Monthly outlook for Markham, IL: average high 79°, average low 65.4°. That is 0.5° below the climate normal for August. Precipitation is expected on 6 days (about 2.6 inches), against a normal of 6 days and 1.9 inches. The warmest day looks like 11 August (86.5°), the coolest 23 August (55.3°).
79°/65.4°
Average high / low
−0.5°
Below normal
6 / 6
Wet days / normal
2.6 in
Precipitation this month
Ten-day blocks vs climate normal
Period
Forecast high
Normal
Difference
8 August – 17 August
79.3°
80°
-0.4°
18 August – 27 August
78.4°
79.9°
-0.8°
28 August – 6 September
at normal
78.5°
—
August records over 66 years
98.1°24 August 2023All-time high
49.1°29 August 1965All-time low
Climate normals and records are computed from the nearest reference point — Blue Island (4 mi).
Sun, 0983° / 66°1.67
83 °
Markham, Sunday, 9 August
Heavy rain. High 83°, low 66°.
Pressure29.91 inHg
Wind7.2 mph SW
Humidity77%
Visibility6.2 mi
Dew point74°
Mon, 1083° / 73°0.09
83 °
Markham, Monday, 10 August
Rain. High 83°, low 73°.
Pressure29.91 inHg
Wind5.3 mph SE
Humidity70%
Visibility6.2 mi
Dew point71°
Tue, 1186° / 73°0.18
86 °
Markham, Tuesday, 11 August
Light rain. High 86°, low 73°.
Pressure29.77 inHg
Wind15.1 mph SW
Humidity72%
Visibility6.2 mi
Dew point72°
Wed, 1281° / 70°0.01
81 °
Markham, Wednesday, 12 August
Light rain. High 81°, low 70°.
Pressure29.83 inHg
Wind5.9 mph NE
Humidity69%
Visibility6.2 mi
Dew point66°
Thu, 1372° / 64°0.03
72 °
Markham, Thursday, 13 August
Light rain. High 72°, low 64°.
Pressure29.94 inHg
Wind10.9 mph NE
Humidity66%
Visibility—
Dew point60°
Fri, 1474° / 61°0
74 °
Markham, Friday, 14 August
Clear. High 74°, low 61°.
Pressure30.18 inHg
Wind10.5 mph NE
Humidity47%
Visibility—
Dew point51°
Sat, 1576° / 58°0
76 °
Markham, Saturday, 15 August
Overcast. High 76°, low 58°.
Pressure30.33 inHg
Wind6 mph S
Humidity48%
Visibility—
Dew point52°
Sun, 1678° / 59°0
78 °
Markham, Sunday, 16 August
Mostly clear. High 78°, low 59°.
Pressure30.21 inHg
Wind9.6 mph S
Humidity54%
Visibility—
Dew point57°
Mon, 1775° / 68°0.11
75 °
Markham, Monday, 17 August
Light rain. High 75°, low 68°.
Pressure30.00 inHg
Wind6.2 mph S
Humidity90%
Visibility—
Dew point68°
Tue, 1883° / 69°0
83 °
Markham, Tuesday, 18 August
Partly cloudy. High 83°, low 69°.
Pressure30.00 inHg
Wind8.6 mph NE
Humidity54%
Visibility—
Dew point64°
Wed, 1984° / 66°0.39
84 °
Markham, Wednesday, 19 August
Rain. High 84°, low 66°.
Pressure29.91 inHg
Wind8.4 mph SW
Humidity62%
Visibility—
Dew point67°
Thu, 2080° / 67°0.06
80 °
Markham, Thursday, 20 August
Light rain. High 80°, low 67°.
Pressure29.88 inHg
Wind9 mph N
Humidity64%
Visibility—
Dew point65°
Fri, 2179° / 66°0
79 °
Markham, Friday, 21 August
Partly cloudy. High 79°, low 66°.
Pressure29.97 inHg
Wind9.4 mph NE
Humidity54%
Visibility—
Dew point60°
Sat, 2278° / 62°0.02
78 °
Markham, Saturday, 22 August
Light rain. High 78°, low 62°.
Pressure29.91 inHg
Wind14.6 mph N
Humidity63%
Visibility—
Dew point63°
Sun, 2366° / 55°0
66 °
Markham, Sunday, 23 August
Mostly clear. High 66°, low 55°.
