Initial Publication Date: August 12, 2008

# Hurricanes and Heat Transfer

For this lab, you and your team will be moving through a series of stations completing hands-on experiments. These experiments will help you understand how heat moves from one part of a system to another.

As you work through the activities, keep in mind that we're not just interested in heat transfer, but how heat transfer affects hurricane formation and intensification. The take-home message is that warmer ocean water evaporates more easily and that means that more heat energy makes its way into the atmosphere. When that water vapor condenses into rain, it releases the heat and this can fuel storms to produce more wind and rain. This is the power that makes hurricanes possible.

At each station, read the procedure and use the materials to perform the experiment or demonstration.

Based on an activity by Dorothy Merritts of Franklin and Marshall College.

#### Materials

• Can of compressed air
• Infrared thermometer
• Graph paper (Acrobat (PDF) 16kB Nov29 07)

#### Procedure

1. Using the digital thermometer, take the initial temperature of the can and record it on the table on the activity sheet. Take all measurements with the laser dot in the center of the can.
2. Keeping the thermometer pointed at the can, allow gas to spray out of the can for 1 minute. Record the temperature reading every 10 seconds.
3. After 1 minute, stop spraying gas from the can. Continue to take temperature readings at 10 second intervals for another 2 minutes.
4. Use the data you have collected to create a graph of temperature versus time.

### Discussion:

Adiabatic is an adjective that describes a process where heat is not transferred to or from a working fluid in this case the gas in the spray can. What happens during this activity is very similar to what takes place as a packet of air moves from low to high altitude and cools. Atmospheric pressure is higher at low altitude than high altitudes, and the volume of a given amount of air is smaller because of the higher pressure. In the same way, gas in a spray can is compressed, and hence at higher pressure. When the can is sprayed, the gas is released and expands. In both cases, the expanding gas does work on the surroundings, pushing against atmospheric pressure. As the gas expands and does work, its internal energy drops, resulting in cooling.

## Stop and Think

1.You've seen that "adiabatic" applies to the first half of the activity when gas is being released from the can. Does this adjective apply to the second half of the activity after you've stopped releasing gas? How do you know?

## Station 2: Convection Cell

#### Materials

• Clear, 500 mL beaker filled with water containing pencil shavings or parsley flakes settled on the bottom
• Bunsen burner and striker
• Hand and eye protection (safety glasses and oven mitt or tongs, etc.)
• Blue and red colored pencils

#### Procedure

1. Set the beaker on the ring stand.
2. Wearing hand and eye protection, use the striker to light the burner.
3. Position the burner so that the flame is near one edge, not centered.
4. Note what happens to the shavings in the water after you put the burner under the beaker. Draw the behavior of the particles on the activity sheet using the colored pencils. Use the red pencil to show where the particles are moving upwards and blue for where they are moving downward.
5. Extinguish the burner and, using hand protection, move the beaker of water to the side to cool. (Be careful not to touch the ring stand as it will be hot.)

## Stop and Think

2. Where are the particles moving up? Down?
3.What does this say about where the water is hotter and colder?

## Station 3: Measuring Dew Point by Evaporation

#### Materials

• 2 identical thermometers (Fahrenheit)
• 1" piece of cotton shoelace
• Piece of cardboard big enough to accommodate the two thermometers
• Tape
• Marker
• Table of Relative Humidity and Dew Point appropriate for your elevation

#### Procedure

1. Attach the two thermometers side by side on the cardboard with tape.
2. Label one thermometer "wet-bulb" and the other "dry-bulb." (This step will not need to be repeated after the first group.)
3. Wet the piece of shoelace thoroughly and slip it over the bulb of the wet-bulb thermometer.
4. Gently wave the assembly back and forth until the temperature reading on the wet-bulb side has stabilized. (Make sure the thermometers are attached securely!)
5. Record the final temperature readings from both thermometers.
6. Disassemble the apparatus in preparation for the next group.
7. Calculate the Dew Point using the table.

