Meteorology is defined as the study of the atmosphere and its associated phenomena. One of the first recorded discussions of atmospheric phenomena occurred in 340 B.C. with Aristotle’s book Meteorologica. But it wasn’t until the seventeenth and eighteenth centuries that meteorology “came into being” with the advent of the modern thermometer and barometer. By the late nineteenth century, observations of temperature and pressure were being made routinely and began to be transmitted between different areas by telegraph. Scientifically based weather forecasting, however, did not occur until the 1930s because of the inability of scientists to collect atmospheric data in enough time to make predictions about the weather. It was the father and son team of Vilhelm and Jacob Bjerknes who, beginning in the 1920s, developed a weather station network that permitted the collection of regional weather data. As a result, by the 1930s scientists were able to collect data from a weather station network in a timely enough fashion to begin forecasting atmospheric events. The ability to forecast atmospheric events changed significantly after World War II when meteorological radars were implemented and again in the 1950s and 1960s when computers and satellites were added as meteorological tools.
In order to better understand “the weather” or meteorology, it is important to have a working knowledge of the atmosphere. The atmosphere is a thin layer of gases that surrounds the Earth. It extends over 400 kilometers from the surface of the Earth and is held in place by the force of Earth’s gravity. It is hypothesized that 4.6 billion years ago, the Earth’s atmosphere consisted of two of the most abundant gases found in the universe, hydrogen and helium. Then, through a process in which gases were released from the Earth’s interior, water vapor, carbon dioxide, and nitrogen were released into the atmosphere. This process of outgassing occurred over millions of years and supposedly led to the current composition of the atmosphere: 78% nitrogen, 21% oxygen, 0.9% argon, and a smaller amount of gases such as water vapor, carbon dioxide, methane, nitrous oxide, ozone and particulate matter. The term “air” is often used to refer to this collection of gases. Figure 1 shows the percentage by volume of the gases that comprise the atmosphere.
Although a part of the atmosphere is a homogenous mixture of the gases listed in Figure 1, there are sections of the atmosphere that are heterogeneous in nature, such as the thermosphere, one of the upper layers of the atmosphere. This difference as well as differences in the way in temperature changes with altitude have resulted in scientists dividing the atmosphere into several layers. These are the troposphere, the stratosphere, the mesosphere, and the thermosphere. Figure 2 illustrates the organization of the layers within the atmosphere. Meteorologists have given special names to the boundaries between the four layers: tropopause, stratopause, and mesopause.
The troposphere extends from the surface of the Earth to approximately ten to twelve kilometers above the Earth’s surface. It is the layer in which most of the Earth’s weather occurs. The jet stream, which is a high altitude wind, is found in the upper areas of the troposphere. Within the troposphere, there is an inverse relationship between temperature and altitude such that higher levels of the troposphere are colder than lower levels. The decrease in temperature with increasing troposphere altitudes results from the way in which the air in the troposphere is heated. One common misconception is the troposphere is directly heated by the electromagnetic energy from the Sun. In other words, people often mistakenly believe that the air in the troposphere is heated as UV, visible, and other electromagnetic energy passes through it. This is not the case. Ultraviolet (UV) and other forms of electromagnetic energy from the Sun do travel through the layers of the atmosphere and reach the surface of the Earth. However, the air in the troposphere is indirectly heated by the electromagnetic energy that interacts with the Earth’s surface. After passing through the atmosphere, UV and other electromagnetic energy is either absorbed or reflected by the water and land on the Earth’s surface. Some of the electromagnetic energy absorbed by water and land is converted to heat energy and transferred to the air directly above the water and land surfaces by the processes of conduction and radiation. Thus the air in the troposphere is heated as a result of the conduction and radiation of the electromagnetic energy received by the Earth. As a result, the temperature of the troposphere decreases as troposphere altitude increases because higher levels of the troposphere are farther from the source of heat- the surface of the Earth.
