1. Heat Transfer
8th Grade Weather

2. Thermal Energy and Temperature
All substances are made up of tiny particles that are constantly moving.
The faster they move, the more energy they have.
Temperature is the average amount of energy of motion of each particle of a substance.
Temperature is the measure of how cold or hot a substance is.

3. Thermal Energy
In contrast, the total energy of motion in the particles of a substance is called thermal energy.

4. Measuring Temperature
Air temperature is usually measured with a thermometer.
Contains mercury or colored alcohol.
Thermometers work because liquids expand when they are heated and contract when they are cold.
The liquid in the instrument expands as it is heated and moves up the column.

5. Temperature Scales Measured in units called degrees.
Two scales are used. Celsius and Fahrenheit.
Celsius is used by scientists. The freezing point of pure water is 0. the boiling point is 100.
Weather reports in the United States use Fahrenheit. Freezing point of water is 32, and the boiling point is 212.

6. How Heat is Transferred
Heat is transferred in three ways: radiation, conduction, and convection.

8. Radiation The transfer of energy through space is called radiation
Heat transfer by radiation takes place with no direct contact between the heat source and an object.
Sunlight, warmth from a fire, heater etc…

9. Conduction
Heat transfer within a material or between materials that are touching is called conduction.
A spoon in a pot of soup. The particles near the bottom of the spoon vibrate faster as they are heated, so they bump into other particles and heat them too.
Gradually the entire spoon heats up.
When your hand touches the spoon, conduction transfers heat from the spoon directly to your skin.

10. Convection
Heat can also be transferred by the movement of fluids-liquids and gases.
Convection is heat transfer by the movement of currents within a fluid.
During convection, heated particles of fluid begin to flow. This flow transfers heat from one part of the fluid to another.
Heat transfer by convection is caused by differences of temperature and density within fluid.
Density is a measure of how much mass there is in a volume of a substance.

11. Convection cont…
When a liquid or a gas is heated, the particles move faster and spread apart. As a result, the particles of the heated fluid occupy more space.
The fluid’s density decreases.
When it cools its particles move more slowly and settle together more closely.
As the fluid cools its density increases.

12. Convection Currents
When you heat up soup on a stove, the soup on the bottom of the pot gets hot.
It expands and becomes less dense.
The warm less dense soup moves upward and floats over the cooler denser soup.
At the surface the warm soup cools, becoming denser.
Then gravity pulls this cooler, denser soup back down to the bottom of the pot, where it is heated again.
A confection current is the flow that transfers heat within a fluid.
This continues as long as heat is added. Without heat, convection currents eventually stop.

13. Check your Neighbor! List the three types of heat transfer.
Give an example of each.
Radiation
Conduction
Convection

14. Heating the Troposphere
All three work together to heat the troposphere.
During the day, the sun’s radiation hits the Earth’s surface. The land becomes warmer than the air.
Air near the surface is warmed by both radiation and conduction. Only the first few meters of the troposphere are heated by conduction. Making the air close to the ground warmer than the air a few meters up.

15. Heating the Troposphere
Within the troposphere, heat is transferred mostly by convection.
When the air near the ground is heated, the particles move faster. They bump into each other and move far apart.
The air becomes less dense.
Cooler denser air sinks toward the surface, forcing warmer air to rise.
The upward movement of warm air, and the downward movement of cooler air forms convection currents.
These currents move heat through the troposphere.