Presenter Notes: Slide 1: Humidity and Moisture Welcome to this distance learning module on Student Notes: Humidity and Moisture. My name is Dale Morris. I’m a meteorologist at the Oklahoma Climate Humidity and Moisture: Survey and work as a program manager in An Introduction for Science Teachers outreach.

This module is presented as a service by the Climate Survey for its outreach customers, including participants in EarthStorm. Although this module is designed specifically for science teachers to provide content knowledge about humidity, others may find some use of the materials and lesson which accompany this module.

This module should take approximately 20 minutes to complete.

Learning Objectives Presenter Notes: Slide 2: Learning Objectives • Describe qualitatively the relationship between temperature and average molecular velocity. By the conclusion of this lesson, you should be Student Notes: able to perform these learning objectives. • Describe the processes of condensation and evaporation. • Describe relative humidity in terms of condensation and evaporation rates.

• Recognize the relationships among temperature, relative humidity, and dew point temperature.

• Identify examples of real-world applications of humidity.

Oklahoma Climatological Survey www.ocs.ou.edu Introduction Presenter Notes: Slide 3: Introduction • Humidity is routinely measured and reported, The relative humidity is typically presented Student Notes: but not well understood during every weather report, which gives an by lay audiences. indication of its importance to the weather • Humidity is critical forecast. However, relative humidity is also a component of weather concept not necessarily well understood by lay forecasts and the water audiences. This lack of understanding is cycle. represented by common questions like: • Humidity is a component Oklahoma Mesonet Station (above) • “The relative humidity is 100%. Why isn’t of photosynthesis, and is Humidity probe (below) it raining?” important to sustain life. • “It’s raining, but why is the relative humidity not 100%?”

Indeed humidity is so important that nearly every weather observation station (such as an Oklahoma Mesonet station which is pictured here) measures either relative humidity or the dewpoint temperature. Humidity is a part of the hydrologic cycle and is also a by-product of photosynthesis in plants, through a process called transpiration. Thus, humidity is critical to sustain life on our planet.

Temperature and Molecular Motion

• All molecules always move. Presenter Notes: Slide 4: Temperature and •The kinds of motion depend upon the phase of the To understand humidity, we need to review basic substance (solid, liquid, or gas) Molecular Motion •The amount of motion is related to the amount of concepts about temperature and molecular Student Notes: energy the molecule has – more energy, faster motion. motion. Of course, every substance contains molecules that constantly move. The type of this molecular motion depends on the phase of the substance (solid, liquid, or gas). Molecules in solids vibrate in place. Liquid and gas molecules can also rotate and translate. Liquid molecules have greater attraction for each other but can flow around one another. Gas molecules have little attraction for each other and their translation motion is only restricted by the container of the gas.

The amount of motion depends only upon the temperature of the substance. Temperature is

Oklahoma Climatological Survey www.ocs.ou.edu related to the kinetic energy of the molecules – their energy of motion. Warmer temperatures correspond to faster motions.

Temperature and Molecular Motion in Air • In a gas, molecular motion depends upon temperature and mass. • Lighter molecules move faster than heavy ones. Presenter Notes: Slide 5: Temperature and • Warmer molecules move faster the cooler ones. • Note the distribution of speeds among one type of molecule. Air is a mixture of gases. The approximate recipe Molecular Motion in Air for air is: Student Notes: • 78% nitrogen • 21 % oxygen • 1% is water vapor, carbon dioxide, argon, and other trace gases

This interactive applet presents an illustration of how molecules in a small box of air might move. Adjust the temperature to see the response of the kinetic energy of the molecules. Note that the lighter molecules (water vapor) move faster than heavier molecules (oxygen and nitrogen). Also, within a given species of molecules, there is a spectrum of motions, but the average motion (and thus the kinetic energy) is related to the temperature.

This illustration does not include vibrational or rotational motion.