The stratosphere is the layer of the atmosphere that extends from the tropopause to approximately fifty kilometers above the Earth’s surface. The stratosphere contains the Earth’s ozone layer. In contrast to the troposphere, the temperature of the stratosphere increases as the height of the stratosphere increases. The direct relationship between stratosphere altitude and temperature occurs because the ozone within this layer absorbs the UV energy from the Sun and then re-emits the energy as heat. The heat released from the reaction between UV energy and ozone warms the upper layers of the stratosphere.
Beyond the stratosphere is the mesosphere which extends from the stratopause to approximately 85 kilometers above the Earth’s surface and the thermosphere which extends from the mesopause to over 400 kilometers above the Earth’s surface. The upper level of the thermosphere is filled with large concentrations of ions and free electrons and is often referred to as the ionosphere. Because of the abundance of ions and free electrons, the ionosphere is important for the propagation of radio waves.
With regards to the Earth and the atmosphere, the term weather is used to describe the state of the atmosphere at a particular time and place. Weather is characterized by pressure, temperature, humidity, clouds, precipitation, visibility, and wind. The term climate, on the other hand, refers to the average weather over a particular region. Climates can change, but do so over long periods of time. As stated earlier, nearly all of the Earth’s weather occurs in the troposphere. Therefore within the remainder of this background and the material within the CELL, when the terms air or atmosphere are used, they will refer to the composition of gases within the troposphere.
Changes in the atmosphere follow some basic physical principles that govern the behavior of all gases. Understanding weather therefore involves an understanding of these principles. Within this CELL, you will investigate how changes in the temperature of air affect the volume, density, and pressure of air. An understanding and application of the relationship between temperature, volume, and density, between density and pressure, as well as an understanding of pressure (concentration ) gradients, will help you to decipher the changes in the atmosphere that accompany high and low-pressure systems, movement of air masses, and formation and passing of cold and warm fronts.
One of the basic laws that govern gases is Charles’ Law. Charles’ Law states that when a gas is heated, it tends to expand. Figure 3 illustrates Charles’ Law. This law applies to gases in the atmosphere as well as to gases studied in a laboratory. How does understanding Charles’ Law increase our understanding of atmospheric events? Before this question can be completely answered, it is important to take a closer look at the heating of the atmosphere over the entire surface of the Earth.
All surfaces of the Earth are not heated to the same extent by the electromagnetic radiation of the Sun. Because the Earth is tilted 23.5 degrees on its axis, the equator and regions close to it receive more direct light (electromagnetic radiation) than the poles and regions close to them. As a result, the warmest area on Earth is at the equator. As the distance from the equator increases, the surface temperature decreases. The coldest areas on Earth are the north and south poles. This unequal heating of the Earth by the Sun also results in inequalities in the temperature of the atmosphere around the globe because the atmosphere is heated by conduction and radiation of heat from the Earth’s surfaces. The end result is that the air above the equator is warmer than that above the poles.
Here is where Charles’ Law comes into play. Like any other gas, air expands as it is heated. On a molecular level, this means that the molecules that comprise air experience an increase in kinetic energy and move faster and farther apart as the temperature of the air increases. The result is an increased volume or the space those molecules occupy. In contrast to the change in volume, heating a gas does not result in a change in its mass. Recall that density is the ratio of the mass of a substance to the volume it occupies. In other words, density is a measure of the “compactness” of matter. Because the same amount of matter (number of molecules of air) now occupies a larger volume as compared to prior to heating, the density of the “warm” air decreases. As the density of the “warmed air” decreases, the warm air rises above cooler and more dense air. How long does the warm air rise? Recall that the temperature of the troposphere decreases with altitude. As the warm air rises higher into the troposphere, it encounters areas of lower temperature. Heat is transferred from the warmer air to the cooler air around it. At some point, the warm air has transferred enough heat that it is no longer less dense than the surrounding air and it stops rising. As it continues to cool in the upper levels of the atmosphere, the decrease in temperature decreases the kinetic energy of the gas molecules such that their movement is slowed. As a result, the cooled air contracts and now occupies a smaller volume than prior to cooling. Because the decrease in temperature changes the volume of air without changing its mass, the cooler air is now denser than the air surrounding it. As a result, the cooler air sinks back towards the surface of the Earth. If these simple events are drawn with respect to the Earth’s surface and the atmosphere, the pattern of a convection current is apparent.
