Freeze-Thaw Cycles

Summary

• Freeze-that cycles (FTCs) have important impacts for a variety of economic sectors across the region, including agriculture, transportation, and infrastructure.
• FTCs have decreased over time, with the largest region-wide decreases occurring between 2000 and 2020.
• FTCs are likely to continue decreasing in the future, though the amount varies by location.
• The variability of freeze-that cycles shows large fluctuations station to station.

Definition of Freeze-Thaw Cycles

A freeze-thaw cycle (FTC) occurs when air temperature drops low enough to freeze water (32°F), then increases enough for it to thaw again. FTCs usually occur most frequently in the wintertime, though have the potential to occur at any time of year. Typically, FTCs are defined as the total number of times temperature readings crossed the 32°F (or freezing) threshold during a given time period, divided by two. However, other definitions of FTCs exist, often depending on the sector or geographic location they’re used in. Although definitions may vary within the field, the following are the most commonly seen: 

• An annual FTC with freezing in winter and thawing during any particular summer ¹
• A FTC of several years’ duration 
• FTCs which occur on a less-than-daily basis 
• When the daily maximum temperature exceeds the 32°F threshold and the daily minimum temperature is less than the 32°F threshold ²

For GLISA’s purposes, we have defined FTCs as the total number of temperature readings that cross the freezing threshold (32°F) within a calendar year, divided by 2:

FTC = N/2 

where, in this case, N is the annual number of 32°F threshold crossings. FTCs can have important implications for various industries such as agriculture, infrastructure, and tourism, and the calculation is necessary in order to assess how the frequency of current freezing conditions affect these sectors. ³

Impacts on Various Sectors

Water expands when it freezes, so the freezing, thawing, and re-freezing of water over time can cause significant damage to roadways (e.g., the formation of potholes), sidewalks, and other outdoor infrastructure. The report written by the Wisconsin Initiative on Climate Change Impacts (WICCI) on surface transportation and water transport suggested that FTC projections could predict changes in the lifespan of concrete. ⁴

FTCs contribute to additional runoff, erosion, weathering, and plant establishment. ⁵ The Agriculture Sector of the Midwest Technical Input Report for the Third National Climate Assessment concluded that though temperatures are rising, the seasonal freeze free period (FFP) has only lengthened slightly. ⁶ Thus early buds resulting from warmer temperatures could increase the risk of crop destruction due to exposure to relatively normal freezing conditions later in the season. 

A previous GLISA Small Grants project details the possibility of such an impact regularly occurring in the Michigan Tart Cherry Industry, and possible mitigation efforts. This report was motivated by the occurrence of two devastating weather events the industry experienced in 2002 and 2012 that drastically reduced crop yields, mainly due to early spring temperatures that accelerated the start of the tart cherry season, followed by freezing conditions typical for that time of year. As a result of this early spring warm up followed by this typical FTC, nearly the entire harvest of tart cherries was lost as the early buds froze and died. These events are crippling to the industry, as Michigan supplies 70-75% of the nation’s tart cherries. This highlights the need for improved protection against both expected and unexpected FTC events after the start to the growing season, and methods to effectively manage increasingly variable growing conditions. 

The Responding to Climate Change in New York State Synthesis Report calls for new research to integrate weather forecasts into early warning systems for hard freeze and spring frost events, in order to link those systems to susceptibility of crops. ⁷ The Recreation and Tourism Sector of the Midwest Technical Input Report for the Third National Climate Assessment provides examples of the impacts that lengthened spring or fall seasons on Outdoor Recreation and Tourism (ORT), with an increased need for more staffing and possible restructuring of commercial enterprises to support year round service ⁸

GLISA’s Methodology

In GLISA’s FTC calculations, we use raw data from the Global Historical Climatology Network Daily (GHCN-D) observations to create annual FTC values. We aggregate minimum and maximum daily temperature data to represent the diurnal cycle, or daily temperature fluctuation, that is consistent with what is experienced in the area. We apply a requirement that no more than 5% of data in a given year are missing, and if more than 5% are missing, we exclude that year from our analysis.

By comparing the minimum and maximum temperatures for each day to the freezing threshold, we calculated the number of FTCs for each year. However, measurements within ±1°F of freezing are subtracted from our annual FTC count. This was done intentionally to ensure that a given surface did indeed freeze and thaw, as sometimes a 32°F surface may not produce a frozen or thawed surface. ⁹

The number of freeze-thaw crossings is divided by two, yielding the annual number of FTC events for that year. We repeated this analysis for each year from 1951-2020, and additional calculations provided statistics for certain periods like the 1951-1980 Climate Normal.  The results of the analysis provides a picture of the trends found in the Great Lakes region.

Observed and Projected Changes in Freeze-Thaw Cycles

References