Weather Foklore: Fact Or Fiction?

October 2012Teacher's Guide for

Weather Foklore: Fact or Fiction?

Table of Contents

About the Guide

Student Questions

Answers to Student Questions

Anticipation Guide

Reading Strategies

Background Information

Connections to Chemistry Concepts

Possible Student Misconceptions

Anticipating Student Questions

In-class Activities

Out-of-class Activities and Projects

References

Web sites for Additional Information

About the Guide

Teacher’s Guide editors William Bleam, Donald McKinney, Ronald Tempest, and Erica K. Jacobsen created the Teacher’s Guide article material. E-mail:

Susan Cooper prepared the anticipationand reading guides.

Patrice Pages,ChemMatters editor, coordinated production and prepared the Microsoft Word and PDF versions of the Teacher’s Guide. E-mail:

Articles from past issues of ChemMatters can be accessed from a CD that is available from the American Chemical Society for $30. The CD contains all ChemMatters issues from February 1983 to April 2008.

The ChemMatters CD includes an Index that covers all issues from February 1983 to April 2008.

The ChemMatters CD can be purchased by calling 1-800-227-5558.

Purchase information can be found online at

Student Questions

1. Why was the invention of the telegraph important to weather forecasting?
2. What is the definition of dew point?
3. Why is it less likely that water vapor will condense from warmer air?
4. Explain the force that causes water vapor to condense.
5. Why does dew form more often on clear nights?
6. What color light is scattered the most by the Earth’s atmosphere?
7. What are aerosols?
8. Why do flowers have odor?
9. The article cites two reasons why cats lick their fur. What are they?

Answers to Student Questions

1. Why was the invention of the telegraph important to weather forecasting?

The telegraph enabled weather forecasters to send local weather observations over long distances, providing information for other forecasters to predict the kind of weather headed their way.

1. What is the definition of dew point?

Dew Point is the temperature at which the air is saturated with water. That is, it’s the temperature at which water vapor condenses. Note that scientists often use the term “point” to refer to a temperature (as in melting point or boiling point).

1. Why is it less likely that water vapor will condense from warmer air?

In air at a higher temperature both the molecules making up the air and water molecules are moving at greater velocities. These higher velocities prevent the attractive forces between the molecules from “taking over”. At lower temperatures the reduced velocities allow the forces to attract water molecules to each other, condensing the water to liquid form.

1. Explain the force that causes water vapor to condense.

The force is primarily hydrogen bonding—the intermolecular force between hydrogen in one molecule (like water) and a very electronegative atom (like the oxygen in water) in another molecule. The polar covalent bonding between hydrogen and oxygen in the water molecule creates a partial positive charge near the hydrogen and a partial negative charge near the oxygen. Note that the electronegative atom has one or more unshared electron pairs, and the positive charge of the hydrogen is attracted to one of those unshared pair of electrons in the oxygen.

1. Why does dew form more often on clear nights?

At night the earth radiates back into space the energy gained during the day. This happens most easily on a clear night. If, however, there are night-time clouds, some of the radiated energy is reflected back to Earth, keeping the overnight temperatures higher. On a clear night overnight temperatures are lower, making dew formation possible (making it more likely that the overnight low temperature will be lower than the dew point).

1. What color light is scattered the most by the Earth’s atmosphere?

Blue light is scattered the most (actually violet light is scattered the most, but our eyes are not as sensitive to violet light as they are to blue light so we see the blue scattering more). Rayleigh scattering is inversely proportional to the wave length of light raised to the fourth power. Since blue light has a short wave length compared to the other colors in the visible spectrum, we see the sky as blue—blue light is scattered in all directions resulting in a blue color when we look skyward.

1. What are aerosols?

Aerosols are small particles of dust, water, salt and other particulates that are suspended in the atmosphere. Aerosols scatter light and cause sunsets to appear red, much like the scattering caused by gases in the atmosphere.

1. Why do flowers have odor?

Some molecules of any substance or object that has an odor escape from the substance in the form of a gas, either as a result of evaporation or sublimation. These molecules diffuse through the atmosphere, and if they reach people’s noses in sufficient concentration, people detect an odor. Odor, then, is gas molecules migrating from an object to our nose.

1. The article cites two reasons why cats lick their fur. What are they?

Since cats do not have sweat glands they lick themselves to cool off. The water in cats’ saliva is deposited on their fur, and as the water evaporates it absorbs heat from the fur and cools them. The second reason is to reduce static electricity in their fur. Water reduces static electricity in the cats’ fur. For a more complete explanation of this phenomenon see “More on cats and weather.”