If this behavior of air is taken into account along with the unequal heating of the Earth, the following global circulation pattern would emerge: warm air from the equator would rise and move towards the colder poles where changes in its density would cause it to sink back to the surface of the Earth. As this air sinks it would push the existing air towards the equator where it would encounter warmer temperature and again rise.
Although global convection currents exist, the single convection current described above is not the scenario for Earth. Instead of a single convection current, three smaller convection currents exist around the Earth and set into play differences in pressure and other atmospheric events. One of the reasons for three rather than one convection current or convection cell is the shape and rotation of the Earth. The Earth is a sphere. This means that its circumference is largest at its center and smallest at its poles. Therefore when the Earth rotates, the rate of rotation is greatest at the equator and smallest at the poles. Figure 5 illustrates this concept. The different velocities of rotation gives rise to the Coriolis Force or Coriolis Effect. The result of the Coriolis Force is that air that would move in a straight line from the poles to the equator is deflected to the right of its intended path in the northern hemisphere and to the left of its intended path in the southern hemisphere (remember that the Earth rotates from West to East). As a consequence, the single convection current described above is broken up into three smaller convection currents in each hemisphere. Figure 6 illustrates those currents.
At the equator, warm air rises and diverges toward each hemisphere’s poles. However, around 30 degrees north and south latitudes, the air has cooled as a result of its rising into cooler levels of the atmosphere and begins sinking. As it moves towards the surface of the Earth it spreads out and heads back towards the warmer equator. This first convection cell in both hemispheres is called a Hadley Cell. Between 30 and 60 degrees latitude a second convection cell occurs. This second cell is referred to as a Ferrell Cell. Some of the warm air from 60 degrees latitude rises and draws in the cooler air from 30 degrees latitude to replace it. This air, which is not as warm as that over the equator, moves towards the equator. As it moves towards the equator it cools and sinks around 30 degrees latitude, combining with the sinking air from the Hadley Cell. In addition, some of the warm air rises and moves towards the poles. This sets into motion a second Ferrell Cell between 60 degrees latitude and the poles. As the warmer air reaches the poles it cools and sinks, moving towards the warmer 60 degrees latitude. Thus, the unequal heating of the Earth sets into motion changes in air density that are responsible for global atmospheric circulation patterns. Understanding Charles’ Law therefore provides a basis for understanding some of those changes.
Thus far, air movement, atmospheric events, and global circulation patterns have been discussed in terms of changes in air temperature and density. How does atmospheric pressure fit into the picture? Is there a relationship between air temperature, air density, and pressure? Atmospheric pressure is defined as the sum of forces exerted on the surface of the Earth by the molecules that compose the atmosphere per unit area. Atmospheric pressure can be represented by the following equation and is measured in units of millibars:
For simplicity sake, atmospheric pressure is often described as the weight of the atmosphere exerted on the surface of the Earth. When discussing atmospheric pressure, a model called a “parcel of air” is used to help conceptualize changes in atmospheric pressure. A parcel of air is an arbitrary means of illustrating the movement of air. A parcel of air can be thought of as a column of air. Figure 7 shows a column of air and its relationship to atmospheric pressure.
