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Notes on Wisconsin Climate Data

There are 169 weather stations in Wisconsin. "First Order" stations record hourly observations and are usually staffed by professional observers. These can be identified by having WSO or FAA in their name. Other stations are called "cooperative stations." These usually are manned by volunteer observers and record daily data only. Figures and letters following a station name generally indicate a rural location and refer to the distance and direction of the station from the nearest US Post Office.

Climate data are presented from one weather station in each county. Be aware that the data from one station may not be completely representative for other parts of a county due to local variations in elevation, topography, land use and proximity to open water. For counties without a weather station, we present data from a nearby station in an adjacent county. For these, the county of the reporting station is indicated in parentheses after the station name.

Available data is incomplete for a few stations, and we have had to fill out the climate table with data from another, nearby station. When this has been necessary it is indicated in the footnote directly beneath the climate table.

In the footnote to every county climate table we indicate the reporting station's name, latitude (angular distance north or south from the equator), longitude (angular distance east or west from the Prime Meridian which passes through Greenwich, England) and elevation in feet above mean sea level (MSL).

Below are explanations and citations for the data presented in the county climate tables. Data are sometimes referred to as "normals" because they are averages for a 30-year "normals period." By international agreement, the arithmetic mean (the average) of a climatological element computed for a 30-year period (the most recent three whole decades) is considered to constitute a climate "normal." The current normals period is 1961 to 1990. Data for the 1971-2000 normals period will be available in 2001.

Monthly normals for February are based on a 28-day month.

What does "median" mean?
Temperatures, degree days and inches of precipitation can be added up and averaged, but dates can't be added up or averaged. Instead, the median date is used. In a data set, if you sort all the data in order and then go to the middle value, you have the median. In other words, half of all the values in the data set are lower than the median, and half of the values are higher than the median. For dates, half of all the dates are earlier than the median date, and half of the dates are later.

Last Spring Frost and First Fall Frost

The median date of the last spring frost is the day by which half of all years in the data period have reached a minimum temperature of 32° F for the last time in the spring. The median date of first fall frost is the day by which half of all years in the data period have reached a minimum temperature of 32° F for the first time in the fall.

The spring 10 percentile date indicates that the last 32° temperature in the spring was recorded on this day or later in 10 percent of the years in the data period. The fall 10 percentile date indicates that the first 32° temperature in the fall was recorded on this day or earlier in 10 percent of the years in the data period. These dates complement the median last and first frost dates. Taken together, they indicate the "typical" beginnings and endings of the growing season for their locales.

Air temperatures are measured about 1.5 m (about 5 feet) off the ground, so when the air temperature is recorded as 32° F, it is even colder at the surface and is damaging to most annual plants, rather than just sensitive ones. Gardeners know that different plants have different sensitivites to cold. Some plants will survive a light frost while others quickly succumb.

Source: US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center reports available from the Midwestern Regional Climate Center, Champaign Illinois. Data is for the normals period 1961-1999.

[Also see our statewide maps showing zones of Typical Dates of Last Spring Frost and Typical Dates of First Fall Frost.]

Growing Season

The growing season is the number of continuous days during which the temperature remains above the 32° F base temperature for a frost. "Median" means that, for the data period, half the years had longer growing seasons and half the years had shorter growing seasons that the number of days indicated. The range indicates the lengths of growing seasons in the years of the data period, after both the shortest 10 percent and the longest10 percent have been excluded from the sample.

Source: US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center reports available from the Midwestern Regional Climate Center, Champaign Illinois. Data is for the normals period 1961-1999.

[For perennials, landscape plantings and fruit trees, also see our map of USDA Plant Hardiness Zones for Wisconsin.]

Average Daily High and Low Temperatures

More correctly referred to as "mean daily maximum" and "mean daily minimum" temperatures, these figures are computed from weather station reports for the 30-year period 1961-1990 and are reported as degrees Farenheit (F°).

Source: Climatography of the United States No. 81. Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days 1961-90: Wisconsin. Published by the US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center (1962).

Some Equivalents in Farenheit and Celsius

   
 -40  -40     59  15  
 -22  -30     68  20  
 -4  -20     77  25  
 14  -10     86  30  
 32  0     95  35  
 41  5     104  40  
 50  10     212  100  
NOTES:

  • To convert Farenheit to Celsius, subtract 32 from the temperature in F° and divide by 1.8.

  • To convert Celsius to Farenheit, multiply the temperature in C° by 1.8 and add 32.

  • Water boils at 212° F at sea level. For every 550 feet above sea level, the boiling point of water is lowered by about 1° F. Thus, in most of Wisconsin the actual boiling point of water is in the range of 210° to 211°.

