Energy and heat Sources of heat in the atmosphere Heat transfer in the atmosphere
Energy and Heat
Energy is a very difficult concept to come to terms with. A simple scientific definition of energy is the ability or capacity to do work on some form of matter (and remember that matter is anything that has mass and occupies space). Work is do ne when matter is either pushed, pulled, or lifted over some distance.
Energy can be classified as potential energy or kinetic energy. Kinetic energy refers to energy of motion whereas potential energy represents the potential for k inetic energy. An objectÕs kinetic energy is equal to the product of its mass and its velocity squared.
The amount of heat an object has actually refers to the kinetic energy of the atoms and molecules that make up the object. As the ob ject acquires more energy, its atoms and molecules move faster, thus the object heats up. Heat is actually a form of energy.
Temperature thus is actually a measure of the average speed of the molecules and atoms of a substance. Ene rgy stored within the atoms and molecules of a substance is referred to as internal energy.
Note again that it is the average speed of the molecules that is important. To illustrate this point, consider two half full buckets of water. E ach have the same finite number of water molecules. If you pour the contents of one bucket into the other, you will be increasing the total amount of internal energy in that bucket by a factor of 2, however, the average speed of the molecules will remain unchanged hence the temperature will remain unchanged. The important point being illustrated here is that temperature is related to the average speed of the molecules of a substance.
Keeping this in mind, consider that the words hot and cold are quite relative terms. Something that is cold simply has slower moving molecules than something hot. In actuality, nothing is really cold from an absolute point of view, however it may just lack heat.
The temperature at which all molecular motio n ceases to exist is called absolute zero which is -461 F, -273 C, 0 K. The Kelvin (K) temperature scale is often referred to as the absolute temperature scale.
Energy occurs in many other forms aside from heat energy: chemi cal energy, mechanical energy, nuclear energy, and electrical energy are a few examples:
Energy in the universe is conserved. A very important principle of physics is the First Law of Thermodynamics which is also called the Law of Conservation of Energy. It states mathematically that the total amoun t of energy in the universe remains constant and that energy cannot be created or destroyed. It merely changes from one form to another.
Energy is quantified by many different units. The Metric System uses the units Joules which represents one kg m^2/s^2. This makes sense when we recall from above that energy is mass (kg) times velocity squared (m^2/s^2).
The First Law of Thermodynamics can be reworded as: for a process, the total change in energy equals the amount of heat lost or gai ned minus the work performed.
Consider a block being slid across a rough surface. The amount of work performed on it will depend on the distance it is moved. That amount of work will equal the amount of heat that is released as a result of fri ction such that for the entire process, energy is conserved. This is an example of the conversion of mechanical energy into heat energy. The important concept to grasp here is that heat and energy are interchangeable.
Specifi c heat
Specific heat refers to the amount of heat energy that must be added to a substance to raise its temperature. For instance, in the Metric System, one gram of water has a specific heat of 4.18 J/C. That means that in order t o raise the temperature of one gram of water by one degree Celsius, you must apply 4.18 Joules of heat energy.
Different substances have different specific heats. For instance, air and water have quite different specific heats. This is why oce an and lake temperatures are cooler than the air during the day however warmer at night. Air heats up and cools off much more rapidly than water.
The specific heat of a substance has a lot to do with its chemical makeup.
Latent heat
Latent heat refers to heat that is absorbed or released upon the change of phase of matter. In meteorology, we primarily are concerned with changes of phase of water:
| Phase | ------> | Phase | Phase Change Name |
|---|---|---|---|
| Solid | ----> | Liquid | Melting |
| Liquid | ----> | Gas | Evaporation |
| Solid | ----> | Gas | Sublimation |
| Gas | ----> | Liquid | Condensation |
| Gas | ----> | Solid | Depo sition |
To better understand latent heat, we will once again recall that temperature is a measure of the average speed of the molecules that comprise an object or substance. Intuitively, one would reason that molecules in a liquid m ove faster than in a solid and molecules in a gas move faster than in a liquid. Hence gases can be thought of as being in a higher energy state than liquids or solids. Consider a liquid that is evaporating. Its molecules will need to attain energy to g o from the lower energy liquid phase to the higher energy gas phase. As a result, once a liquid gets to its boiling point, some of the energy that it acquires does not go toward changing its temperature, but to giving it enough energy to escape to the hi gher energy gas phase.
Latent heat is then the amount of heat that must be applied to a substance or that is released by a substance such that it undergoes a particular phase change. The amount of heat needed for a substane to evaporate to the gas phase is the Latent Heat of Vaporization. The amount of heat released when a substance condenses from the gas phase to the liquid phase is the Latent Heat of Condensation.
We will now consider an example of how evaporation is a cooling proc ess. When you get out of the shower, you have drops of water all over your body. As that water evaporates, it must be acquiring energy to jump from the lower energy liquid phase to the higher energy gas phase. It attains that energy by pulling heat ene rgy out of your body. As a result, the surface of your skin cools. This is actually the mechanism by which sweat acts as a natural air conditioner for your body.
When condensation occurs however, such as when raindrops form, water is going from the higher energy gas phase to the lower energy liquid phase. In this case, heat is released. This process drives storm systems and hurricanes and helps to drive the Global Energy Circulation.
Sources of heat in the atmosphere - the Sun an d Latent Heat
The Sun is the primary source of heat on the Earth. Located about 93,000,000 miles from Earth, it takes energy approximately 8.5 minutes to reach Earth from the sun. The sun is tremendously bigger than the Earth. With a di ameter of 835,200 miles, some 1.2 million Earths would fit in the sun.
The sun is essentially a giant fusion reactor. It is composed of primarily hydrogen and helium with some other heavier elements present. Hydrogen atoms are fused together in a process called fusion. Each time this occurs, heat energy is released. This occurs on the sun trillions of times every millionth of a second.
The core of the sun is extremely hot as temperatures there are believed to be nearly 15 million deg rees C. The surface layer of the sun is called the photosphere and is much cooler at a temperature of nearly 6,000 C.
Blemishes on the sunÕs surface are called sunspots. They are actually cooler regions on the sun. During period s of high sunspot activity (every 11 years), there is believed to be a lessening of the amount of heat energy released from the sun. Scientists as a result believe that sunspots may play a role in EarthÕs climate.
The other significant source o f heating in the atmosphere is latent heating from the condensation of water vapor which was discussed earlier.
Heat transfer
Heat has a tendency to move from warmer regions to cooler regions. There are different means by wh ich this is accomplished however.
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