When discussing the pressure exerted by air molecules, the assumption is made that the only force exerted by the molecules of air is in a downward direction. This assumption is partially correct. Air molecules exert a force in all directions, including in an upward, sideways, and downward direction. In addition, the force of the Earth’s gravity exerts a downward force on the molecules in the atmosphere. When the SUM of the forces exerted on or by the “air” molecules is in a downward direction, a parcel of air will move in a downward direction. If the SUM of forces exerted on or by the “air” molecules is in an upward direction, a parcel of air will move in an upward direction. For example, when air is heated, it expands and its density decreases. As a result this parcel of air rises in the atmosphere. The opposite is true when the air cools. As a parcel of air cools, its volume decreases and density increases. As a result, the air sinks in the atmosphere.
If one thinks of atmospheric pressure in terms of the movement of different parcels of air, then the SUM of forces exerted over an area could change as the movement of parcels of air change. In this case, movements that decrease the downward force of air would decrease atmospheric pressure, and those that increase the downward force of air would increase atmospheric pressure. Take the cases of rising and sinking air. Rising air decreases the force acting downward on Earth. As a result atmospheric pressure under an area of rising air decreases. Sinking air increases the force acting downward on Earth. As a result atmospheric pressure under an area of sinking air increases.
When the atmospheric pressure at one area of the Earth is higher than the areas that surround it, the term high-pressure center or system is used. When the atmospheric pressure at one area of the Earth is lower than the areas that surround it, the term low-pressure center or system is used. Therefore, air rising as a result of temperature or other causes generally leads to the development of a low-pressure center. Air sinking as a result of temperature or other causes generally leads to the development of a high-pressure center. In addition to temperature, air is often “forced down” or “sucked up” as a result of changes in the flow of the jet stream, high-velocity winds in the upper levels of the troposphere.
In addition to the local high and low-pressure systems described above, the effect of rising and sinking air has an effect on global air circulation. If these principles of changes in force and pressure are now applied to the convection currents (Hadley and Ferrell Cells) caused by the unequal heating of the Earth, the result are areas of high and low pressure along the equator, and 30, 60, and 90-degree latitude. The areas of high pressure are associated with cooler air sinking. The areas of low pressure are associated with warmer air rising. These areas are referred to as semi-permanent highs and lows and are relatively stable atmospheric phenomena in that that they persist for months at a time and occur year after year at the same locations.
In addition to the vertical movements of air described above, meteorologists also study winds which are considered horizontal movements of air set into motion by pressure differences that exist within the Earth’s atmosphere. It was previously mentioned that differences in atmospheric pressure exist along the surface of the Earth. Some of these differences result from the semipermanent highs and lows created by the unequal heating of the Earth. Others result from differences in the topography and composition of the Earth’s surface. Meteorologists map these pressure differences across the globe by collecting data about atmospheric pressure from weather stations across the world. Before plotting the atmospheric data, they “correct” or “calibrate” the pressure readings to take into account the differences in altitude of the locations of the weather stations. This calibration is important because atmospheric pressure decreases as altitude increases simply because there are fewer gas molecules higher in the atmosphere. Once the calibrations have been made, meteorologists connect areas of the same atmospheric pressure with a line. These lines of equal pressure are called isobars. The map below shows an example of two isobars. Observation of isobars can help meteorologists determine the direction and strength of winds.
Regardless of the cause of the differences in atmospheric pressure across the Earth, the result is the same. Air moves between areas of different pressure and always flows in the same direction from areas of higher pressure to areas of lower pressure. It is important to note here, that, the terms higher and lower pressure are used as opposed to high pressure center and low pressure center. This is to indicate that air will move between two areas anytime there is a pressure difference. Air movement is NOT limited solely to movement between high pressure centers and low pressure centers.
The strength of winds depends upon two factors: 1) the difference in pressure between the higher and lower areas of pressure and 2) the distance between the two areas. Meteorologists refer to the force that sets air into motion as a result of the pressure differences as the Pressure Gradient Force (PGF). The equation below illustrates the two factors that affect PGF.
A quick look at the equation shows that larger differences in the pressure between two areas or a smaller distance between the two areas of different pressure result in a larger PGF. Larger distances between two areas of different pressure or smaller differences in those pressures result in a smaller PGF. Winds with larger PGFs are described as stronger, higher velocity winds. Meteorologists used Isobar Maps to determine the direction and strength of winds. Figure 11 shows one example of how the PGF is calculated.
