Introduction-basics
Nitrogen and oxygen make up 99% of the atmosphere. Yet the remaining 1% contains many of the atmospheric gases that are critical to weather and climate. The three major trace gases in the atmosphere are carbon dioxide, water vapor, and ozone. These gases play important roles in the energy cycle of the atmosphere. Other important trace gases for atmospheric studies include methane and chloroflurocarbons (CFCs). These gases are important because they interact with other gases and modify the energy balance of the atmosphere. The CFCs do not occur naturally. The CFCs were invented by chemists, and used as coolants for refrigerators and air conditioners. These human-made or anthropogenic gases were found to destroy ozone. For climate predictions, it is important to know how and why the concentrations of these gases and particles change over time.
In meteorology, the pressure is given in millibars or Pascals rather than inches of mercury. One millibar (mb) equals 100 Pascals and 0.76 millimeters (0.03 inches) of mercury. The average surface pressure at sea level is 1013.25 mb or 29.92 inches of mercury. Pressure is also expressed in terms of pounds per square inch (psi): air pressure in tires is frequently measured in psi. A common pressure in automobile tires is 32 psi, which converts to 2206mb or 65.3 inches of mercury-more than twice the atmospheric pressure at sea level.
The atmospheric pressure always decreases with increasing altitude (1mb drop for every 10m height). When comparing the pressure of different locations, meteorologists always adjust the pressure measurements to a single altitude: sea level. This removes the effect of altitude on pressure and allows the meteorologists to focus on the smaller, but important, pressure differences due to weather systems.
Meteorologists find it useful to divide the atmosphere into layers. Based on the temperature, the atmosphere is divided into four main layers: the troposphere, stratosphere, mesosphere and thermosphere (refer Fig 1.1). From the surface up to approximately 10 to 16km, the temperature generally decreases with altitude. This is because the atmosphere is nearly transparent to the Sun’s energy and is instead heated from below by the Earth’s surface, like a pot of water on a stove. The region of the atmosphere closest to the Earth, where the temperature decreases as you go up, is called the troposphere. The top of the troposphere is referred to as the tropopause. It acts as an upper lid on most weather patterns, Above the tropopause lies the stratosphere, where temperature increases with altitude. Temperature is increasing because ozone molecules in the stratospheric “ozone layer’ are absorbing solar energy near the top of the stratosphere. Air flow in the stratosphere is much less turbulent than in the troposphere. The stratopause marks the top of stratosphere. On average the stratopause occurs at an altitude of about 50km. Above the statopause, lies the mesosphere. Temperature decreases with altitude in the mesosphere as it does in the troposphere. In the thermosphere, the temperature again increases with altitude. From the mesosphere on up, the atmosphere becomes more and more affected by high-energy particles from the Sun. These particles break apart atmospheric molecules, which then form ions. For this reason, the region of the upper mesosphere and thermosphere is sometimes called the Ionosphere. Some interesting visual phenomena occur in the mesosphere and thermosphere due to the interaction of solar particles and the atmosphere, such as the aurora. The ionosphere reflects radio waves, permitting radio stations to be heard far beyond the horizon.
The Energy Cycle
Introduction
At any particular moment half of the Earth is facing the Sun and half isn't. Over a year, the tropical regions of the planet experience a net energy gain while the poles have a net energy loss. This global imbalance of energy is largely due to differences in how high the Sun gets in the sky and the length of day. The atmosphere and oceans respond to this energy imbalance by transporting heat from the equatorial regions toward the poles. Transport of heat is the reason we have weather.
Doing work requires energy. Energy is the capacity to do work. The amount of energy needed depends on the amount of work to be done. It does not take much energy to push a book or lift a glass of water. We will study four different forms of energy: heat energy, electrical energy, kinetic energy, and potential energy. Energy can be converted from one form to another, but the total energy is always conserved.
In meteorology and oceanography, we are primarily concerned with kinetic and potential energy. Kinetic energy is the work that a body can do by virtue of its motion. A major part of this energy is related to how fast a mass is moving. The kinetic energy of a moving object is also related directly to its mass. A freight train movingat only 16 km/hr (10 mph) has 36 times less kinetic energy than the same train moving at 96 km/hr (60 mph). A sport-utility vehicle on a highway has more kinetic energy than a car moving at the same speed.
Potential energy is the work an object can do as a result of its relative position. You do work as you lift a book off the desk. Once off the desk, the book has potential to do work because of gravity. The higher you lift the book the greater the potential energy. When you let go of the book it falls and potential energy is converted to kinetic energy. If you drop the book from the tenth floor of a building, it becomes a dangerous missile. If you drop the same book from a height of an inch it can't do much damage. So, potential energy represents stored energy that can be converted to other forms of energy, such as kinetic energy. The potential energy of the book is represented by its height from the surface of the desk.