Oklahoma Climatological Survey www.ocs.ou.edu Motion in a Fluid • Gases and liquids are both fluids. Presenter Notes: Slide 6: Motion in a Fluid • Molecules exhibit similar random motions. In liquids, a similar relationship exists between Student Notes: • Liquids have less energy than gases. molecular motion and temperature. Molecules in warm water move faster than those in cool water.

Activity: Motion in a Fluid This simple activity proves: (1) liquid molecules move, and (2) molecules of warmer fluids move faster (have more energy) than those in colder fluids. Presenter Notes: Slide 7: Motion in a Fluid (Activity)

Procedure: This simple activity helps to demonstrate the two Student Notes: 1. Fill two small clear containers with relatively equal amounts of hot and cold water. key concepts presented thus far: 2. Heat one container and leave the other at room temperature or slightly cool it. 3. Carefully release one drop of food coloring in each container at approximately the 1. Molecules of a fluid constantly move same time. Do not stir. Observe. 4. The coloring in both containers disperses on its own (proving that the molecules 2. Molecules of warmer fluids move faster do indeed move). The coloring in the warm water disperses much faster. than those of cooler fluids. This faster motion also means that molecules in warmer fluids have greater kinetic The photos in this sequence were taken approximately 30 energy than their cooler cousins. seconds apart. The last photo was taken after about 15 minutes.

Humidity •Humidity: The amount of water (vapor) molecules in the air. Presenter Notes: Slide 8: Humidity • There is no difference in the liquid and vapor molecules except for the amount of kinetic energy they possess. • Surface tension: The force in a liquid that holds the liquid together and The next key concept is to consider the Student Notes: prevents all the molecules from escaping. concentration of water vapor molecules in a volume of air. This closed container of water has water molecules moving about in the liquid and water molecules in the vapor state moving about in the air. Molecules of any substance in the vapor state are invisible.

Steam is not water vapor. It is composed of tiny liquid water drops that can be seen and touched. There is no difference between a liquid water molecule and a water vapor molecule except in

Oklahoma Climatological Survey www.ocs.ou.edu the amount of energy that it possesses.

In this illustration, note that the vapor molecules in the air bounce off the liquid surface. Likewise, the liquid molecules do not penetrate the surface. This is because liquids exhibit a force called “surface tension” that holds the liquid together and prevents most liquid molecules from escaping rapidly.

This illustration is an idealized case; in reality, some molecules do penetrate the surface which is the topic of the next slide. An activity on surface tension is also ahead.

Evaporation and Condensation •Evaporationoccurs when, by random chance, a liquid molecule gains enough kinetic energy to break through the surface and escapes to the ambient air. Presenter Notes: Slide 9: Evaporation and This happens when the molecule’s energy is greater than the energy associated with surface tension. When liquid water molecules by random chance Condensation • Condensation occurs when a water molecule in the free air collides and acquire enough kinetic energy to break through adheres to the liquid surface or breaks completely through the surface. Student Notes: the liquid surface (in other words, to overcome the surface tension force), the molecules evaporate. If vapor molecules join the liquid from the air by adhering to the surface, they condense.

The energy that is acquired by a liquid molecule to change to vapor is termed “latent” heat. The word latent means “hidden”. This energy must come from someplace. Indeed, it comes from the ambient environment, or air. Thus, from the perspective of the surrounding air, evaporation is a “cooling process” because heat from the air is supplied to the liquid to change the phase to vapor.

Oklahoma Climatological Survey www.ocs.ou.edu Surface Tension • Surface tension may seem like an abstract concept, but it can be observed. Presenter Notes: Slide 10: Surface Tension (Activity) • This simple hands-on activity demonstrates surface tension and "Surface Tension" is the force that "holds" a produces a counter-intuitive result: a paper clip that seems to float on Student Notes: water. liquid together. The paper clip (mass about 1 gram) weighs less than the surface tension force of the water, so it can ride on top of the water surface. If it weighs too much (extra large paper clip) or if the surface is disturbed, then the paper clip will sink.

