DW Test

Tremendous quantities of energy are fed into the troposphere, setting it in motion and making it work in many ways to create our everchanging weather. At any time and place, the energy may be in any one form or a combination of several forms. All energy, however, comes either directly or indirectly from the sun.

Simply defined, energy is the capacity to do work. Its more common forms are heat or thermal energy, radiant energy, mechanical energy (which may he either potential or kinetic), chemical energy, and electrical energy. There are also atomic, molecular, and nuclear energy.

Energy can be, and constantly is being, transformed from one form to another, but energy is always conserved in the process. It cannot be created nor destroyed, although a transformation between energy and mass does occur in atomic reactions.

Kinetic energy is energy of motion, whereas potential energy is energy due to position, usually with respect to the earth's gravitational field. The motion of a pendulum is a good example of the interchange of potential and kinetic energy. At the end of its swing, a pendulum has potential energy that is expended in the down stroke and converted to kinetic energy. This kinetic energy lifts the pendulum against the force of gravity on the upstroke, and the transformation back to the potential energy occurs. Losses caused by friction of the system appear in the form of heat energy. The sun is the earth's source of heat and other forms of energy.

The common storage battery in charged condition possesses chemical energy. When the battery terminals are connected to a suitable conductor, chemical reaction produces electrical energy. When a battery is connected to a motor, the electrical energy is converted to mechanical energy in the rotation of the rotor and shaft. When the terminals are connected to a resistor, the electrical energy is converted to thermal energy. When lightning starts forest fires, a similar conversion takes place.

Energy is present in these various forms in the atmosphere. They are never in balance, however, and are constantly undergoing conversion from one form to another, as in the case of the pendulum or the storage battery. Their common source is the radiant energy from the sun. Absorption of this energy warms the surface of the earth, and heat is exchanged between the earth's surface and the lower troposphere.

[add image - caption: Chemical energy can be transformed into electrical energy, which in turn can be transformed into mechanical energy or thermal energy.]

Heat Energy and Temperature

Heat energy represents the total molecular energy of a substance and is therefore dependent upon both the number of molecules and the degree of molecular activity. Temperature, although related to heat, is defined as the degree of the hotness or coldness of a substance, determined by the degree of its molecular activity. Temperature reflects the average molecular activity and is measured by a thermometer on a designated scale, such as the Fahrenheit scale or the Celsius scale.

If heat is applied to a substance, and there is no change in physical structure (such as lee to water or water to vapor), the molecular activity increases and the temperature rises. If a substance loses heat, again without a change in physical structure, the molecular activity decreases and the temperature drops.

[add image - caption: All forms of energy in the atmosphere stem originally from the radiant energy of the sun that warms the surface of the earth. Energy changes from one to another in the atmosphere; so does energy in a swinging pendulum.]

Heat and temperature differ in that heat can be converted to other forms of energy and can be transferred from one substance to another, while temperature has neither capability. Temperature, however, determines the direction of net heat transfer from one substance to another. Heat always flows from the substance with the higher temperature to the one with the lower temperature, and stops flowing when the temperatures are equal. In this exchange of heat, the energy gained by the cooler substance equals that given up by the warmer substance, but the temperature changes of the two are not necessarily equal.

Since different substances have different molecular structures, the same amount of heat applied to equal masses of different substances will cause one substance to get hotter than the other. In other words, they have different heat capacities. A unit of heat capacity used in the English system of measures is the British thermal unit (B.t.u.). One B.t.u. is the amount of heat required to raise the temperature of 1 pound of water 1°F.

The ratio of the heat capacity of a substance to that of water is defined as the specific heat of the substance. Thus, the specific heat of water is 1.0-much higher than the specific heat of other common substances at atmospheric temperatures. For example, most woods have specific heats between 0.45 and 0.65; ice, 0.49; dry air, 0.24; and dry soil and rock, about 0.20. Thus, large bodies of water can store large quantities of heat and therefore are great moderators of temperature.

If heat flows between two substances of different specific heats, the resulting rise in temperature of the cooler substance will be different from the resulting decrease in temperature of the warmer substance. For example, if 1 pound of water at 70°F. is mixed with 1 pound of gasoline, specific heat 0.5, at 60°F., the exchange of heat will cause the temperature of the gasoline to rise twice as much as this exchange causes the water temperature to lower. Thus, when 3 1/3 B.t.u. has been exchanged, the pound of water will have decreased 3 1/3°F. and the pound of gasoline will have increased 6 2/3°F. The temperature of the mixture will then be 66 2/3°F.

With minor exceptions, solids and liquids expand when their molecular activity is increased by heating.

They contract as the temperature falls, and the molecular activity decreases. The amount of expansion or contraction depends on the size, the amount of temperature change, and the kind of substance. The expansion and contraction of liquid, for example, is used in a thermometer to measure temperature change. Thus, volume changes with temperature, but at any given tempera, lure the volume is fixed.

The reaction of gases to temperature changes is somewhat more complex than that of liquids or solids. A change in temperature may change either the volume or pressure of the gas, or both. If the volume is held constant, the pressure increases as the temperature rises and decreases as the temperature falls.

[add image - caption: If the volume of a gas is held constant, the pressure increases as the temperature rises, and decreases as the temperature falls.]

Since the atmosphere is not confined, atmospheric processes do not occur under constant volume. Either the pressure is constant and the volume changes, or both pressure and volume change. If the pressure remains constant, the volume increases as the temperature rises, and decreases as the temperatures falls. The change in volume for equal temperature changes is much greater in gases under constant pressure than it is in liquids and solids. Consequently, changes in temperature cause significant changes in density (mass per unit volume) of the gas. Rising temperature is accompanied by a decrease in density, and falling temperature is accompanied by an increase in density.