Growing Degree Days

Growing degree days (GDD) are used as an indication of the atmospheric heat-based development potential for plants and insects. The basic concept is that development will only occur if the temperature exceeds some minimum developmental threshold (base) temperature. Above that base, higher temperatures make for more plant growth and more insect activity (up to a practical limit, or "ceiling"). Base and ceiling temperatures are different for different organisms, and are determined experimentally. For instance:

Reported base temperatures for different crops

Base TemperatureCrops
40° Fwheat, barley, rye, oats, flaxseed, lettuce, aparagus
45° Fsunflower, potato
50° Fsweet corn, corn, sorghum, rice, soybeans, tomato

Reported base temperatures for different insects

Base TemperatureInsects
44° FCorn Rootworm
48° FAlfalfa Weevil
50° FBlack cutworm, European Corn Borer
52° FGreen Cloverworm

To calculate growing degree days (GDD), the mean temperature for the day is subtracted from the base. The mean temperature is obtained by adding the high and low temperatures for the day and dividing by two. In our tables, we show "modified" growing degree days (base 50° F, ceiling 86° F) derived from station data. The data is modified in that, if the day's low temperature is less than 50° F it is reset to 50° F (to avoid negative growing degree days), and if the high is greater than 86° F it is reset to 86° F (indicating no growth benefit from temperatures above 86°).

Michael Palecki of the Midwest Regional Climate Center explains, "When the GDDs are accumulated over a growing season, you can compare the running totals with previous years to see if a normal amount of heat energy has been made available to a growing crop. If GDDs are running behind normal, this usually means that plant development is delayed. The total GDDs received by the end of the growing season is often related to crop yield, assuming adequate moisture supplies are available."

Lyle Anderson of the Wisconsin State Climatology Office adds that "modified growing degree days are often called Corn Growing Degree Days, because corn is such an important crop and is especially vulnerable to frost." In addition, corn's development is limited when the temperature exceeds 86° F.

In our tables you will see that even some winter months provide growing degree days, but the proximity of killing frosts still limits gardening and crop development to the summer growing season.

US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center reports available from the Midwestern Regional Climate Center, Champaign Illinois. Data is for the normals period 1961-1999.

Heating Degree Days

Heating degree days are a useful index of heating demand and fuel requirements. When the daily mean temperature is below 65° F, most homes and buildings require heating to maintain in inside temperature of 70°. The daily mean temperature is obtained by adding the high and low temperatures for the day and dividing by two. Each degree of mean temperature below 65 is counted as one heating degree day.

For each additional heating degree day, more fuel is needed to maintain a comfortable 70° indoor temperature. Of course, if you don't require 70° indoor to be comfortable, you can conserve one heating degree day of fuel for each degree below 70° that you set your thermostat.

Gas, electric and fuel oil companies use heating degree days to anticipate demand for energy consumption for heating. The amount of fuel required to maintain a given temperature, and consequently the cost of fuel, is directly proportional to the heating degree days (assuming the same cost per unit of fuel). Some energy companies report heating degree days for the billing period on consumer bills.

NOTE: Heating degree days and cooling degree days do not cancel each other out! Some months require heating on some days and cooling on other days. Both require energy consumption.

Source: Climatography of the United States No. 81. Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days 1961-90: Wisconsin. Published by the US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center (1962).

Cooling Degree Days

Cooling degree days are a useful index of air-conditioning (cooling) requirements during warm months.

The need for cooling generally begins to be felt when the daily high temperature climbs to 80° or higher. The base of 65° (for consistency with heating degree days, see above) is subtracted from the day's mean temperature. If the daily mean temperature is 65° or lower, the cooling degree days total is zero for that day.

A higher number of cooling degree days indicates a need for more energy to maintain indoor temperatures at a comfortable level. However, this relationship is less precise than that for heating degree days due to the influence of other factors — including relative humidity — which appear to influence cooling demand more significantly than they do heating demand.

NOTE: Heating degree days and cooling degree days do not cancel each other out! Some months require heating on some days and cooling on other days. Both require energy consumption.

Source: Climatography of the United States No. 81. Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days 1961-90: Wisconsin. Published by the US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center (1962).

Precipitation

Precipitation is reported in inches ("). Precipitation includes rainfall and the liquid water equivalent of frozen precipitation (snow, sleet, hail). Depth of snowfall is reported separately, but the liquid water equivalent of the snowfall is included in these precipitation figures.

Source: Climatography of the United States No. 81. Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days 1961-90: Wisconsin. Published by the US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center (1962).

Snowfall

Mean monthly snowfall for the normals period 1961-1999, reported in inches (").

Source: US Department of Commerce, National Oceanic and Atmospheric Administration, National Climatic Data Center reports available from the Midwestern Regional Climate Center, Champaign Illinois. Data is for the normals period 1961-1999.

Special thanks to Peter Palecki and Traci Westfall of the Midwestern Regional Climate Center, and to Lyle Anderson of the Wisconsin State Climatology Office, for assistance obtaining data and for their helpful explanations of the data elements. Any remaining errors of data or interpretation — in spite of their best efforts — are those of Wisconsin Online Inc. Please notify David Falck at Wisconsin Online of any errors or to suggest improvements in content or presentation.


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