Relative Humidity • In a closed container of water and air, some of the liquid molecules evaporate (become vapor) and some of the vapor molecules condense (become liquid). • Eventually, the condensation rate equals the evaporation rate. In this equilibrium state, the air is saturated, Presenter Notes: Slide 11: Relative Humidity and the relative humidity is 100%. • Before equilibrium occurs, the relative humidity is less than 100% and is the ratio of the condensation and evaporation rates. Dry air has fewer water vapor molecules to condense so less condensation than As we have previously seen, both vapor and Student Notes: evaporation occurs. • The evaporation rate depends upon temperature. When the temperature is higher, more molecules can liquid molecules are continually in motion. By evaporate because they can gain more energy. random chance, some of the vapor molecules condense into the liquid and some of the liquid molecules evaporate into the air. Humidity is a measure of the amount of vapor molecules in the air. There are several different ways to express humidity. Perhaps the most common is “relative humidity”.

If you have a closed container of liquid, eventually the condensation rate and the evaporation rate will reach an “equilibrium” such that the number of evaporating molecules exactly balances the number of condensing molecules. This equilibrium state is called “saturation”, and the relative humidity at this state is 100%. It turns out that this equilibrium state is depends upon the temperature, with higher temperatures producing greater evaporation rates and cooler temperatures producing fewer evaporating molecules.

The relative humidity is a ratio of the condensation and evaporation rates. In practice, we don’t deal with closed containers of liquid, and the relative humidity is usually less than Oklahoma Climatological Survey www.ocs.ou.edu 100%. When the relative humidity is less than 100%, less condensation than evaporation occurs because there is a smaller concentration of vapor molecules in the air. In other words, the air is drier than it would be if it were saturated. However, when the relative humidity is 100% in free air (saturated), then there must be an equal exchange of vapor molecules and liquid molecules which occurs in clouds and fog. This exchange occurs between vapor molecules in the humid air and the liquid droplets that make up the cloud. Fog, by the way, is simply a cloud in contact with the ground.

This interactive applet can help you visualize different relative humidity states. Remember that this illustration is oversimplified because the molecules are always in random motion, not the organized motion that is depicted. Also, the relative humidity that is visualized is the “beginning” state, and the applet depicts the response for the given relative humidity value.

It also is possible for the condensation rate to be greater than the evaporation rate. This condition is known as supersaturation, and occurs when cloud droplets grow.

Can it rain when the relative humidity is less than 100%? The answer is “yes”. The relative humidity in a cloud is 100%, but there may be dry air beneath the cloud through which the rain falls. So at the surface the relative humidity may be less than 100%, but not where the rain originates. If the air beneath the cloud is too dry, the raindrops may evaporate before reaching the ground, a phenomenon known as “virga”.

Oklahoma Climatological Survey www.ocs.ou.edu Dew Point Temperature

• Relative humidity is sometimes a difficult way to express moisture content because it depends on temperature. In other words, it is related to both the condensation and Presenter Notes: Slide 12: Dew Point Temperature evaporation rates. • Dew point temperature: The air temperature when relative humidity is 100%, Meteorologists often prefer to express humidity which is when clouds and/or dew would form. Student Notes: • Higher dew point temperatures indicate more absolute moisture content is in the air. in terms of dew point temperature rather than relative humidity. The dew point is defined as Question to Consider the temperature the air must be cooled for If the dewpoint does saturation to occur. At the dewpoint, the relative not change much humidity is 100%. Any further cooling would throughout the day, should the relative require that vapor molecules must condense so humidity reported on dew forms at the dewpoint. the 10 p.m. newscast be higher or lower than the 6 p.m. As you can see from playing with this applet, newscast? relative humidity has a strong dependence upon temperature. In fact, the parameter that the relative humidity is “relative” to is the temperature (because the temperature is the prime factor in the determination of the evaporation rate). On the other hand, the dew point is a way to express the actual moisture content in the air (it is related to the condensation rate). However, since the dewpoint is actually a temperature, it is expressed in degrees (Fahrenheit, Celsius, or Kelvin). The dewpoint is an indication of the absolute moisture content. Larger dewpoint values are related to greater moisture content.