Anticipation Guide

Anticipation guides help engage students by activating prior knowledge and stimulating student interest before reading. If class time permits, discuss students’ responses to each statement before reading each article. As they read, students should look for evidence supporting or refuting their initial responses.

Directions: Before reading, in the first column, write “A” or “D” indicating your agreement or disagreement with each statement. As you read, compare your opinions with information from the article. In the space under each statement, cite information from the article that supports or refutes your original ideas.

Me / Text / Statement

1. Scientific weather forecasting has been around for more than a thousand years.

1. Water molecules are more likely to stick together in warm air.

1. Dew forms for the same reason you can see your breath on a cold morning.

1. Clouds help keep the earth warmer at night.

1. The Earth’s atmosphere scatters red light more than blue light.

1. When the sun is low on the horizon, the sun’s light must pass through more than 400 km of low atmosphere before reaching your eyes.

1. Low pressure is associated with good weather.

1. Gases escape from liquids and solids more readily with high pressure.

1. Faster-moving fluids exert less pressure than still or slow-moving fluids.

1. Cats do not have sweat glands.

1. When cats lick themselves, the static electricity in their fur is reduced.

Reading Strategies

These matrices and organizers are provided to help students locate and analyze information from the articles. Student understanding will be enhanced when they explore and evaluate the information themselves, with input from the teacher if students are struggling. Encourage students to use their own words and avoid copying entire sentences from the articles. The use of bullets helps them do this. If you use these reading strategies to evaluate student performance, you may want to develop a grading rubric such as the one below.

Score / Description / Evidence
4 / Excellent / Complete; details provided; demonstrates deep understanding.
3 / Good / Complete; few details provided; demonstrates some understanding.
2 / Fair / Incomplete; few details provided; some misconceptions evident.
1 / Poor / Very incomplete; no details provided; many misconceptions evident.
0 / Not acceptable / So incomplete that no judgment can be made about student understanding

Directions: As you read, complete the chart below to describe the science behind the weather-related folklore.

Folklore / Scientific explanation / Is the folklore accurate?
“When grass is dry at morning light, look for rain before the night. Dew on the grass, rain won’t come to pass.”
“Red sky at night, sailor’s delight. Red sky in the morning, sailors take warning.”
“Flowers smell best just before rain.”
“If cats lick themselves, it means good weather is on its way.”

Background Information

(teacher information)

More onweather history

The premise of this article is to examine samples of traditional weather lore to determine if they have any scientific basis. Prior to the advent of modern meteorology and scientific weather forecasting people made personal observations of natural events in order to predict short-term weather. As early as 650 BC the Babylonians used the appearance of clouds to predict upcoming weather. Recurring astronomical events such as moon phases and other cyclic patterns were also often used as the basis for attempting weather predictions.

In 340 BC Aristotle wrote Meteorologica, which included theories about the formation of rain, clouds, hail, wind, thunder, lightning, and hurricanes. Aristotle’s ideas were used as the basis for weather predictions until at least the 1600s, when scientific developments like the thermometer and barometer permitted scientists to actually measure properties of the atmosphere and correlate them to changes in weather. Even in ancient timessome crude weather measurements were made.Forexample, during the Shang Dynasty in China humidity was measured by exposing charcoal to the air and measuring the increase in weight.

But the consistent use of measurements to predict weather began in the 17th century. Galileo invented a rudimentary thermometer around 1600, and in 1643, Torricelli invented the barometer. Galileo’s thermometer was called a thermoscope, and it relied on what we now know as Charles’ Law to measure “heat” (temperature). This description from a contemporary of Galileo illustrates how it worked:

“He took a small glass flask, about as large as a small hen's egg, with a neck about two spans long [perhaps 16 inches] and as fine as a wheat straw, and warmed the flask well in his hands, then turned its mouth upside down into the a vessel placed underneath, in which there was a little water. When he took away the heat of his hands from the flask, the water at once began to rise in the neck, and mounted to more than a span above the level of the water in the vessel. The same Sig. Galileo had then made use of this effect in order to construct an instrument for examining the degrees of heat and cold.”

By the mid-1600’s a liquid-in-glass thermometer had been invented and by the mid-1700s both the Fahrenheit and Celsius temperature scales were established.

Torricelli, who was an assistant to Galileo in the final three months of Galileo’s life, was working on improving water pumps of the time. Instead of a column of water, Torricelli used mercury in a glass tube and discovered that the column of mercury supported by the atmosphere was about 76 cm and that the length of the column varied from day to day. In this way Torricelli invented the first barometer in 1643. These and other measurement devices were improved and refined over the next 150 years, and ultimately enabled research-based predictions of weather as opposed to relying onlocal observations by individuals.

