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  • Tim McGill

Surviving Summer On Chicago's Heat Island

The Heat Is Higher In Urban Areas



Anyone who lives in or around Chicago is familiar with the phrase "cooler by the lake". A phrase that you don't hear is "hotter in city". Maybe it should be a part of our lexicon. It's a phrase that sums up the idea of an urban heat island.


Here's how the Environmental Protection Agency describes an urban heat island:


Heat islands are urbanized areas that experience higher temperatures than outlying areas. Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. Daytime temperatures in urban areas are about 1–7°F higher than temperatures in outlying areas and nighttime temperatures are about 2-5°F higher.


There are a number of factors that can contribute to the heat disparity when comparing a city to its suburbs and more rural areas. For instance a surface's albedo is a measurement of how much sunlight that surface reflects. Darker surfaces absorb more heat while lighter ones are cooler because they reflect more of the sun's ultraviolet rays. The amount of trees, other forms of vegetation and water surface also play a role. They have a net cooling effect because they provide shade and water evaporating from the surface cools the air above it. Last but not least, the dimension of buildings and how they are spaced is also important.


Climate Central considered these factors and more to come up with an index to evaluate the intensity of a city's urban heat island effect. The index helps evaluate how urban heat island intensity varies across cities.

The index score for each city is a temperature representing the potential difference in average temperature for the city compared to its less developed surroundings.



Climate Central explains that the index score for each city is a temperature representing the potential difference in average temperature for the city compared to its less developed surroundings. Chicago's intensity score of 7.2° fell towards the middle of the range that starts as low as less than 5° and goes as high as around 9°. They computed the index for over 150 cities and Chicago, New Orleans, Newark, N.J., New York City, Houston, and San Francisco were among the five highest scoring cities in their study.


Climate Central notes that the impacts of this are serious:


Extreme urban heat is a public health threat. It amplifies air pollution and creates dangerous conditions for people working outside or living in buildings without air conditioning. Discriminatory housing practices like redlining along with other socioeconomic factors mean that communities of color are often in areas with fewer trees and parks and thus are exposed to higher urban heat.

The city and surrounding areas will all be warming up this weekend. It will once again feel more like July. 8 of the last 9 days have been below average and the month is running nearly 3° below average. Today will still be below average but warmer than Friday.


The HRRR Model has highs today in the middle to upper 70s. A wind off the lake will keep it cooler lakeside with highs there in the lower 70s. The average high for today is 85°.



We bounce back into the lower 80s on Sunday but the wind will continue to keep things cooler along the lakefront with middle 70s expected there.


Dew points will be in the lower 60s so it should not be overly muggy this weekend. Dew points are the preferred measurement of moisture for meteorologists. When they climb into the middle to upper 60s it begins to feel very muggy. When they climb to 70° or above it becomes oppressive.



The National Blend Of Models has our highs still in the 70s today. Once the 80s return on Sunday they stick around for the next ten days in a row.



The forecast signal for a warmer than average end of July is still fairly strong. The 6-10 day forecast has outlooked us for above average temperatures overall from July 22nd through July 26th. The longer range 8-14 day forecast also favors us for above average temperatures overall from July 24th through July 30th. Average highs are in the middle 80s for the middle of July.



The even longer range Subseasonal Experiment (SubX) temperature anomaly forecasts keeps us sandwiched between an expected warmer than average plains and western half of the country and a cooler middle and eastern part of the US. A close examination of these forecasts shows we are expected to be right around average overall for the end of July as start to August. The temperature anomaly forecast for the week ending July 30th has us outlooked for temperatures near normal. The forecast for the following week ending on August 6th also has Chicago outlooked for about average temperatures too.


Back To Dry Pattern Again

Even though rain has officially fallen at O'Hare 8 out of the last ten days we are still running -.42" below average for the month. O'Hare is now nearly five inches of rain below average since the start of meteorological spring on March 1st.


The US Drought Monitor released its latest analysis last week and the news was mixed for Illinois. Here is my recap from Friday's post:


The area of Illinois in a moderate drought (level 1 out of 4) has dropped slightly from 7.40% last week to 7.32% this week. The area of the state in a severe drought (level 2 out of 4) has increased slightly from 2.22% a week ago to 2.40% this week. The area experiencing an exceptional drought (level 3 out of 4) remains unchanged at .96". All in all, very little change in our drought status from last week.


The latest GFS model has us entering another dry period. A spotty shower or isolated thunderstorm can't be ruled out today or tomorrow but we will basically be dry through next Thursday.


The GFS model total rainfall forecast through this weekend doesn't offer much hope for drought relief. The highest rainfall forecast during this period is south of the city where a couple tenths of an inch could come down.



The last few runs of the longer range precipitation probability forecasts were discouraging for drought stricken areas in Illinois with forecasts of below average precipitation. The latest run of the models has some slight improvement. We move from below average precipitation to above average for the end of the month. The 6-10 day forecast has the Chicago area favored for about average precipitation overall from July 22nd through July 26th with the highest probabilities just to our west. The longer range 8-14 day forecast also has us favored for near normal precipitation overall from July 24th through July 30th.




Here is my 7 day forecast:


Today: Partly to mostly sunny, spotty shower possible High: 78 (cooler lakeside)


Sunday: Partly to mostly sunny, spotty shower? Low: 67 High: 82 (cooler lakeside)


Monday: Mostly sunny Low: 68 High: 87 (cooler lakeside)


Tuesday: Mostly sunny Low: 69 High: 89 (cooler lakeside)


Wednesday: Mostly sunny Low: 71 High: 84 (cooler lakeside)


Thursday: Partly to mostly sunny, sct. t-storm possible late Low: 68 High: 88


Friday: Partly to mostly sunny, sct. shower/t-storm possible Low: 67 High: 86

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