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

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:

• Check the online daily climate data at the Chicago NWS office selecting the Preliminary Climatology Data under product and then your location and date.
• Monthly and annual data for Chicago can be found in our monthly climate database.
• Normals and extremes for Chicago can be found in our station summaries.
• A useful site for free hourly data is a private site called Weather Underground - here is an example of their hourly data (notice that you can change the date at the top of the page or in the link itself): http://www.wunderground.com/history/airport/KORD/2018/3/26/DailyHistory.html
• Additional data can be found at the National Climatic Data Center.
• If all else fails, send me an email at jimangel@illinois.edu

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 15°F

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.

Comparison of the old and new wind chill charts.

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 -30°F, wind chills below -30°F will not harm your car (only if the air temperature drops below -30°F are you in trouble).