During the second half of the 18th century chemists contributed to what was known about the atmosphere with the discovery of many gases like oxygen and nitrogen, and discovery of the chemical composition of water. The work of Priestley (Experiments and Observations on Different Kinds of Air), Cavendish (composition of the atmosphere), Scheele (discovery of oxygen), Lavoisier (the role of oxygen) and others were important here.

The invention of communication devices like the telegraph also permitted observations made at multiple locations to be shared quickly, this giving rise to weather maps and systemic predictions of the weather. Formal weather observing stations followed and, by the time of the Civil War, modern weather forecasting had begun.

A major development in weather forecasting was the invention of the radiosonde, a small lightweight box that is tied to a hydrogen or helium balloon that rises in the atmosphere. As it rises the radiosonde measures temperatures and pressures at regular altitudes and transmits the data back to earth for use in developing weather maps. (See image at left for an early U.S. weather map from NOAA.) For a detailed history of the radiosonde see

By the early 1900’s attempts were made to forecast weather by means of developing and solving mathematical equations that were based on known weather patterns. These early mathematical models required more computing power than was available until the advent of an early version of a computer that enabled a 24-hour prediction of the weather by a team of scientists in Princeton, New Jersey, in 1950. By 1960, weather satellites were being used to help predict weather, and today satellites are the major tools for forecasters.

More ondew formation

Your students will know from reading the article that the formation of dew is a change of phase from gas to liquid. They will also likely know about the hydrologic cycle—that interconnected system of evaporation and condensation that determines so much of the world’s weather. Dew formation is one small sub-cycle of that larger process. Evaporation and condensation are physical changes that results from molecules interacting with each other and gaining or losing energy. This excerpt from the ChemMatters Teachers Guide from the October, 2005edition provides some theoretical background on change of phase:

This review will remind students that in all phases of matter molecules are in constant random motion. As a result of this motion, molecules have kinetic energy, which can be shown by the equation: K.E. = ½ mv2

The equation could be used to calculate the kinetic energy of a single molecule. However, the molecules in a sample of a matter have a range of kinetic energies with some molecules moving faster and some moving more slowly. The conventional method of indirectly measuring the kinetic energies of all the molecules in the entire sample is by measuring the temperature of the sample and assuming that this represents the average of all these energies.

A range of intermolecular attractions constrain molecular motion in liquids (see below). These attractive forces are London dispersion forces, dipole-dipole interactions and hydrogen bonding. Each of these forces is relatively weak compared to intramolecular covalent or ionic bonds, but each is strong enough to influence the motion of molecules in solids and liquids. So the intermolecular forces hold molecules together and limit their motion.

In liquids the molecules are free to move around in a limited way, and in gases the molecules move independent of each other (in ideal gases). The process in which a liquid undergoes a phase change to a vapor is called vaporization. If the process takes place at or near room temperature, we tend to call the process evaporation, even though a liquid can evaporate over a wide range of temperatures.

If we look at evaporation at the molecular level, we can focus on the surface of the liquid, which is where evaporation occurs. Molecules on the surface of the liquid are in motion, like all liquid molecules, and they have a range of kinetic energies. They are also held together by one or more of the intermolecular forces. Energy must be added to the molecules in order to overcome these intermolecular forces, so evaporation is, therefore, an endothermic process. In the case of a liquid at ambient temperature, the added heat is drawn from the immediate environment in contact with the liquid. Because of this, we say that evaporation is a cooling process. A better statement of the phenomenon is that an evaporating liquid cools its surroundings.

When dew forms the phase change is, of course, from gas to liquid, and the process is called condensation. There are two important ways in which this process takes place in the atmosphere. The first is the cooling of water-laden air as it rises and expands higher up in the atmosphere. The cooling of the air causes the water vapor molecules to slow down, allowing their intermolecular forces to attract them together into liquid droplets,and when droplets of water become massive enough they fall to earth as rain. The second case of atmospheric water vapor condensation occurs near the Earth’s surface. As the temperature of the atmosphere decreases during overnight hours the water molecules in the lower atmosphere lose kinetic energy. Reducing the velocities of the water molecules allows the intermolecular forces referred to in the excerpt above to take over and bring the molecules closer together forming a liquid on solid surfaces at or near the ground. The resulting liquid is called dew, or in the case of very cold atmospheric and ground temperatures, frost.