The 1995 Chicago Heat Wave

• Very high apparent temperatures that persisted
day and night over a 48-hour period.
• Minimum apparent temperature did not drop
below 89 degrees. EXTREMELY RARE!
• Large number of deaths caused by persistently
high heat index (apparent temperature)
• What kind of effect did the urban heat island
have on this unusual event? Can we expect more
events like this?
• Heat wave is
centered around
the northern half
of Illinois.
• The highest
temperatures are
centered around
• Paper focuses on the severity and duration of
extreme heat waves.
• Researched probability of similar events of the
same or greater magnitude.
• Used the apparent temperature, heat index, to
evaluate historical significance of heat wave. Will
show on diagram later.
• Used an annual series instead of partial duration
series to model temperature data from each
summer. (Human acclimatization)
• Data was gathered for the observed heat
index during this event and adjusted for
growth of urban heat island.
• Algorithm used to model heat island; uses
population to model effects on mean monthly
max and min temperatures.
• Analysis conducted based on Monte Carlo
simulations of detrended autoregressive
moving-average (ARMA) models.
• The chances of this event happening again are very rare.
• As seen in the previous diagram, even with an increase in
average temperatures the chance of an extended heat
wave of this magnitude is still quite rare. (Generally less
than 4%)
• Two straight days with a minimum apparent temperature
greater than 80 degrees is a once in a life time event.
• Even with increased variance, increased temperature, and
changed persistence, 0.4% chance of this happening
• Temperatures in Chicago and
the Midwest have increased
over time, most likely as a
result of urbanization.
• The HO-83 weather instrument
provided some bias in the daily
maximum temperatures after
• The persistence and variance
are just as critical to the heat
wave as the apparent
What will be discussed?
• Increase in human energy
production (HEP)
• Urban Heat Island effect
• Global climate impacts;
specifically precipitation
• Future outlook
• On average, 235 Wm^-2 of energy is conveyed
from the sun to the earth and its atmosphere
• Human energy production (HEP) is responsible
for the release of 0.025 Wm^-2 of energy
• HEP accounts for less than 1/3 of the total
internal terrestrial energy flux
• 2 to 5-fold rise in HEP
by the end of the
• Release of air pollutants
and CO2
• Variable energy
production densities
Does HEP affect global climate? Does the release of
pollutants and aerosols impact precipitation?
Claim A
• High concentrations of cloud
condensation nuclei
• Slowing down coalescence
• Suppression of “warm rain”
• This has been seen in
California and Israel
Claim B
• At higher altitudes, can lift
formation of precipitation to
the mixed phase region
• Enhanced convection via
release of enthalpy during
• Increased precipitation and
40-85% increase in lightning
frequency were seen in and
downwind 16 cities of the
United States
Factors Contributing to
1. Higher UHI temperatures
(0.6-6.0o C)
2. Enhanced urban frictional
3. Variable cloud
microphysical conditions
from high levels of
Consequences of
Stronger Convection
• Transportation of aerosols,
water , and pollutant gases
to upper troposphere and
potentially the
• How does this impact global
• We don’t know
So which claim has been proven?
• Satellite observations showed existence of
aerosol overseeding effect for pollution plumes at
cloud tops warmer than -10o C
• Also observed in the Amazon region and in
French Guyana
• It has been hypothesized that this effect may be
more prominent during dry seasons of the tropics
due to high Bowen ratios
• Suppression of warm cloud formation in the
Amazon; however no correlation was seen
between lightning frequency and increase HEP
• Urbanization is projected to double in the next
two decades!
• This would put nearly 80% of the world’s
population in cities and mega-cities
• Social, logistic, medical, and urban heat and
air quality consequences of urbanization could
be significant
• Altering of regional meteorology and possibly
global climate
Daniel Rosenfeld
What will be looked at?
• Evidence supporting the
hypothesis that urban and
industrial air pollution
suppresses precipitation
• Satellite data showing
reduced cloud particle size
• Tropical Rainfall Measuring
Mission satellite imagery
What impact do aerosols from urban and industrial
air pollution have on precipitation?
• Smoke released by burning vegetation has been proven to
suppress precipitation
• Initially it was assumed that aerosols from air pollution also
suppressed precipitation
• Recent studies showed that certain urban settings and also
areas downwind of paper mills were experiencing
enhanced rainfall
• Studies attempting to correlate enhanced rainfall to air
pollution have been unsuccessful
• Pollution from ships was the first evidence of pollution
suppressing precipitation
• Pollution tracks over land were not included in previous
So how do we make sense of this?
• The Advanced Very High Resolution
Radiometer (AVHRR) on the NOAA orbiting
weather satellite
• YELLOW is the most important color, as it
denotes clouds with small drops
• RED represents clouds with larger cloud
• GREEN indicates clouds with cloud droplets
between the yellow and red sizes
• This pollution track was
seen in 47 images on
different days
• Stratocumulus and
cumulus cloud tops
warmer than -12oC
• Port Augusta (5) is home
to a 520-MW power plant
operating on brown coal
• Port Pirie (6) has the
world’s largest lead
smelter and refinery
• Adelaide (7) holds
industry for processing
minerals mined in the
• Major oil refinery and
power plant (8)
• 43 kg/hour of ash particles
• 1108 kg/hour of SO2
• 1655 kg/hour of NOx
Things to Note:
1. Peak near 2 km
2. Box 2 had no
3. Boxes 4 and 7 were
outside the radar’s
• Cloud top heights and horizontal dimensions had no differences
• Data off of the TMI-measured T85 showed that lack of cloud water was NOT the
reason for precipitation suppression
• First, for those who do not know, coupling is defined as
the pairing of two items.
• Most of the recent European summer heat waves have
been preceded by a pronounced spring precipitation
deficit. (10 warmest European summers)
• Heat waves are generally associated with quasistationary anticyclonic circulation anomalies.
Subsidence, clear skies, warm-air advection, and
prolonged hot conditions at surface.
• How do land-atmosphere interactions in addition to
circulation patterns contribute to heat waves?
• Simulations were used to model the four most
extreme European heat wave events of the past
decades. (Climate High-Res Model CHRM)
• They model previous European heat waves; uses
the same boundary layer conditions that
occurred during event.
• One model run uses the actual soil moisture
content during the event, the other model run
uses the climatological soil moisture content.
• We will look at the results of the model runs on
the next slide.
• Data from the study shows more soil moisture =
cooler temperatures.
• Temperature amplification through landatmosphere coupling is found to be substantially
stronger for daily maximum temperatures
compared to min and mean temperatures.
• Drought conditions themselves may contribute to
higher persistence of the heat wave.
• Estimated 90% of surface net radiation was
transferred to sensible heat and 10% to latent
heat in driest years (Normal: 60%/40%)
• The heat waves of 1976,
2003, and 2005 were
preceded by a mean
precipitation deficit during
at least four antecedent
• Land-atmosphere coupling
is found to increase mean,
max, and as well to a lesser
extent the min
temperatures averaged
over anomalously warm

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