Convection is the transfer of heat through the movement of liquids and gases.
Why do I care? Convection is sometimes responsible for the formation of thunderstorms, and these thunderstorms during the summer can provide a substantial amount of rain for growers. Convection also contributes to the wind chill effect, which can endanger people working outside on cold, windy days.
I should already be familiar with: Temperature
Convection is the transfer of heat from a warmer region to a cooler one by moving warm liquid or gas from the heated area to the unheated area. In a boiling pot of water, hot water in the bottom of the pot rises to the surface, leading to bubbles of heated water and sometimes steam visible on the surface. In meteorology, convection is often associated with rising air and clouds, and at times, thunderstorms. Air that is rising cools as it reaches lower pressures, and may reach the point where water vapor in the air condenses and forms clouds. These rising columns of air are called "thermals". Fair weather cumulus clouds often form on the top of these rising columns of air. Thunderstorms can sometimes form where there is a lot of water vapor and heating. You may have heard the TV meteorologist mention afternoon convection leading to thunderstorms on hot summer afternoons.
Convection is not just limited to meteorology. In fact, many of our daily activities involve or observe convection. For example, if you drink coffee (or any hot beverage), you may notice steam rising up from your hot beverage. In this case, we can visibly observe convection as the steam is transferring heat into the air. Another example would be a heater during the winter. The heater emits warm air which will rise to the top of the room (Figure B). The warm air will eventually cool and sink to the bottom of the room before getting pulled back into the heater. Over time, this process will warm and mix the air as long as the heater remains on.
How does this relate to agriculture?
Convection is responsible for many of the naturally occurring processes that we see in nature every day. For example, convection can affect fog layers that are often seen on cool fall mornings, when the air close to the surface is warmer than the atmospheric air higher up. Smoke rising from a fire can also show the conduction currents present as the heated air goes up. Also, old flue-cured tobacco barns worked on the convection principle, where the heated air from the botton rose through the air to dry the tobacco that was hanging (Figure C). Convection of air is also a factor in chilling warm bodies in winter when frigid air is brought into contact with skin and the heat is convected away, potentially leading to hypothermia and frostbite when wind chills are extreme.
Links to National Science Education Standards:
5th grade science: 5.P.3.1 : Explain the effects of the transfer of heat (either by direct contact or at a distance) that occurs between objects at different temperatures. (conduction, convection, or radiation).
7th grade science: 7.E.1.5 : Explain the influence of convection, global winds, and the jet stream on weather and climatic conditions.
Earth Science: EEn.1.1.4 : Explain how incoming solar energy makes life possible on Earth. (extension)
Physical Science: PSc.3.1.1 : Explain thermal energy and its transfer.
Activities to accompany the information above:
Activity: Convection - Modeling How Air Moves (Link to original activity).
Teacher Set-up Instructions
Description: This activity shows how currents move through water using food coloring and hot and cold water. This simulates how air can act as a fluid. Students will fully understand the process of convection and how heat is transferred through this process.
Activity: Convection Discrepant Event
Description: This activity poses a demonstration of convection and potentially water currents that should be conducted in front of the students for the students to think about what is happening during this activity. At the beginning the activity is not fully explained to students, but instead placed in front of them to create thought. This activity could be an opening into heat transfer through convection.
Activity: Energy Webquest
Description: In this activity, students review several websites that walk them through the layers of the atmosphere and energy transfer in the atmosphere.