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:
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
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.
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).
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.
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.
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.
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.
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.
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:
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.
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.