In studies of the atmosphere, meteorologists find it useful to refer to an isolated parcel of air. A parcel of air is a hypothetical balloon-like bubble of air, flexible but impermeable, and perhaps as large as a parking lot. Inside this bubble, basic weather variables such as temperature and moisture are the same. As this parcel of air moves around in the atmosphere, mass and energy do not cross its imaginary boundary. The air around the parcel is the "environment" or the "surroundings." Even though the real atmosphere is not composed of parcels, meteorologists can make sense of the atmosphere by looking at it in this way. If we imagine such a parcel of air, we can move it through the atmosphere as an intact unit and compare its temperature to the temperature of its environment.
Temperature relates to energy, because temperature is a measure of the average kinetic energy of a substance. A thermometer in a glass of ice water records a lower temperature than a thermometer in a pan of boiling water, because the molecules of H2O in the boiling water are much more energetic. That energy is imparted to the thermometer and shows up as a higher temperature.
With this definition of temperature, we can now define the calorie (abbreviated cal) as the unit used to measure amounts of energy. A calorie is the energy needed to raise the temperature of 1 gram of water 1 degree Celsius (from 14.5° C to 15.5° C.) The dietary 'Calorie' (with a capital "C") used in quantifying the energy content of foods is actually a kilocalorie or 1000 calories (with a small "c"). A Joule is another unit used to measure amounts of energy. One Joule equals 0.2389 calories.
The term power refers to the rate at which energy is transferred, received, or released. The watt (W) is a unit of power that represents the transfer of one joule of energy per second.
Heat is energy produced by the random motions of molecules and atoms; it is the total kinetic energy of a sample of a substance. Both heat and temperature are related to kinetic energy and therefore to one another.
the temperature change of an object depends on:
- How much heat is being added-
- The amount of matter-the more matter, the more heat is required to change its temperature.
- The specific heat of the substance-but what is specific heat?
The specific heat of a substance is the amount of heat required to increase the temperature of one gram of that substance one degree Celsius. Because it takes alot of energy to raise the temperature of water, water has a high specific heat (Table 1). A low specific heat means that a substance heats up and cools down easily, requiring little energy. Table 1 indicates that it takes more than four times as much heat to warm one gram of water one degree Celsius than it takes to warm one gram of air by the same amount.
Transferring Energy in the Atmosphere
To change the temperature of a substance, such as air, we need to add or remove heat. Methods of heat transfer important to weather and climate are: conduction, convection, advection, latent heating, adiabatic cooling, and radiation. In discussing energy transfer, we will concentrate on the direction of the energy transfer and the factors that determine how fast the energy transfer occurs.
Conduction: Requires Touching
Conduction is the process of heat transfer from molecule to molecule; energy transfer by conduction requires contact. An example of energy transfer by conduction is when we touch an object to feel if it is warm or cold. Heat is transferred from the warmer object to the colder one. The amount of heat transferred by conduction depends on the temperature difference between the two objects and their thermal conductivity. The ability of a substance to conduct heat by molecular motions is defined by its thermal conductivity.
Water is a good conductor of heat, while air is a poor heat conductor when it is not moving. This is why air between two pieces of glass in a storm window keeps a room insulated from the cold in winter. Dry sand is a poor conductor because of the air between the sand grains. Wet sand is a better conductor than dry sand because the air spaces are filled with water, which is a good conductor, as are the individual grains of sand. This is why running barefoot on a dry, sandy beach on a hot, sunny day can be a painful experience!
Since air is a poor conductor, conduction is not an efficient mechanism for transferring heat in the atmosphere on a global scale. But conduction is good for transferring energy over small distances, and is an important form of heat transfer near the ground.
Convection: Hot Air Rises
If the ground is hot, heat is transferred to air molecules in contact with the surface via conduction. The heated parcel of air rises and cooler air sinks to replace the rising warm air. This results in a net transfer of heat upward, away from the surface. Warmth moves upward, far away from the surface, because of the movement of the fluid (air in this case), not because the air high up is in direct contact with the ground. This process of transferring energy vertically is called convection.
In the atmosphere the rate of energy transfer by convection depends on how hot the rising air parcel is and the vertical temperature pattern of the surrounding atmosphere. In certain regions of the globe where the ground is much warmer than the air above it, convection is an important process for moving heat vertically. Convection is strong over deserts during the summer, where energy from the Sun rapidly heats up the sand. Convection is an inefficient mode of heat transfer in polar regions, where the surface air is in contact with a surface that is often cooler than is the air above it.