The discussion above should make it easy to see that the measurement of atmospheric pressure is an essential component to understanding and predicting atmospheric phenomena. One of the key instruments used by meteorologists, therefore, is a barometer. The mercury barometer is one of several different types of barometers and one that the general public is familiar with. In a mercury barometer, changes in pressure push down on the mercury that surrounds the tube and causes a change in the level of mercury inside the tube. Atmospheric or barometric pressure is thus described in mm Hg. Figure 12 shows a diagram of a mercury barometer and briefly describes its operation.
In this CELL, you will actually build and use a model of a barometer in order to enhance your understanding of atmospheric pressure.
Information in this Introduction section has thus far introduced some basic concepts to understanding the atmosphere. The next section illustrates how Charles’ Law and the relationship between temperature, density, and pressure be used to understand another atmospheric event: the movement of air masses and the formation of fronts.
Air masses are large bodies of air that possess certain temperature and humidity characteristics. Air masses tend to develop over areas of the Earth that are flat and where the atmosphere is calm with few strong winds so that the air remains relatively stationary and takes on the temperature and humidity characteristics of the surface below. The polar and tropical regions of the Earth are areas in which air masses form. These areas are found from 0 to 30 degrees latitude and 60 to 90 degrees latitude. Air masses can form over both land and oceans and are classified by meteorologists according to temperature, and humidity/Earth surface characteristics. The letters “m” for maritime or “c” for continent are used to designate whether the air mass formed over water (m) or a continent/land ( c). In general, maritime air masses contain more humidity than continental air masses. The capital letter A, P, or T is also used to identify the air mass as a tropical (T), polar (P), or arctic (A) air mass. Tropical air masses are significantly warmer than polar or arctic air masses. Arctic air masses are generally colder than polar air masses. Figure 10 illustrates the major air masses that affect Earth’s weather.
Air masses do not remain stationary all of the time. Winds high in the atmosphere such as the jet stream and other winds associated with high and low-pressure centers move air masses. As air masses move over other areas, they can take on the temperature and humidity characteristics of the surface beneath them. Air masses tend to collide in the middle latitudes of the Earth (30 to 60 degrees). When air masses collide a front is formed.
The term front was first used by Jacob Bjerknes to describe the boundary between air masses of two different densities. He used the term front because the behavior of the air masses at the boundary reminded him of the clashing of two different armies on a field of battle as there is often severe weather along a front.
There are several different types of fronts including cold fronts, warm fronts, stationary fronts, and occluded fronts. Cold fronts represent a boundary between a cold air mass and warm air mass. In a cold front, the colder air mass moves in behind the warmer air mass. Because the colder air is less dense than the warm air in front of it, the colder air mass sinks, pushing into the warmer air mass. As a result, the warmer air mass rises into higher levels of the atmosphere. As this happens, the rates of evaporation and condensation of the water and water vapor in the “warm” air mass change because of the change in temperature. As the temperature of the air decreases, the kinetic energy of the gas molecules that comprise air decreases. The result is that the rate of condensation becomes greater than the rate of evaporation and cloud formation and precipitation can occur.
On a weather map, a cold front is represented by a solid line with triangles. The triangles point in the direction of the warmer air and the direction in which the front is moving. If the map is colored, a cold front is represented by the color blue.
A warm front also represents the boundary between a cold and warm air mass. In a warm front, the warmer air mass moves in behind the colder air mass. Because the warmer air behind the cold air mass is less dense, the warmer air rises and slides over the colder air mass. As the warmer air mass rises into the colder, upper levels of the atmosphere the rates of evaporation and condensation of the water and water vapor in the air mass change because of the change in temperature. As the temperature of the air decreases, the kinetic energy of the gas molecules that comprise air decreases. The result is that the rate of condensation becomes greater than the rate of evaporation and cloud formation and precipitation can occur. On a weather map, a warm front is represented by a solid line with semi-circles pointing in the direction of the colder air and in the direction in which the front is moving. If the map is colored, a warm front is represented by the color red.
