What Causes Heat Waves, And Why Are They Dangerous?
Heat waves are a poorly recognised form of severe weather, but rate second in weather related causes of death. But just what are heat waves?
A heat wave is somewhat loosely defined as a period of abnormallyand uncomfortably hot, and usually humid weather.
That's OK so far, but let's look into it a bit further.
Firstly, most weather and health authorities would not regard occasional hot days as heat waves. The effects of high temperatures and high humidity are magnified with time, andthe dangerous heat waves last from several days to two or more weeks.
The other significant words in the definition are "abnormally"and "uncomfortably". What may be abnormal and uncomfortable heatto a resident of Wisconsin may not be anything out of the ordinary to someone living in Houston. Similarly a run of hothumid days in late spring or early summer may feel much more unpleasant than the same conditions in late summer, when people are more accustomed to higher temperatures.
Although significant, these regional or seasonal differences do not concern the forecasters and public health people. There is always a proportion of the population who are at risk during heat wave conditions, and will benefit from warnings and the provision of emergency facilities.
The Heat Index
I'm sure everyone has noticed that some days feel hotter than others, even though the recorded temperature may be the same, or even lower. This is not just imagination - it's a very realphenomenon that corresponds to how well our body is coping with the heat.
Our core temperature is normally 98.6F, or 37C. Our systemsstruggle when our temperature rises to 102F (39C), and we areat risk if temperatures are sustained at or higher than 104F(40C). So if the air temperature is higher, we need some means to cool us down.
Our systems have a number of ways of doing this, but the mosteffective is through evaporation of perspiration. Blood isdirected close to the skin, perspiration is produced, and asit evaporates it cools the skin and the nearby blood, which in turn keeps our body temperature at safe levels.
On hot days with low humidity, perspiration evaporates quicklyand we stay reasonably comfortable - even more so if thereis reasonable air movement.
But on days of high relative humidity, the capacity of the air toabsorb more moisture is low. Perspiration evaporates slowly, and not only do we feel sticky and clammy, but we are not cooling downas fast as we'd like. Consequently we sense the temperature as being higher than it is.
The difference between the apparent temperature, which we feel,and the measured temperature can be calculated using a formulato produce the Heat Index. It is shown in chart form below, and any combination of actual temperature and relativehumidity can be converted to an apparent temperature, giving a better guide to what we feel, and the effect the conditions are having on us.
This chart, developed by NOAA, shows temperature in Fahrenheit
- as a rough guide, 82F= 28C, 100F = 38C, and 110F is a
shade over 43C.
The National Weather Service issues heat advisories for days
when the heat index is expected to reach values between 100
and 114F (38-46C). These are days when the heat and humidity
is expected to be an inconvenience for most of us, and a
problem for some.
Excessive Heat warnings are issued when the Heat is expected
to exceed 115F, when conditions become dangerous for a large
proportion of the population.
Recently, it has been recognised that health problems and
fatalities are not just related to maximum heat index values
during the day, but are strongly linked to high minimum
temperatures at night. The effects of heat build with time,
and the stresses of hot days can be partly overcome by
rest in the cool of the night.
But on very humid days, the water vapour in the atmosphere
limits the effect of radiation of heat into the atmosphere,
and night time temperatures, particularly apparent
temperatures, stay high. People under heat stress during
the day have little opportunity to recover overnight.
Heat advisories and warnings now take this into account.
How Dangerous are Heat Waves?
Heat waves are among the most lethal of weather conditions,
although it took authorities a long time to recognise this.
Direct health problems come from dehydration, particularly
affecting babies who lose fluid at a higher rate than older
children and adults, and heat stroke, when a person's body
reaches 105F, perspiration stops, and they ultimately lose
Heat stroke can happen during exertion outdoors - an
unexpected trap for runners, joggers and cyclists
trying to maintain a fitness program during a heatwave-
or by being confined in a poorly ventilated room.