Heat Advection: Horizontal Movement of Air
The horizontal transport of heat in the atmosphere is referred to as heat advection. Warm air advection occurs when warm air replaces cooler air. On weather maps, temperature advection is occurring wherever the wind "flagpoles" are pointed across the isotherms. In winter snowstorms, warm air advection moves warm air poleward while cold air advection brings cold air towards the tropical regions. Advection is an important process throughout the troposphere.
Latent Heating: Changing the Phase of Water
In the atmosphere, only water exists in all three phases: solid, liquid, and gas. Ice is the solid form of water and water vapor is the gas phase. In everyday language, the liquid form of water is generally referred to as "water". In this section, however, using this common term would lead to some confusion. So, in this section only, water in the liquid phase is referred to explicitly as liquid water.
Changing the phase of a substance either requires or releases energy. Changing the phase of water adds or removes energy from its surroundings. Since the Earth's surface is 70% liquid water, understanding the phase changes of water is important for understanding atmospheric energy. How does a change of phase occur?
Ice cubes melt and puddles evaporate because heat is added or removed from water, causing a change of phase. Latent heat is the heat absorbed or released per unit mass when water changes phase. This change of phase does not necessarily result in an increase in the water temperature. For example, adding heat to an ice cube may result in a change of phase of the water, from a solid to a liquid. However, the temperature of a liquid water-ice mixture will not increase until all the ice is melted. Energy cannot be destroyed, so what happens to the heat energy if it isn't used to change the temperature of the water?
Water molecules of ice and liquid water are bound together by molecular forces. In the ice phase, the water molecules have low enough kinetic energies that intermolecular attractions bind them into a highly ordered, crystalline form. To break these intermolecular attractions and transform ice into liquid water, energy must be added. Latent heat of melting is the amount of energy absorbed by water to change one gram of ice into liquid water, and is equal to 80 cal for each gram of ice.
If heat must be added to melt ice, the opposite occurs when water freezes. The amount of energy released into the environment when water freezes is also 80 cal per gram of ice, and is referred to as the latent heat of fusion.
The transition of water from the liquid phase to the gas phase is called vaporization or evaporation. The molecules of a gas are essentially free of one another, having no bonds between them. To convert liquid water to a vapor requires the addition of a sizable amount of energy (more than seven times the energy required to melt ice) to break the binding forces that keep the molecules in a fluid state. The amount of heat required to evaporate one gram of liquid water is referred to as the latent heat of vaporization. The latent heat of vaporization is a function of water temperature, ranging from 540 cal per gram of water at 100° C to 600 cal per gram at 0° C. That is, it takes slightly more energy to evaporate cold water than to evaporate the same amount of warmer water. (In addition, to raise the temperature of liquid water from 0° C to 100° C also requires an input of energy; this is where the specific heat of water comes in.)
Water vapor condenses to form liquid water. Condensation is the opposite of evaporation. Latent heat of condensation represents the amount of energy released when water vapor condenses to a liquid form. The latent heat of condensation is a function of temperature and has the same range as the latent heat of vaporization.
Water vapor may change directly to ice in a process known as deposition. Conversely, ice may also directly enter the gas phase without melting (called sublimation). The latent heat of sublimation equals the latent heat of deposition. A total of 680 cal are required to change one gram of ice at 0° C (32° F) into vapor. Notice that this number is equal to 600 cal + 80 cal. This means that the latent heats of sublimation/deposition are simply the sum of the latent heats of melting/fusion and the latent heats of vaporization/condensation. However, on Earth the sublimation and deposition of water happen less frequently, and on a far smaller scale, than do the processes of evaporation, condensation, melting, and freezing.
On the other hand, condensation, the opposite of evaporation, is a heating process that supplies energy to the environment. When water vapor changes into the liquid water or ice phase to form clouds, energy is released into the atmosphere. Changes of phase are also important in the energy gains and losses at ground level. The formation of dew (condensation) or frost (deposition) releases heat.
Changing the phase of water is an efficient method of transferring energy globally and provides an energy source for much of our weather. However, we are interested in some phase changes more than others. The reason is simple: 600 cal per gram is a lot more than 80 cal per gram! In other words, the phase changes of evaporation and condensation have by far the largest latent heats of the common phase changes of water. This obvious fact means that throughout this text we will be very interested in where, when, and how much evaporation and condensation occur. Wherever these two processes occur, there is a large transfer of energy going on.