In general, the differences in the amount of precipitation that forms along fronts are attributed to differences in the humidity characteristics of the air masses rather than to differences in the temperature differential between the warm air mass and the cold upper atmosphere. In other words, once the temperature difference is large enough so that the rate of condensation exceeds the rate of evaporation, cloud formation will occur. Changes in temperature beyond that simply ensure that condensation continues to occur. Instead, greater amounts of precipitation occur with air masses that contain more moisture.
Those air masses that form over water tend to have higher levels of humidity than those that form over land. Thus, there is generally more precipitation that occurs with cold and warm fronts that form from the meeting of cold dry air masses and warm moist air masses than those formed from the meeting of cold dry air masses and warm dry air masses.
In addition to the changes in precipitation that accompany a front, there are also changes in atmospheric pressure. Recall that fronts form as air masses of different temperatures collide. When this happens the difference in densities of the air masses results in warmer air masses rising over the colder air masses. Recall also that atmospheric pressure is a measure of the downward force per area exerted by the atmosphere on the Earth. Cool air sinking increases the downward force of air on the Earth and thus increases atmospheric pressure at that area on the Earth. Warm air rising decreases the downward force of air on the Earth and thus decreases the atmospheric pressure at that area on the Earth. Thus, the pressure changes that accompany the formation and passing of a front result from the movement of different masses of air.
To understand the changes in pressure, it is beneficial to think of the passing of a front in stages. In both cold and warm fronts, the rising of the warmer air mass is accompanied by a decrease in the atmospheric pressure. In a cold front, there is a decrease in pressure as the front forms because the cool air mass slides under the warmer air mass and pushes it upwards. The rising warm air results in a decrease in atmospheric pressure. This decrease is followed by an increase in pressure once the front has passed because of the cooler sinking air that moves in behind the warmer rising air.
In a warm front, there is a decline in the atmospheric pressure as the front moves into an area because a warm air mass moves in behind a cooler air mass. As the warm air mass slides over the cooler air mass there is a decrease in pressure. The pressure continues to drop as the front passes because of the warmer, rising air that moves in behind the cooler air mass.
As evident from the description of the movement of air masses and the changes in pressure that accompany the passing of fronts, analysis and forecasting of atmospheric events is tied to comprehension of Charles’ Law and the relationship between the air temperature, density, and atmospheric pressure. Therefore, in this CELL, you will have the opportunity to perform experiments centered around Charles’ Law, the relationship between the temperature and density of air, and the relationship between the density of air and atmospheric pressure. In addition, you will conduct experiments that simulate the creation of high and low-pressure centers and initiate pressure gradients forces. In doing so, you should gain a better understanding of atmospheric events, weather phenomena, and the scientific principles that govern them.
- Fun Facts: Thermometers
- More Facts: Storms
- Learn the Lingo
- Get Focused
Evolution of the Thermometer
Every LabLearner student, from elementary school forward, knows the importance of time and temperature in science. Temperature is involved in countless LabLearner experiments. Time is involved in even more. Can you even imagine doing science without clocks or thermometers? LabLearner students know how difficult this would be.
When we talk about the subject of this CELL, Atmosphere, two instruments needed to be developed in order to bring the study of the atmosphere around us into the scientific realm – the barometer and the thermometer. You will actually build and test a simple barometer in the lab. Let’s talk about the evolution of the thermometer here.