In this applet, because the dewpoint is fixed, the actual concentration of water vapor in the air is constant. When you adjust the temperature, you see the resulting change in relative humidity, and vice versa. Using this applet, answer this question:

On a relatively quiet weather day (no storms, fronts, or strong winds) the absolute moisture content in the air doesn’t change much (i.e., the dewpoint is relatively constant through the day). If this is the case, would you expect the relative humidity reported on the 10 or 11 p.m. newscast to be different than the value reported on the evening newscast (5 or 6 p.m.)? If the values are different, is the afternoon value greater or

Oklahoma Climatological Survey www.ocs.ou.edu smaller than the night value? Why? We will revisit this topic when we look at an application of humidity to forecasting.

Dew Point Temperature • Another way to look at moisture is the dew point depression, which is the difference between the temperature and dew point temperature. Presenter Notes: Slide 13: Dew Point Temperature • Small differences between the temperature and dew point temperature (a few degrees) indicate the air is moist. Here is another way to see the relationships • Meteorologists consider dew point temperatures greater than 55°F to be somewhat Student Notes: moist and dew point temperatures above 70°F to be extremely moist. between temperature, dewpoint and relative humidity. In meteorological practice, when we know two of these quantities, we can always calculate the third. Sometimes we measure temperature and relative humidity and calculate dewpoint. Other weather stations measure temperature and dewpoint and calculate relative humidity.

By using this applet, you can see that the relative humidity is higher when the difference between the temperature and dewpoint is small. In fact, some meteorologists use a parameter called dew point depression which is temperature minus dewpoint. When the dew point depression is small (only a few degrees), the air is humid. When the depression is large (big separation between temperature and dewopint), the air is dry. Many humidity sensors become inaccurate when the dewpoint depressions become large (on the order of 30, 40, or 50 degrees Fahrenheit or more).

Because the dewpoint is related to an absolute measure of moisture content, it is helpful to have some ballpark figures for comparison’s sake. Dewpoint values above 55°F are considered somewhat moist (sufficient as one of the ingredients for severe thunderstorms). Values

Oklahoma Climatological Survey www.ocs.ou.edu above 70°F are considered extremely moist. Meteorologists sometimes begin to think of heavy, excessive, or flooding rainfall becoming possible when dewpoint values are 65°F to 70°F or higher. (Other ingredients are also important floods to occur.) Dewpoint values over very vigorous vegetation (like mature corn in the midwestern United States in mid summer) can be 80°F. Dewpoint values above 85°F are rarely observed.

Clouds • Evaporation and Condensation are two important processes in the hydrologic cycle (or water cycle). Presenter Notes: Slide 14: Clouds • Evaporation is the same process and occurs the same way whether the liquid is from a puddle, pond, lake, or a cloud. This illustration helps to visualize that Student Notes: evaporation is the same process whether the liquid is on the surface (pond, puddle, or lake) or in the air. Remember that a cloud is composed of millions of liquid water droplets. Condensation is the same process no matter what liquid surface is available (cloud drops, water bodies, blades of grass, etc.) for vapor molecules to lose enough energy to become liquid. Cloud drops form from water that adheres to the surface of a condensation nucleus – grain of salt, aerosol, dust, etc, or from an ice particle that melts. Clouds may be entirely ice particles, entirely liquid water droplets, or (most likely) a combination of the two.

