Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

Frequently Asked Questions

Dr. Jim Angel, State Climatologist

The most frequently asked question I get is Where can I find data for Chicago for a particular day/month/year? Here are a few answers:

What is the Heat Index?

Those who have experienced summer in Illinois realize that warm, dry air seems more comfortable than warm, moist air. To alert the public to the dangers of exposure to extended periods of heat and the added effects of humidity, the National Weather Service has developed the Heat Index. Values of the heat index can be determined from the Heat Index calculator (NWS) or in the table below.

RH                       Temperature (F)
(%)   90  91  92  93  94  95  96  97  98  99  100  101  102  103  104  105
--------------------------------------------------------------------------
90   119 123 128 132 137 141 146 152 157 163  168  174  180  186  193  199
85   115 119 123 127 132 136 141 145 150 155  161  166  172  178  184  190
80   112 115 119 123 127 131 135 140 144 149  154  159  164  169  175  180
75   109 112 115 119 122 126 130 134 138 143  147  152  156  161  166  171
70   106 109 112 115 118 122 125 129 133 137  141  145  149  154  158  163
65   103 106 108 111 114 117 121 124 127 131  135  139  143  147  151  155
60   100 103 105 108 111 114 116 120 123 126  129  133  136  140  144  148
55    98 100 103 105 107 110 113 115 118 121  124  127  131  134  137  141
50    96  98 100 102 104 107 109 112 114 117  119  122  125  128  131  135
45    94  96  98 100 102 104 106 108 110 113  115  118  120  123  126  129
40    92  94  96  97  99 101 103 105 107 109  111  113  116  118  121  123
35    91  92  94  95  97  98 100 102 104 106  107  109  112  114  116  118
30    89  90  92  93  95  96  98  99 101 102  104  106  108  110  112  114

**Exposure to full sunshine will increase these values by up to 15F

What is the Wind Chill Index?

Anyone who has experienced winter in Illinois knows that your outdoor comfort depends on several factors, only one of them being temperature. Factors such as whether it's sunny or cloudy, windy or calm, damp or dry, can also play an important role. One popular approach in measuring the level of discomfort (and potential danger from frostbite) is the wind chill index.

The original work on wind chill was done by Antarctic explorers Paul Siple and Charles Passel in the winter of 1941. They measured the amount of time it took a pan of water to freeze and found that the rate of heat loss from the container could be determined from the air temperature and wind speed. However, we are more complex than a pan of water and each person responds differently depending on age, size, health, degree of physical activity, etc. As a result, a revised chart was issued in the fall of 2001 (below). In general, the wind chills of the new chart are less severe than the old chart.

Wind Chill Chart

Wind Chill Chart Thumbnail Comparison of the old and new wind chill charts.

More information on the new wind chill

Other facts:

The important thing to remember is that the wind chill index is a measure of the rate of heat loss, and is not a temperature.

A glass of water will not freeze if the air temperature is above freezing and the wind chill is below freezing.

If your car's antifreeze is good down to -30F, wind chills below -30F will not harm your car (only if the air temperature drops below -30F are you in trouble).

What are heating/cooling/growing degree days?

Heating Degree Days (HDD)

Used to estimate the amount of energy required for residential space heating during the cool season. To calculate the HDDs you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or above 65F, then the HDD amount is zero. If the mean temperature is below 65F, then the HDD amount equals 65 minus the mean temperature. For example, if the mean temperature was 55F then the HDD amount equals 10.

Cooling degree days (CDD)

Used to estimate the amount of air conditioning usage during the warm season. To calculate CDDs, you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or below 65F, then the CDD value is zero. If the mean temperature is above 65F, then the CDD amount equals the mean temperature minus 65. For example, if the mean temperature was 75F then the CDD amount equals 10. You can think of cooling degree days as the flip side to heating degree days.

Growing degree days (GDD)

Used to estimate the growth and development of plants and insects during the growing season. The basic concept is that development will only occur if the temperature exceeds some minimum developmental threshold, or base temperature. The base temperatures are determined experimentally and are different for each organism. Here are some common bases and their target.

  • 40F - wheat, barley, rye, oats,flaxseed, lettuce, asparagus
  • 45F - sunflower, potato
  • 50F - sweet corn, corn, sorghum, rice, soybeans, tomato
  • 44F - corn rootworm
  • 48F - alfalfa weevil
  • 50F - black cutworm, European corn borer
  • 52F - green cloverworm

To calculate GDDs, you must first find the mean temperature for the day. This is usually done by taking the high and low temperature for the day, adding them together and dividing by two. If the mean temperature is at or below the base temperature, then the growing degree day value is zero. If the mean temperature is above the base temperature, then the growing degree day amount equals the mean temperature minus the base temperature. For example, if the mean temperature was 75F then the growing degree day amount equals 10, using a base temperature of 65F. You can think of growing degree days as similar to cooling degree days, only the base temperature can be something besides 65F.

Modified growing degree days

Similar to growing degree days with several temperature adjustments. If the high temperature is above 86F, it is reset to 86F. If the low is below 50F, it is reset to 50F. Once the high and/or low temperature has been modified if needed, the average for the day is computed and compared with the base temperature (usually 50F). Modified growing degree days are typically used to monitor the development of corn, the assumption being that development is limited once the temperature exceeds 86F.

What is Normal?

In terms of climate, the word "normal" refers to the 30-year average of a climate parameter (e.g., temperature, precipitation). These averages are updated every 10 years. The current average covers the period 1961-1990. The next one will cover the period 1971-2000 and will be introduced in 2001.

Originally, it was thought that climate was constant with only random fluctuations around the normal climate. If you took enough observations, your confidence in finding the normal climate would increase. This has not worked out for two reasons:

  1. One is that climate is now understood to fluctuate over many time scales. This can cause our uncertainty in the average to increase instead of decrease over time.

  2. The second is that longer records are prone to data inhomogeneities caused by changes in instrumentation, exposure, and method of observation.

Therefore, the concept of normal climate is somewhat slippery and probably the best we can do is periodically update the 30-year average.

Illinois State Water Survey

2204 Griffith Dr
Champaign, IL 61820-7463
217-333-0729
jimangel@illinois.edu

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