to Earth. This phenomenon is called the Bergeron process and the collision-coalescence process.

The Bergeron process relies on two peculiar properties of water. One is that pure water suspended in air does not freeze until it hits -40°C. This is called being supercooled. "Supercooled water will readily freeze if it is sufficiently agitated. This explains why airplanes collect ice when they pass through a liquid cloud made up of supercooled cloud droplets. Also, supercooled cloud droplets will freeze around solid particles that have a crystalline form. These particles are named freezing nuclei and have given rise to the technology of cloud seeding." 7

The second property, saturation, states that, "when air is saturated (100 percent relative humidity) with respect to water, it is supersaturated (relative humidity greater than 100 percent) with respect to ice. Therefore, when the relative humidity is 100 percent with respect to water, then the relative humidity, with respect to ice is nearly 110 percent." 8 Ice crystals cannot coexist with water droplets and eat up the excess water vapor; this lowers the relative humidity near the droplets and the droplets then evaporate making the ice crystals grow larger. They grow large rapidly and then fall; on their descent, cloud droplets adhere to these ice crystals. This chain reaction continues until larger crystals form producing snow crystals. The reason why snowflakes have a hexagonal shape is that the water molecules bind together to form a hexagonal shape and the snowflake is formed on particles called freezing nuclei. Therefore, even in summer months, snowflakes have formed, but melt before they reach the Earth's surface as rain droplets.

The spherical shape of rain droplets is due to another property of water called surface tension. "Surface tension is a force that tends to pull adjacent parts of a liquid's surface together, thereby decreasing the surface area to the smallest possible size. Surface tension is the attractive force between particles in a liquid." 9 Water has a higher surface tension than most liquids because of the hydrogen bonding between water molecules.

Another, less obvious property of water is capillary action whereby the same properties of water that exist in surface tension exist here, as well, and that is the strong attraction between water molecules. Capillary action is the attraction of the surface of a liquid to the surface of a solid. This can be evidenced by roots of plants taking up water to its leaves. How this fits into weather is that trees transpire, or give off some of this water to the atmosphere.

Still another property of water is specific heat. Specific heat is defined as the amount of heat needed to raise 1 gram of a substance 1°C at sea-level atmospheric pressure. Water has a relatively high specific heat. This can explain why water takes much more heat to raise its temperature than land. There are other factors to explain the heat differential of water versus land. Water moves around where as land does not. This will cause water to heat more slowly than land. Another reason is that heat does not penetrate land deeply; therefore, only a thin layer above the land is heated. This is called conduction. Conduction is heat that is transferred from the ground to the air. Molecularly speaking, conduction is the transfer of heat through matter by molecular activity. On land, there is a thin layer from the ground to the air above it that is being heated. Since water is moving, a much thicker layer of water is heated to just moderate temperatures. That is why it is much more desirable to be near water in the hot summer months and the same holds true in the cold winter months. Land cools much more quickly because of the thin layer and since land is not mobile as water, land cools much more quickly than water. Also, the cooler water at the surface sinks and displaces the warmer water underneath; therefore, a larger mass of water will be cooled. Since air is a poor conductor of heat, conduction is only important to the area of the ground; the air directly above the ground is, therefore, considered the least significant factor in heat transfer.

Radiation plays a significant role in the transfer of heat from the land-sea surface to the atmosphere and vice versa. The heat that is gained by the lowest level of the atmosphere through radiation or conduction is usually transferred by convection. "Convection is the transfer of heat by the movement of a mass or substance from one place to another. It can only take place in liquids and gases." 1 0

Relating specific heat and humidity, water vapor stores heat because it took energy to evaporate liquid water. This will explain why in the desert there are relatively colder nights to warmer days opposed to an area that is near a large body of water where increased evaporation can exist. Another factor that contributes to the differences in heating of land and water is that land is opaque and heat is absorbed only at the surface, whereas water is transparent and allows more solar radiation to penetrate at a deeper level. Another factor is that evaporation, which is a cooling process, happens more from bodies of water than that from land. All of these factors help to explain why water warms more slowly and cools more slowly than land.

The chemistry behind evaporation acting as a cooling agent is that it takes energy to evaporate and that energy comes from heat energy, thus making it feels cooler. Perspiration uses this principle. When you perspire and the perspiration evaporates, you feel cooler. Conversely, condensation releases heat energy. This energy helps to produce violent weather and can transfer a lot of heat energy from the tropical region to the polar region.

Condensation and evaporation are examples of phase changes, changing form one state of matter to another; this occurs through either the addition or the reduction of heat, but there are more phases such as freezing, which was discussed earlier in the supercooling of water droplets forming snowflakes. Then there is melting, where given enough heat energy, solid changes to a liquid and heat is being absorbed, thus creating a cooling effect like that of evaporation. Lastly, there is sublimation which is when a gas is converted directly to a solid and vice versa and the liquid form is skipped. Water vapor converting directly to a solid can be seen as frost and snowflakes. This is also known as deposition. Ice converting directly to water vapor can be seen when snow banks and ice seem to disappear. These phase changes are what drives the hydrologic cycle (water cycle) of the Earth since water from the oceans continuously is evaporating, reaching such heights as to condense forming clouds that eventually precipitate, which is in the form of either snow, rain, sleet, or hail.

Water vapor in the air is termed as humidity. Humidity is the amount of water vapor in the atmosphere. Humidity can be quantitatively expressed as humidity, absolute humidity, specific humidity, and the most common in meteorology, as relative humidity. "Absolute humidity is stated as the weight of water vapor in a given volume of air (usually as grams per cubic meter). As air moves from one place to another, even without change in moisture content, changes in pressure and temperature causes changes in volume and consequently the absolute humidity, thus limiting the usefulness of this index." 1 1 This is why meteorologists use specific humidity to express water vapor content in the atmosphere. "Specific humidity is expressed as the weight of water vapor per weight of a chosen mass of air, including the water vapor." 1 2 Since it is measured in units of weight (usually grams per kilogram), specific humidity is not affected by changes in pressure or temperature. "Relative humidity is the ratio of the air's water vapor content to its water vapor capacity at a given temperature." 1 3 Therefore, on days where the air is saturated, there is 100 percent relative humidity. Moisture being added to the air through evaporation will increase the relative humidity.

Dew point temperature is another important concept related to relative humidity. 1 4 Dew point is the temperature to which a parcel of air would have to be cooled in order to reach saturation. Simply, dew point is the temperature at which condensation begins. If the dew point is above freezing, the water vapor will change to water droplets. If the dew point is below freezing, the water vapor may change to ice crystals. For condensation to occur, a surface must be provided for this condensation to adhere to. In the case of dew, objects near or on the ground provide the needed surface. High in the atmosphere, tiny particles serve as condensation nuclei and provide the surfaces needed for the condensation of water vapor to form clouds. Without these condensation nuclei, "a relative humidity of 400 percent would have to be reached in order for clouds to be produced." 1

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