Oklahoma Climatological Survey www.ocs.ou.edu Application: Severe Weather • Several ingredients are required to produce severe weather. One of these ingredients is moisture. In fact, many forecasters use a “rule-of-thumb” dewpoint value of 55°F as a limiting Presenter Notes: Slide 15: Application to Severe factor. If dewpoint values are less than 55°F, thunderstorms generally are not severe. • One of the key challenges in forecasting severe weather is determining if enough moisture Humidity is a key ingredient required for the will be available. Weather atmosphere to produce severe weather, defined Student Notes: as thunderstorms with damaging winds (above 58 mph), hail above ¾ inch in diameter, or tornadoes. If the surface dewpoint temperature is less than 55°F the likelihood of severe weather becomes very small. If the dewpoint values are slightly below 55°F, other factors may help to overcome the lack this ingredient. If the values

Storm Prediction Center: http://www.spc.noaa.gov are much below 55°F, then severe thunderstorms can almost always be ruled out. Interactive Materials: http://cig.ocs.ou.edu/~dale/distance/humidity/case1

One of the challenges in forecasting severe weather is determining where moisture will be located. This sequence of images shows dewpoint values over the southern plains at 2:00 p.m. in the afternoon for four consecutive days. Notice how the moisture in the atmosphere is transported first into Kansas and Nebraska, and then eventually into Arkansas, Tennessee, Kentucky, and Indiana. Exploratory Question: What other weather maps could you look at to see why the moisture was transported this way?

Now a set of companion images are added to the sequence. These images are daily reports of severe weather from an archive maintained by the Storm Prediction Center in Norman, OK. The SPC website contains a wealth of information about severe weather. On the first day, severe weather was reported mainly in moist areas of south Texas. On the second day, only two reports of large hail were reported across the entire country, but they were in and near the moist area in Kansas and Nebraska. On the third day, a major severe weather outbreak occurred, and all 402 reports are located within the area of moisture. On the last day of the sequence, both the moisture and the storm system producing the severe weather moved east.

Oklahoma Climatological Survey www.ocs.ou.edu

More materials about this brief case study are available at the website indicated here. These materials include imagery, loops, and interactive weather maps.

Application: Wildfires • The lack of atmospheric moisture can be a key factor in the ignition and spread of wildfires. Presenter Notes: Slide 16: Application to Wildfires • Wildfires sometimes ignite from and spread through dead/dormant vegetation. Smaller fuels (dead grass and leaves) can respond to changes in humidity in only a Fuels for wildfires can be classified into live fuels Student Notes: few hours. Wildfires ignite easily and are harder to control when the air is dry. (green vegetation) and dead/dormant fuels. Dead fuels are further categorized by the amount Dry and Windy Dry Leaves and of time required for the fuel to adjust to changes RH < 30% Grasses (Fuels) in humidity. This time is specified in hours and is related to the size of the fuel.

Fires would Higher move faster 1 hour dead fuel dead grass and leaves Fire than 120 Danger ft/min! 10 hour dead fuel twigs and small branches Low Fire 100 hour dead fuel large branches and small Danger 8-10 foot flames logs 1000 hour dead fuel large logs

In this series of images, we see that the western two thirds of Oklahoma had dry and windy conditions with the relative humidity less than 30%. In response to the dry conditions, the amount of moisture in dry grasses and leaves was very low (less than 5 to 10%), making them easy to ignite.

The fire danger, expressed in terms of flame length, was very high in the western part of the state, where some areas could experience flame lengths of 8 to 10 feet if a wildfire were to ignite. In the same areas, range fires would move faster than 120 ft/min, or greater than an entire football field in 3 minutes!

Oklahoma Climatological Survey www.ocs.ou.edu Application: Human Comfort • The heat index combines temperature and relative humidity to estimate the heat stress on the human body. Presenter Notes: Slide 17: Application to Human

Heat Index Humidity plays a major role in how the human Comfort how hot the combination of temperature and humidity feels Relative humidity (percent) body maintains its temperature. Student Notes: 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Dry air: perspiration evaporates easily; the body 140 110 Extreme Danger (130°F or higher). Heatstroke or sunstroke likely