As the illustration below indicates, there are a number of innovations that occurred over the years in our ability to accurately determine temperature. While the so-called Galileo thermometer dates back to the sixteenth century (around 1596), one can imagine how inconvenient and restricted its use would have been for scientific experimentation. It worked on the principle that the lower the temperature of water, the denser it is. Thus, glass balls of different masses would either sink or float depending on the temperature of the water in the column they are placed in. The problems with such an instrument are obvious. For example, it must be held in an upright position, it has a slow response time as water in the column slowly comes to the temperature of the experiment, as well as many other accuracy and practical issues.
For all of these and other reasons, the first practical thermometer that was both accurate and practical was the fluid-filled capillary thermometer. As the temperature of the fluid contained within a sealed tube with a fine tunnel (capillary) increases, it expands because the liquid molecules move faster and faster with the increased kinetic energy associated with heat. This causes an increase in the volume of the fluid, which forces it up the glass capillary, where the temperature can be read on a temperature scale. In the LabLearner CELL Heat and Heat Transfer, you will (or perhaps already have) calibrate a scale on your own thermometer based on the boiling and freezing points of water.
The credit for the first usable capillary thermometer historically goes to the Polish-Lithuanian expert glassblower and scientist, Daniel Fahrenheit. In 1709, Fahrenheit invented a alcohol-filled capillary thermometer and then a mercury-fill version a few years later. Such instruments have been responsible for uncountable numbers of scientific discoveries, not to mentions their contributions to medicine over the centuries. And they are still doing so today. As a result, Fahrenheit was immortalized by naming one of the major temperature scales after him.
The illustration below shows three major temperature scales. While the Fahrenheit scale was once the only scale used, the Celsius Scale was introduced later in the century (1742). Capillary thermometers work the same regardless if they are calibrated in the Fahrenheit or Celcius scale. However, today, and for well over a hundred years, only the Celcius Scale is used in science. Another scale, put forward by the British scientist, Lord Kalvin, a century after the Celcius Scale (in 1848), is also used for specific scientific purposes today. The Kalvin Scale introduces an important concept known as absolute zero, that we will not discuss further at this time.
Celsius, Fahrenheit, and Kelvin Scales
Even though the Fahrenheit Scale is no longer used in science and therefore never used in LabLearner, we are addressing it here because one major country in the world still uses the Fahrenheit Scale in day-to-day life. That country is the United States! But remember, even in the United States, ALL scientific measurements of temperature use the Celcius Scale.
The Celcius Scale is based on setting the temperature of boiling water at 100oC and the freezing point of water at 0oC. The distance between these two points on the capillary tube is then divided to assign all of the other temperature points on the scale. Simply put, the temperature exactly halfway between zero degrees and one hundred degrees on the Celcius capillary thermometer is 50oC. Study the illustration of the differences between the temperature scales above. And remember, when you think that normal human body temperature is 98.6 degrees, the rest of the world thinks 37 degrees!
Although we will make no measurements with anything but the Celcius Scale in LabLearner, below are two simple algorithms to convert between the two scales for your information.
As shown in the three videos below, storms occur everywhere on Earth. In the examples below, we see time-lapse videos of storms in the atmosphere over a city, mountains, and even a desert.
Storm of the City of Singapore
Storm of the Mountains
Storm on the Desert
In the desert storm, which is often unaccompanied by rain, you can see air masses moving in different directions depending on altitude. The storm clouds above appear to be moving away from the camera and to the right. On the other hand, the air at the Earth’s surface is clearly moving to the left and causing a sandstorm or dust storm. Below is a picture taken from a fleeing news helicopter recently in Phoenix, Arizona.
Dust Storm Moving into Phoenix, Arizona (August 16, 2020)
During the 1930s, across large areas of the farm belt in the United States, conditions favoring dust storms prevailed for many months on end. The results were devastating and a major contributor to the deepest economic recession in the country’s history. Topsoil was blown away leaving unfertile land unable to support productive agriculture. This, in turn, caused widespread poverty and the migration of thousands of Americans. The classic novel by John Steinbeck, The Grapes of Wrath, was written about an Oklahoma farm family (the Joad family) about this time, a time often referred to as the “Dust Bowl”.
