Types Of Heat Energy Transfer & Temperature Variations

The heat source for our planet is the sun. Energy from the sun is transferred through space and through the earth’s atmosphere to the earth’s surface. Since this energy warms the earth’s surface and atmosphere, some of it is or becomes heat energy. There are three ways heat is transferred into and through the atmosphere:

• Radiation
• Conduction
• Convection

Radiation

If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this transfer of heat. Heat lamps, that keep food warm, work in the same way. Radiation is the transfer of heat energy through space by electromagnetic radiation.

Most of the electromagnetic radiation that comes to the earth from the sun is invisible. Only a small portion comes as visible light. Light is made of waves of different frequencies. The frequency is the number of instances that a repeated event occurs, over a set time. In electromagnetic radiation, its frequency is the number of electromagnetic waves moving past a point each second.

Our brains interpret these different frequencies into colors, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different colors at the same time, it is interpreted as white. Waves from the sun which we cannot see are infrared, which have lower frequencies than red, and ultraviolet, which have higher frequencies than violet light. It is infrared radiation that produce the warm feeling on our bodies.

Most of the solar radiation is absorbed by the atmosphere and much of what reaches the earth’s surface is radiated back into the atmosphere to become heat energy. Dark colored objects, such as asphalt, absorb radiant energy faster than light colored objects. However, they also radiate their energy faster than lighter colored objects.

Conduction

Conduction is the transfer of heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time, the handle of the spoon will become hot. Even the heat transfer between the stove and the pot is conduction.

This is due to transfer of heat energy from molecule to molecule or from atom to atom. Also, when objects are welded together, the metal becomes hot (the orange-red glow) by the transfer of heat from an arc.

This is called conduction and is a very effective method of heat transfer in metals. However, air conducts heat poorly.

Convection

Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen with a boiling liquid.

Air in the atmosphere acts as a fluid. The sun’s radiation strikes the ground, thus warming the rocks. As the rock’s temperature rises due to conduction, heat energy is released into the atmosphere, forming a bubble of air which is warmer than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools with the heat contained in the bubble moving into the atmosphere.

As the hot air mass rises, the air is replaced by the surrounding cooler, more dense air, what we feel as wind. These movements of air masses can be small in a certain region, such as local cumulus clouds, or large cycles in the troposphere, covering large sections of the earth. Convection currents are responsible for many weather patterns in the troposphere.

Daily (Diurnal) Temperature Variations

Daily cycle of temperature is very much like a seasonal cycle

• Warming begins in the morning (spring)
• Hot in the afternoon (summer)
• Cooling in the evening (fall)
• Coolest late evening/early morning (winter)

Just like seasons – due to differential between incoming (solar) and outgoing (earth) radiation.

Daytime Warming

Solar radiation heats the earth’s surface which in turn heats the air by conduction.

How this heat is distributed depends greatly on the wind.

• Calm wind – hot air remains near the surface
• Thermals are small and don’t bring much hot air up
• Temperatures 5-6 feet above ground can be 30º cooler!
• This is why temperature measurements are not taken at the surface (also in the shade and over grass)

On a windy day, temperature does not change as much with height.

• The wind creates turbulent “eddies” which transfer heat from the near the surface upward and cooler air from above downward

Maximum Daily Temperature

The sun is highest at noon, however, the maximum daily temperature doesn’t usually occur until around 4 PM or so. This has to do with absorption and emission. This is also referred to as thermal lag.

Incoming solar radiation is maximum at noon, but incoming radiation is greater than outgoing radiation until late afternoon. So warming continues until outgoing becomes greater than incoming.

Although 4 PM is the average time the maximum temperature occurs, It can be at other times due to several factors.

Clouds keep temperature lower and causes the max temp to occur earlier. Radiation is reflected by bodies of water – air over water is generally cooler than air over land during summer. Air can move ashore during the day and drop the temperature. Sea breezes are a good example.

Large storms and their fronts. During winter, the passage of warm fronts can cause the highest temp to be late at night or early in the morning.

Other Factors Influencing Max Temperature

• Soil Type -Some types absorb more energy due to color & texture.
• Soil Moisture -Wet soil heats up slower. Some energy goes into evaporation.
• Vegetation -More Evaporation.
• Clouds, Humidity, Haze -Reflect radiation.

The ideal conditions to get the hottest daytime temp:

• Soil which is a good absorber (sand for one).
• Dry soil -No vegetation -No moisture in the air.

Based on the conditions above, the desert is the ideal location for the highest daytime temps.

Nighttime Cooling

Around 4PM the earth starts losing more IR energy than it gains in solar energy. Both the air and earth cool by radiating IR energy this is called radiational cooling. The earth radiates at a greater rate so it cools faster.

Just as air near the surface warms by conduction during the day, it cools by conduction at night. By late evening/early morning, air at the surface is cooler than the air above.

Factors Influencing Minimum Temperature

Wind – causes the coldest air near surface to mix with warmer air above. Less chance of an inversion.

• Light wind – colder temps
• Strong wind – warmer temps

Length of night

• Longer nights – more time for radiational cooling.

Moisture and Clouds

• Absorb IR radiation and emit radiation back toward earth causing warming -If air is moist we get dew.
• Condensation – release of latent heat causes warming.

Terrain

Cold air is more dense (heavy) than warm air, so it moves down slopes into valleys. This is called cold air drainage.

The main factors that cause variations in temperature from one place to another

Latitude – determines amount of solar radiation at surface -More near tropics, less toward the poles.
Elevation – already know that T decreases with height in the troposphere.
Land and water distribution – both have different heating properties.

Definition. – specific heat – the amount of energy needed to raise the temperature of one gram of a substance by one degree C.

Water has a higher specific heat than land. It takes more energy to raise its temperature. So in summer, land gets hotter than water. In winter, land gets colder than water.

Land heats and cool 5 times faster than water.

Ocean Currents

• East coasts of continents – warm currents flowing to the north
• West coasts of continents – cold currents flowing to the south
• Coastal cities on the West Coast will likely have lower summertime temps EX. San Francisco – pretty cool (sometimes cold) in the summer due to cool Pacific water.