135 110 121 Danger (105 – 129°F). Sunstroke, muscle cramps, and/or cools heat exhaustion likely. 130 108 117 128 Heatstroke possible with prolonged exposure and/or physical activity. 125 107 114 121 130 140 Extreme Caution (90 – 105°F). In humid air, perspiration does not evaporate Sunstroke, muscle cramps, and/or 120 105 110 116 122 130 138 148 heat exhaustion possible with prolonged exposure and/or 115 103 106 110 115 121 127 134 143 152 physical activity. easily, so muscles force more water out trying to Caution (80 – 90° F). 110 100 102 105 108 112 117 122 129 136 143 152 Fatigue possible with prolonged exposure and/or physical activity. achieve more cooling. Eventually, the body can

Air Temperature(°F) 105 97 98 100 102 104 108 112 116 121 127 134 141 149 1009494959697100102106 109 114 118 124 129 136 143 run out of water and muscles can cramp. 95 91 90 90 91 91 93 94 96 99 102 105 109 113 118 123 128 134 90 87 86 86 86 86 87 88 89 91 92 95 97 100 103 106 109 113 117 122 Heat index is a formula that combines 85 82 82 82 82 82 82 83 84 84 85 86 88 89 91 93 95 97 99 102 104 107

80 78 78 78 78 79 79 79 80 80 80 81 81 82 82 83 84 84 85 86 86 87 temperature and relative humidity to estimate Copyright 2006, Oklahoma Climatological SurveyBased upon Formulation by National Weather Service El Paso Forecast Office Interactive Activity from EarthStorm and The Oklahoman: heat stress on the human body.

Sample Lesson on Humidity • Weather measurements at regular intervals (5 min., 15 min., or hourly) at locations nearby students. Presenter Notes: Slide 18: Sample Lessons • Analysis with graphs is common. A lesson has been developed to supplement this Student Notes: • This exercise is designed to help students improve and reinforce –graphing skills learning module on humidity. The lesson – interpretation of actual data contains teacher’s guides and grade-specific – communication of math and science information Late Elementary (Grades 4-5) student activities. Early Elementary Nearly every weather station records (Grades 2-3) Sample Questions: • What is the high temperature and humidity measurements at temperature? • When did it occur? regular intervals, regardless of the purpose of the • Compare the temperature and relative humidity graphs. Middle and High School station. Thus, lots of data exist. (Grades 6-12) Because relative humidity depends upon both temperature and dewpoint, and because relative Click on the graphics to view the specific lessons. humidity and temperature are inversely related to each other, a set of general-purpose questions has been developed. Teachers may use the pre- packaged data provided as-is, or may choose to use this lesson as a template with data from nearby stations that may seem more relevant to students.

Some recent evidence indicates that some students have difficulty creating, and/or interpreting information presented in graphs. Weather data, routinely collected at regular intervals, provides a mechanism for students to improve analysis skills using graphs. If nearby stations are used, then students can appreciate the graphical representation of the weather they

Oklahoma Climatological Survey www.ocs.ou.edu felt.

Students also need to be able to communicate scientific and mathematical concepts both quantitatively and qualitatively. Some of the questions ask students to summarize what they learn from the graphs in simple, short paragraphs.

The middle and high school lesson also contains a set of additional application/assessment exercises that can be used for further study. The basic exercises contain general questions for every weather station, that can be used in many situations. The additional exercises build upon the basic exercises and contain case-specific questions.

Presenter Notes: Slide 19: Humidity Quiz Here is a short quiz to test what you’ve learned Student Notes: from this lesson. At this time, the score is not reported or recorded in any way, so there’s no risk. Use the quiz to help you learn. There’s

Oklahoma Climatological Survey www.ocs.ou.edu Presenter Notes: Slide 20: The End! Thanks for your time. You may review this Student Notes: Humidity and Moisture: material anytime you wish. Please e-mail me An Introduction for Science Teachers with any comments or feedback at [email protected]. Thanks for your time.

E-Mail Comments Dale Morris: [email protected]

Oklahoma Climatological Survey www.ocs.ou.edu