### Discussion

In this activity you built a simple piece of equipment called a psychrometer. Also known as a wet-bulb/dry-bulb thermometer, the psychrometer was invented by the German scientist Dr. Adolf Assmann in the late 19th century. It allows us to measure how much moisture is in the air. Evaporating a liquid takes heat energy, so the surface the liquid was on is cooled in the process. (Think about a nice breeze on a hot, sweaty summer day.) This causes the wet-bulb thermometer to register a lower temperature. How much water evaporates from the wet-bulb depends on how moist or dry the air around it is, so we can tie the temperature difference to a particular dew point or relative humidity level.

Record the following measurements
Dry-Bulb Temperature ____________ Wet-Bulb Temperature ____________
Use these two temperatures and the chart at your station to find the current dew point and relative humidity.
Dew Point _____________ Relative Humidity _____________

## Station 4: Measurement of Dew Point by Condensation

#### Materials

• 250 mL beaker of room temperature water
• Bucket of crushed ice
• Thermometer (Fahrenheit)
• Stir-stick or spoon

#### Procedure

1. Put the thermometer into the beaker of water and record the initial temperature.
2. Begin adding crushed ice to the water to slowly lower its temperature.
3. Stir the water gently to make sure that the temperature is even over the whole beaker. Don't stir with the thermometer. It might break.
4. Observe the side of the beaker and record the temperature at which the first signs of moisture condensation occur.

## Stop and Think

4.At Station 3, you calculated the dew point based on using a psychometer. How does the value you obtained at this station compare to that calculated value?
5.What are some potential sources of error you might expect to have between these numbers?

## Station 5: Energy Calculation

#### Materials

• Color print outs of sea surface temperature before (Acrobat (PDF) 531kB Jun22 22) and after (Acrobat (PDF) 532kB Jun22 22) Hurricane Dennis in 2005.
• Ruler
• Calculator

#### Procedure

In Lab 2, you calculated how much energy was released by Hurricane Isabel when moisture condensed out as rain. This time we want to look at the other end of that process, namely how much heat energy the storm absorbs when it sweeps up warm, moist air from the surface of the ocean. To do this, we are going to compare images showing the sea surface temperature before and after Hurricane Dennis in 2005.

1. Use your ruler and the scale bar on the "after" image to estimate the surface area of the water that was cooled. (It will be helpful later on if your answer is in square meters.)
2. The data that were used to create these images was gathered by satellites whose infrared instruments are measuring the temperature within the top 1 mm of the water. If we take 1 mm to be the minimum depth over which the cooling happens, what is the volume of water involved? (Show your work and include all units.)
3. By comparing the two images, you should be able to determine how much the water cooled after the hurricane's passage. Estimate the average temperature in the region in question both before and after the hurricane, then compute the change in temperature. Note that the scale is in °C. The amount of heat energy absorbed by the storm can be found by using this equation: H (in kilojoules) = (VHC) x (volume in m3) x (temperature change in °C) Where VHC is the Volumetric Heat Capacity for water (4.181 x103 kJ/m3 °C). This is the amount of heat in kilojoules gained or lost in changing the temperature of 1 cubic meter of water by 1°C. Use this equation to calculate H. (show your work)

4. Multiply the volume of water by the change in temperature to calculate the amount of heat absorbed by Hurricane Dennis from that portion of the Gulf of Mexico.

## Stop and Think

6.Besides the condensation of rain, how else do hurricanes expend the energy they have picked up?
7.Do you think that we made a valid assumption when we said that the hurricane only interacts with the top 1 mm of the ocean? Why? Bearing this in mind, do you think that the actual amount of energy absorbed by Hurricane Dennis over that stretch of ocean is less than, the same as, or greater than what you just calculated? Justify your answer.

## Station 6: Science Article Review

#### Materials

Copies of these news articles:

#### Procedure

1. Everyone in the group should pick an article to read. Everyone should take a different one unless there are more group members than articles.
2. Spend the first few minutes reading your article and then write a paragraph summary (on your activity sheet) of what the main points of the article were and what you learned from it.
3. When everyone is finished, each person should spend 1 minute telling the rest of the group about the article and fielding any questions their group-mates might have about the material.
4. Keep an eye on the time so that everyone gets a chance to share what they learned!
5. In your own words, write a couple of sentences based on the summary that your group-mates give of their articles.
6. Leave the articles for the other groups to use when you move on to your next lab station.