This report describes the historical weather record at the O'Hare International Airport (Chicago, Illinois, United States) during 1979. This station has records back to December 1947.
Chicago, Illinois has a humid continental climate with hot summers and no dry season. The area within 25 mi of this station is covered by built-up areas (37%), grasslands (36%), oceans and seas (22%), forests (3%), and croplands (3%)
Daylight saving time (DST) was observed at Chicago, Illinois during 1979. There were two time changes during 1979:
1979 was not a leap year, so it has 365 days and no February 29th. The first leap year before 1979 was 1976 and the first after was 1980.
The summer and winter solstices and the spring and fall equinoxes mark the passing of the seasons. They fall on nearly the same day each year, with differences of a day or two depending on the year. In 1979 they occurred on:
|Spring Equinox||Wednesday, 21 March 1979.|
|Summer Solstice||Thursday, 21 June 1979.|
|Fall Equinox||Sunday, 23 September 1979.|
|Winter Solstice||Saturday, 22 December 1979.|
The hottest day of 1979 was July 23, with a high temperature of 94°F. For reference, on that day the average high temperature is 84°F and the high temperature exceeds 92°F only one day in ten. The hottest month of 1979 was July with an average daily high temperature of 84°F.
Relative to the average, the hottest day was December 23. The high temperature that day was 62°F, compared to the average of 33°F, a difference of 29°F. In relative terms the warmest month was December, with an average high temperature of 42°F, compared to an typical value of 35°F.
The longest warm spell was from December 19 to December 31, constituting 13 consecutive days with warmer than average high temperatures. The month of December had the largest fraction of warmer than average days with 77% days with higher than average high temperatures.
The coldest day of 1979 was January 11, with a low temperature of -16°F. For reference, on that day the average low temperature is 16°F and the low temperature drops below -2°F only one day in ten. The coldest month of 1979 was January with an average daily low temperature of 5°F.
Relative to the average, the coldest day was January 15. The low temperature that day was -16°F, compared to the average of 16°F, a difference of 32°F. In relative terms the coldest month was January, with an average low temperature of 5°F, compared to an typical value of 16°F.
The longest cold spell was from January 29 to February 23, constituting 26 consecutive days with cooler than average low temperatures. The month of February had the largest fraction of cooler than average days with 96% days with lower than average low temperatures.
The longest freezing spell was from January 29 to February 20, constituting 23 consecutive days with temperatures strictly below freezing.
The clearest month of 1979 was September, with 60% of days being more clear than cloudy. The longest spell of clear weather was from September 15 to September 20, constituting 6 consecutive days that were clearer than they were cloudy.
The cloudiest month of 1979 was April, with 73% of days being more cloudy than clear. The longest spell of cloudy weather was from April 19 to May 1, constituting 13 consecutive days that were cloudier than they were clear.
This station reports both the quantity of liquid precipitation and categorical observations of precipitation (e.g., moderate rain, or heavy snow). Both are subject to erroneous reports, but the former is particularly prone to false reports, especially ones indicating an excessive quantity of precipitation. Please bear this in mind when reading the extrema reported in this section.
The day with the largest quantity of precipitation was August 29. That day saw 2.236" of liquid (or liquid equivalent) precipitation, compared to a median value of 0.189". The month with the most precipitation was August, with 7.031", compared to a median value of 3.783".
As determined by quantitative measurements, the longest dry spell was from August 30 to September 24, constituting 26 consecutive days with no measured precipitation. The month with the largest fraction of dry days was September, with 97% of days reporting no measured precipitation at all.
The month with the largest fraction of days with at least some measured precipitation was March, with 52% of days reporting some measured precipitation.
This station reports when significant weather events (including precipitation) are visually observed at or near the station. Such events do not always correspond to measured quantities of liquid equivalent precipitation, such as when the event is near by not at the station, or in the case of solid precipitation that does not melt in the collection basin.
The day in 1979 with the most precipitation observations was January 13. There were 24 hourly weather reports that day (out of a maximum of 24) in which some form of precipitation was observated at or near the station. The month with the most precipitation observations was January, with 203 hourly present weather reports involving some form of precipitation.
As determined by the present weather reports, the longest dry spell was from September 14 to September 30, constituting 17 consecutive days with no observed precipitation. The month with the largest fraction of days without observed precipitation was September, with 90% of days reporting no observed precipitation at all.
The month with the largest fraction of days with at least some observed precipitation was November, with 73% of days reporting some observed precipitation.