Pressure30.24 inHg
Wind11.8 mph NE
Humidity43%
Visibility—
Dew point42°
From day 17 onward the outlook is built from 60-year climate statistics
Mon, 2480° / 66°0.14
80 °
Markham, Monday, 24 August
Climate normal for this date — 60-year average.
Average high80°
Average low66°
Average precipitation0.14 in
Chance of precipitation28%
Record high98° (2023)
Record low56° (1966)
Tue, 2580° / 66°0.07
80 °
Markham, Tuesday, 25 August
Climate normal for this date — 60-year average.
Average high80°
Average low66°
Average precipitation0.07 in
Chance of precipitation27%
Record high92° (2021)
Record low53° (2017)
Wed, 2681° / 67°0.17
81 °
Markham, Wednesday, 26 August
Climate normal for this date — 60-year average.
Average high81°
Average low67°
Average precipitation0.17 in
Chance of precipitation35%
Record high94° (2024)
Record low54° (2025)
Thu, 2780° / 67°0.14
80 °
Markham, Thursday, 27 August
Climate normal for this date — 60-year average.
Average high80°
Average low67°
Average precipitation0.14 in
Chance of precipitation37%
Record high96° (2024)
Record low54° (2025)
Fri, 2880° / 67°0.15
80 °
Markham, Friday, 28 August
Climate normal for this date — 60-year average.
Average high80°
Average low67°
Average precipitation0.15 in
Chance of precipitation37%
Record high92° (1984)
Record low51° (1986)
Sat, 2980° / 66°0.1
80 °
Markham, Saturday, 29 August
Climate normal for this date — 60-year average.
Average high80°
Average low66°
Average precipitation0.1 in
Chance of precipitation28%
Record high92° (1984)
Record low49° (1965)
Sun, 3079° / 65°0.09
79 °
Markham, Sunday, 30 August
Climate normal for this date — 60-year average.
Average high79°
Average low65°
Average precipitation0.09 in
Chance of precipitation30%
Record high92° (2013)
Record low54° (1986)
Mon, 3178° / 65°0.06
78 °
Markham, Monday, 31 August
Climate normal for this date — 60-year average.
Average high78°
Average low65°
Average precipitation0.06 in
Chance of precipitation23%
Record high89° (2012)
Record low55° (2023)
Tue, 0178° / 65°0.13
78 °
Markham, Tuesday, 1 September
Climate normal for this date — 60-year average.
Average high78°
Average low65°
Average precipitation0.13 in
Chance of precipitation30%
Record high94° (2011)
Record low54° (1967)
Wed, 0279° / 65°0.06
79 °
Markham, Wednesday, 2 September
Climate normal for this date — 60-year average.
Average high79°
Average low65°
Average precipitation0.06 in
Chance of precipitation27%
Record high92° (2011)
Record low52° (1965)
Thu, 0379° / 65°0.07
79 °
Markham, Thursday, 3 September
Climate normal for this date — 60-year average.
Average high79°
Average low65°
Average precipitation0.07 in
Chance of precipitation23%
Record high93° (2023)
Record low53° (1974)
Fri, 0477° / 64°0.09
77 °
Markham, Friday, 4 September
Climate normal for this date — 60-year average.
Average high77°
Average low64°
Average precipitation0.09 in
Chance of precipitation33%
Record high91° (2023)
Record low50° (2025)
Sat, 0577° / 63°0.07
77 °
Markham, Saturday, 5 September
Climate normal for this date — 60-year average.
Average high77°
Average low63°
Average precipitation0.07 in
Chance of precipitation27%
Record high90° (2023)
Record low53° (2010)
Sun, 0677° / 64°0.1
77 °
Markham, Sunday, 6 September
Climate normal for this date — 60-year average.
Average high77°
Average low64°
Average precipitation0.1 in
Chance of precipitation37%
Record high91° (1990)
Record low49° (2025)
About the monthly outlook
How accurate is a 30-day forecast?
The first 16 days come from weather models — reliable for the first 3–5 days and progressively less so after that. Days 17 through 30 for Markham, IL are built from the climate normal, the 60-year average, so they show typical weather for those dates rather than a specific forecast.
What does "above normal" mean?
We compare the forecast against the climate normal for those exact calendar days, computed from 60 years of observations. The deviation shows how much the coming month differs from a typical one.
Where do the 66-year records come from?
They are real temperature extremes recorded in Markham, IL since 1960: the highest and lowest reading for each calendar date, with the year it happened.
How often is it updated?
The forecast portion refreshes every hour; the climate portion is based on long-term data and stays stable.