The coastal areas of the Great Lakes seem somewhat magical – rolling hills leading to the edge of the lakes, temperate breezes rolling off the water, and fruit orchards, green fields and vineyards dotting the surrounding land. It’s not just your imagination: there is something special happening there, and there’s a scientific reason as to why.

One of the principles of water is that it absorbs and releases heat easily. When there is a basin of water, like the Great Lakes, the ability to hold onto and release heat can have a significant impact on the surrounding coastal areas. Those coastal areas throughout the Great Lakes are often referred to as “The Fruit Belt.” This lesson and accompanying activities explore water’s ability to absorb and release heat, the affect the Great Lakes can have on the surrounding land, and how the relationship between those two influence the economy and culture of Great Lakes coastal areas.

Grade Levels:

  •     National Science Education Standards: 5th-8th grade
  •     Michigan Grade Level Content Expectations: 5th-7th grade

Performance Expectations:

  • MS-ESS2-5 Earth’s Systems: Collect data to provide evidence for how the motions and complex interactions of air masses results in changes in weather conditions.

For alignment, see: Microclimates NGSS Summary

Lesson Objectives

  • Describe the Great Lakes as a heat source and a heat sink.
  • Develop a hypothesis about differences between coastal and inland temperatures in the Great Lakes region.
  • Develop a hypothesis about the relationship between the Great Lakes and fruit growing in the region.
  • Graph coastal and inland temperatures.
  • Analyze data in graphs and tables to support or reject hypotheses.


How do the Great Lakes influence weather and climate?

They influence the climate and weather of the surrounding land in several ways. In this lesson we focus on how the Great Lakes act as a heat source and a heat sink. What does that mean?

  • Heat sink means a substance is absorbing more heat than it is giving off.
  • Heat source means a substance is radiating — or giving off — more heat than it is absorbing.

Figure 1. Water as a heat source and heat sink

Have you ever arrived at the beach to find the air surprisingly cool? You may have experienced the impact of the Great Lakes on air temperatures. During the summer, air temperatures are usually several degrees cooler at the edge of a lake as compared to a few hundred feet inland. That is because water keeps its energy longer than most substances as a result of its high specific heat. (Specific heat refers to the heat required to raise the temperature of a unit by one degree.)

The specific heat of water is 1; most other substances have a specific heat of < 1. Because of its high specific heat, water has a great capacity for storing heat energy. Water absorbs heat slowly and can absorb more energy than most other substances. Other substances act as heat sources and sinks too, but most cannot hold onto heat energy like water does and quickly release it to the atmosphere.

For example, swimming in Lake Superior in May is a very different experience than swimming in Lake Superior in August. Though the air temperature may be the same, the water is much warmer at the end of the summer, making it more hospitable for swimmers.

The Fruit Belt Effect

Because the Great Lakes act as a heat source during the winter, coastal air temperatures are more moderate than the air temperature in inland areas. The Lakes act to smooth out the seasonal changes along the coastal areas, and fewer temperature extremes along the coasts provide ideal conditions for a very profitable fruit industry. Delicate, early-blooming fruits, typically only grown in the south, can thrive in a narrow band of land (30 miles/24 kilometers wide) along the Great Lakes shores. The primary fruit-growing areas in the Great Lakes include the east shore of Lake Michigan, as well as the southeast shores of Lake Erie and Lake Ontario.

 Climate Farming mapClimate Frost Free map

See: Figure 2. Great Lakes Basin, Farming Types and Figure 3. Great Lakes Basin, Frost Free Days

If you can go to your neighborhood farmer’s market and find locally grown peaches, apples, pears, plums, grapes and berries, you probably live near one of these few special areas found in Michigan, Ohio, New York or Pennsylvania.

Here’s how the heat sink and heat source properties of the Great Lakes work throughout the year as well as how they influence the fruit belt areas:

  • Winter: There is less radiation from the sun, generally making temperatures cooler. That means the lakes are warmer than the air. However, the lakes become heat sources, giving up their energy to the atmosphere throughout the winter. As air passes over the lakes, it warms up and carries the warmth for about 30 miles (48 km). Warmer air keeps the fruit trees and plants from being frozen and killed by below-zero temperatures.
  • Spring: The water stays cool longer than the air and land. Air passing over the lake cools off and stays cool around the fruit area. This cooling slows the development of fruit blossoms. The trees and plants blossom later when there is less chance of damage by frost.
  • Summer: The temperature of Great Lakes water tends to increase or becomes warmer during the summer. The lakes are storing up extra solar energy from the atmosphere, and acting as a heat sink throughout the summer.
  • Late summer and early fall: The warmed water of the lake again warms the air, protecting the mature fruit from damage by early frosts.

Figure 4. Great Lakes Climate and Weather (source: U.S. EPA, Great Lakes Atlas)

Figure 5. Acres in Orchards, Great Lakes watershed (source: Michigan Geographic Alliance and the Science/Mathematics/Technology Center, Central Michigan University)


  • Heat Source and Sink
    Summary: Students investigate the Great Lakes as a heat source and a heat sink.
    Time: One 50-minute class
  • Growing Fruit in the Great Lakes
    Summary: Students investigate how the properties of the Great Lakes support fruit growing regions.
    Time: One 50-minute class


Lesson Sources

Great Lakes Climate and Water Movement. Earth Systems – Education Activities for Great Lakes Schools (ES-EAGLS). 1996. Series EP-085. Ohio Sea Grant. Ohio State University, Columbus, OH 43212. 33-45. Authors: Fortner, RW, Miller, H, Sheaffer, AL

Greatest of the Great Lakes (GOGL): A Medley of Model Lessons. 2007. COSEE Great Lakes, Illinois-Indiana Sea Grant, University of Illinois, Champaign, IL 61820 Authors: Goettel, R, Hallesy, T, Murphy, J, White, S, Fortner, R, Stewart, S, Munson, B, Domske, H, Lubner, J, Danielski, A

The Climate of the Great Lakes Basin. 1972. Climatological Studies. Number 20. Environment Canada, Atmospheric Environment. 551-582. Authors: Phillips, DW and McCulloch, J.A. W.

Data Sources

Great Lakes CoastWatch Node. 2008 NOAAPORT Observations. Unpublished data. Great Lakes Environmental Research Laboratory, Ann Arbor, MI.