In this section we consider only those weather reports that indicate liquid precipitation. For the purposes of this analysis, we include thunderstorms even though some thunderstorms are not accompanied by liquid precipitation.
The month of 1979 with the largest number of those reports was April, with a total of 122 reports. The day with the largest number of those reports was December 24, with a total of 22 reports.
This station reports both when snow is observed to be falling and the measured depth of the snow on the ground. Both are subject to erroneous reports, but the latter is significantly less reliable. Please bear this in mind when reading this section.
In this section we consider hourly weather reports that contain an observation of falling snow. These reports do not necessarily correspond to accumulation.
The first reported snow fall in 1979 was on November 3; the last was on April 28. The month of 1979 with the largest number of those reports was January, with a total of 183 reports. The day with the largest number of those reports was January 13, with a total of 24 reports.
Snow depth on the ground is an optional and inconsistently reported part of standard weather reports. It is rarely reported more often than every six hours, it is often skipped, it is often reported erroneously, and a snow depth of zero is normally not distinguished from a missing report. These issues (particularly the last one) make it hard to collect statistics on snow depth with any confidence. To overcome this issue, we base our statistics on only those reports with present non-zero measurements of snow depth. Reports that fail to mention snow that is present, and reports that do not report snow depth because there is no snow on the ground are excluded because they cannot be distinguished from one another.
The first reported accumulation in 1979 was on November 27. The last day of the snow season with snow reported on the ground was March 6. The day with the deepest snow depth was January 14, with an average snow depth of 27.3" over the course of the day. The longest stretch of time during which there was always snow on the ground was from January 1 to February 14 (45 consecutive days).
Humidity is an important factor in determining how weather conditions feel to a person experiencing them. Hot and humid days feel even hotter than hot and dry days because the high level of water content in humid air discourages the evaporation of sweat from a person's skin.
When reading the graph below, keep in mind that the hottest part of the day tends to be the least humid, so the daily low (brown) traces are more relevant for understanding daytime comfort than the daily high (blue) traces, which typically occur during the night. Applying that observation, the least humid month of 1979 was September with an average daily low humidity of 43%, and the most humid month was March with an average daily low humidity of 65%.
But it is important to keep in mind that humidity does not tell the whole picture and the dew point is often a better measure of how comfortable a person will find a given set of weather conditions. Please see the next section for continued discussion of this point.
Dew point is the temperature below which water vapor will condense into liquid water. It is therefore also related to the rate of evaporation of liquid water. Since the evaporation of sweat is an important cooling mechanism for the human body, the dew point is an important measurement for understanding how dry, comfortable, or humid a given set of weather conditions will feel.
Generally speaking, dew points below 50°F will feel a bit dry to some people, but comfortable to people accustomed to dry conditions; dew points from 50°F to 68°F are fairly comfortable to most people, and dew points above 68°F are increasingly uncomfortable, becoming oppressive around 77°F.
To take some examples, and basing our categorization on the daily high dew point in 1979, January had 31 dry days, no comfortable days, and no humid days; April had 24 dry days, 6 comfortable days, and no humid days; July had no dry days, 15 comfortable days, and 16 humid days; and October had 19 dry days, 12 comfortable days, and no humid days.
The highest sustained wind speed was 46 mph, occurring on April 5; the highest daily mean wind speed was 26 mph (February 25); and the highest wind gust speed was 67 mph (April 5).
The windiest month was March, with an average wind speed of 13 mph. The least windy month was July, with an average wind speed of 9 mph.
Visibility is the maximum distance at which a given reference object or light can be clearly discerned. In the United States, visibilities that are greater than or equal to 10 miles are typically reported as 10 miles.
The day of 1979 with the lowest average visibility was January 13, with an average visibility of 0.7 mi. The month with the lowest average visibility was March, with an average visibility of 7.8 mi. With an average visibility of 12.0 mi, the month of September had the highest average visibility.
The cloud ceiling is the altitude of the lowest layer of clouds that are at categorized as broken (mostly cloudy) or overcast (cloudy). If no such cloud layer exists then the ceiling is unlimited and no value is reported.
The day of 1979 with the lowest average cloud ceiling was December 28, with an average cloud ceiling of 271'. The month with the lowest average cloud ceiling was March, with an average cloud ceiling of 4414'. The month of June has the highest average cloud ceiling, with an average cloud ceiling of 12662'.