Dust Cloud Approaching in 1930s Oklahoma
Dust Cloud About to Engulf a Farm in 1930s Texas
While outside the scope of our simple introduction to the atmosphere, tornados and hurricanes are major classes of storms. Look at the satellite video of a hurricane below. You might imagine the many forces and pressures interacting with each other to form such awesome and large air mass movement. The hurricane shown here is well over 400 kilometers (km) across with wind speeds of 200 km/hr or more. The hole in the middle of the hurricane, referred to as the “eye”, is relatively calm. This one looks to be about 10 km in diameter.
Unlike hurricanes, tornados (pictured below) can occur far from coastlines on dry land. They are smaller and much more local than hurricanes. Nonetheless, localized wind speeds easily in excess of 200 km/hr make tornados exceeding dangerous and deadly.
LEARN THE LABLEARNER LINGO
The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.
Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.
- Density: the amount of matter in a given volume or area. Density is also described as a measure of the compactness of matter. It is determined by dividing the amount of mass of a substance by its volume. D=m/v
- Atmosphere: The thin layer of gases that surrounds the Earth. It is held in place by the force of Earth’s gravity.
- Charles Law: The law that describes the relationship between the temperature and volume of a gas under conditions in which the pressure and amount of a gas is constant. Charles’s Law states that the volume of a given amount of dry ideal gas is directly proportional to the Kelvin Temperature. Charles’ Law can be represented by the mathematical formula:
- Troposphere: The layer of the atmosphere extends from the surface of the Earth to approximately ten to twelve kilometers above the Earth’s surface. It is the layer in which most of the Earth’s weather occurs.
- Atmospheric pressure: The sum of forces exerted on the surface of the Earth by the molecules that compose the atmosphere per unit area. Atmospheric pressure can be represented by the following equation and is measured in units of millibars:
- High-pressure system: When the atmospheric pressure at one area of the Earth is higher than the areas that surround it.
- Low-pressure system: When the atmospheric pressure at one area of the Earth is lower than the areas that surround it.
- Barometric pressure: An indication of the atmospheric pressure in an area as measured in millimeters of mercury (mm Hg) with a barometer. The pressure in mm Hg can be converted to millibars using the equation below.
- Barometer: One of the key instruments used by meteorologists to measure changes in atmospheric pressure. A mercury barometer is one type of barometer. Changes in atmospheric pressure measured with a mercury barometer are reported in millimeters of mercury (mm Hg).
- Pressure gradient force (PGF): The difference in pressure between higher and lower areas of pressure and the distance between the two areas.
- Isobar: Lines on a weather or atmospheric pressure map that indicate areas of equal atmospheric pressure.
- Wind: Horizontal movements of air set into motion by pressure differences that exist within the Earth’s atmosphere.
- Air mass: A large body of air that possesses certain temperature and humidity characteristics.
- Front: The boundary between air masses of two different densities.
- Cold front: A boundary between a cold air mass and warm air mass. In a cold front, the colder air mass moves in behind the warmer air mass.
- Warm front: A boundary between a cold air mass and warm air mass. In a warm front, the warmer air mass moves in behind the cooler air mass.
- Evaporation: A change of phase in which the liquid phase of a substance is converted to its gaseous phase.
- Condensation: A change of phase in which the gaseous phase of a substance is converted to its liquid phase.
- Precipitation: Liquid and solid phases of water that fall from the sky. Precipitation includes rain, hail, snow, freezing rain, and sleet.
The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:
- What happens to air as it is heated and cooled?
- How do changes in the temperature of air affect its density?
- Why do changes in density cause air movement?
- How do differences in the temperature of the Earth’s atmosphere affect the movement of air?
- What types of air movements cause areas of high and low pressure?
- Why do differences in pressure cause wind?
- How are differences in pressure measured?
- What factors are necessary for precipitation to form along fronts?
- What changes in atmospheric pressure occur with the passing of a cold or warm front?