But heat waves lead to far more deaths from the stress
imposed on the elderly and ill. The difficulties in
coping with the heat and humidity are often enough to
aggravate other health problems, particularly involving
the heart and lungs, resulting in deaths which otherwise
may not have occurred until well into the future.
These indirect fatalities show up in comparisons between
deaths during and immediately following the heat wave
with those occurring in other years when conditions were
Heat waves are therefore responsible for more deaths, on
average, than any other weather related phenomenon except
extreme cold. Here are some recent figures for the USA
- 1995 -- 1021
- 1998 -- 173
- 1999 -- 502
- 2000 -- 153
- 2002 -- 167
The 1995 figures include the Chicago heat wave of July that
year, which acted as a wake up call to both the severity of
heat wave effects on health, and the inadequacies of
warnings and responses from emergency and health services.
Even worse was the European heat wave of August 2003, where,
depending on how the comparisons are made, between 11,000
and 15,000 people died as a result of prolonged hot humid
Maximum Temperatures, Maryland - Virginia
July/Sugust 1930. Source NOAA
Causes of Heatwaves
Heat waves result from similar causes to
and some types of
although as a
general rule heat waves are accompanied by higher humidity.
They are usually associated with ridges or zones of high
pressure, including upper level highs. These highs
lead to warmer air because of pressure effects, but
are also associated with clear skies, more sunlight,
and higher temperatures as the land and air heat up.
If the high is stationery or blocked, a succession
of hot days results, often with temperatures increasing
on each succeeding day.
Upper level highs may also put a cap on the underlying
hot air, preventing convection and mixing with cooler
air at altitude. Clockwise circulation around the
high may also bring in humid air from elsewhere - a
common feature of eastern US heatwaves is a high over
the south east bringing in humid gulf air to its west.
But usually there are other factors involved in the
severe heatwaves, which normally would not have great
effects on their own. But add them to the mix, and
problems can arise.
As an example, the 1995 Chicago heat wave coincided with
a fairly light airflow from west to east. This carried in
humid air, partly resulting from evaporation from
extensive croplands to the west. But it also stopped
the lake breezes which help cool the city on most hot
days. The result was still, hot humid days and human
tragedy which was unexpected.
A final factor in the Chicago heat wave was simply that
most buildings in the city are designed for the long
winters with sub zero temperatures. Buildings designed
to keep heat in simply do not work well on hot days.
Other types of heatwaves, generally with hot dry heat,
include those associated with winds descending from
mountainous areas. Winds such as the Chinook and
Santa Ana move downslope, and heat up as the air
pressure increases. The combination of high wind
speed and high temperatures can create heat wave
conditions, but humidity is usually low. Fire
becomes a strong concern under these conditions, and
more information is included in the page on
And finally, let us consider the hottest place in the
USA, Death Valley. High temperatures can't really be
described as heat wave conditions, because they are
usual and expected. But Death Valley is affected by
several of the main causes of heat waves - the
frequent presence of a high pressure system (helped by
its below sea level elevation), downslope
air movement from surrounding mountains, and radiation
of heat from the surrounding cliffs. The last feature
has its counterpart in some of the more northern cities
in both North America and Europe. Older stone, brick
and concrete structures make their own contribution to
the street temperatures simply by reflecting heat
during the day, and retaining it at night.
Heat Waves and Home Weather Stations
Well, there's really not much to be said here - the
temperature and humidity records tell most of the story.
However there is something to be gained from wind direction
and the pressure charts. Where is the wind coming from, and
is it bringing in humid air, or helping to cool things
Surface pressure charts will give some clues, and most weather
services have them hidden away somewhere in their sites. But
it is often worth checking the charts based on the 750mb
and 500mb levels. The real culprit behind the heatwave may
be hiding up there.
Like To Know More?
As might be expected, NOAA has a useful resource page covering
, so slip over there if you would like to
Related types of hazardous
weather are included in the
More information on Weather Stations can be found on the
Home Weather Stations
page, and there is also a
page to help you
set your weather station up.
Back to the Top, or return to the Home page.
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Last update 05/24/2011