Southern New England has already had a couple of “official” heat waves this summer. A heat wave is defined in these parts as a period of three or more days where the ambient air temperature is more than 90 degrees. Chances are good that we will see more of them this summer, but what really happens during a heat wave?

Heat waves form when high-pressure systems in the mid-levels of the atmosphere strengthen and remain over a region for at least several days. These high-pressure systems basically compress air toward the earth’s surface; the “sinking” air acts as a “dome” that traps heat instead of allowing it to lift.

Without lift, there are fewer clouds and minimal chance for rain. The end result is a continual build-up of heat on the surface. This compressed air, coupled with high humidity, is what gives the feeling of “thickness” in the air during a heat wave.

When a heat wave descends on notoriously humid southern New England, stepping outside can feel like walking into someone’s mouth.

Cooking the air
Heat waves are by far the deadliest weather phenomenon in the United States. Between 1992 and 2001, deaths from excessive heat numbered 2,190, compared with 880 deaths from floods and 150 from hurricanes.

Fortunately, most heat-related illnesses, ranging from the annoying heat rash to the life-threatening hyperthermia, or heat stroke, are non-lethal if diagnosed in time. Most heat-related illnesses are treated by hydrating the patient with isotonic fluids orally — think sports drinks — or in more severe cases fluids are introduced to the overheated patient intravenously.

As with many other illnesses, heat-related ones disproportionately affect the young, old and poor. These groups are less likely to have access to the preventive measures, such as air conditioning, required to stave off these effects.

Heat waves can affect air quality, triggering respiratory reactions in those suffering from lung ailments such as asthma and emphysema.

Excessive heat basically “cooks” the atmosphere. Coupled with the forcing of air — and all of the pollutants and particles therein — toward the surface under high-pressure domes, this cooked and compressed air becomes a soup of ozone, carbon soot and pollutants from tailpipes to smokestacks.

Heat waves may be worsened in urban areas due to the heat-island effect. Every bit of infrastructure, from roads to skyscrapers, absorbs heat and slowly releases it back into the atmosphere. This heat energy has a cumulative effect on the ambient air temperature in highly developed areas.

High-pressure domes create a sort of temporary, quasi-closed system, at least when it comes to pollutants and heat energy. These systems trap not only the sun’s direct heat energy but also the heat reflected by infrastructure, as well as absorbed and released by every surface under the dome. All of that heat energy accumulates under the high-pressure system.

Soil, water and food impacts
Though a heat wave isn’t necessarily accompanied by drought — nor a drought by a heat wave, as Rhode Island nearly experienced thanks to a virtually snow-less winter followed by a dry spring — each can have a distinct and cumulative impact on the other.

As topsoil dries out in a drought, plants die. When the things that are converting the sun’s energy directly and shading the soil to prevent evaporation die, the exposed soil is more likely to dry out if a drought is accompanied or followed by a heat wave. When soil dries out, it actually absorbs more energy than wet soil, adding another input to the compressed soup under a high-pressure system.

Heat waves and drought don’t really affect water quality so much as demand for water. Whether for irrigation, drinking or showering, water usage goes up in both scenarios, so when coupled, water is at a premium.

The tandem of heat and drought is destroying crops and soil throughout the United States. Drought and heat have hit America’s farmlands hard this summer, and food prices are expected to spike dramatically. CNN has reported that crop insurance claims from American soy, wheat and corn farmers may hit $20 billion this year. Corn prices are expected to hit $9 a bushel by summer’s end. The average is anywhere from $3-$4 a bushel.

Energy usage
As a rule, energy usage goes up in the summer — thank you air conditioners — but a heat wave can see a 30 percent to 60 percent spike in that already elevated rate.

Turning on all of those air conditioners in an area, virtually at once and for long periods of time, wreaks havoc with the local power production and distribution system. On top of that, the heat energy exhausted by those air conditioners is added to the already-heat-energy-rich soup of surface air, exacerbating heat waves. During the last heat wave that hit the Midwest, power demand in the Detroit area rose to the point that rolling blackouts were instituted in some suburbs.

Global and local warming
Until recently, the scientific community has been reticent to link any particular weather event to the effects of climate change, but in the past few months, that has begun to change. From Somalian famine to freak weather events such as floods in Russia, scientists of all stripes have begun to view these events as signals of a larger pattern of change.

Climatologists are indicating that the increase and frequency of severe weather events — such as the windstorm that recently swooped from Chicago to D.C., killing 17 and leaving 2 million without power, and the heat and drought that are gripping much of the United States — are examples of a larger pattern of extreme weather that science has long predicted would be caused by the warming of the planet.

A recent study sanctioned by Environment Rhode Island showed that the occurrence of extreme rainstorms and snowstorms are up 90 percent between 1948 and 2011. In other words, heavy downpours or snowstorms that happened once every 12 months on average in 1948 now happen 6.3 months, on average.

Storms with extreme precipitation also increased in frequency by 85 percent in the New England region during the period studied, and New England ranks first nationwide for the largest increase in the frequency of storms with heavy precipitation.

The biggest rainstorms and snowstorms are getting bigger. The amount of precipitation released by the largest annual storms in the Northeast increased by 26 percent from 1948 to 2011, according to Environment Rhode Island.

Stay cool, but don’t beat the heat
The recommended response to a heat wave is to remain inside and crank up the A/C, but running that machine on “sub-arctic” will actually worsen heat events in the near and long terms.

Air conditioning has a magnifying effect on localized heat events. All that exhaust becomes trapped under the high-pressure dome, but more troubling is the potential worsening of global warming.

As the heat rolls into Rhode Island, on go the A/C units, releasing heat to be trapped in the immediate atmosphere, and drawing more energy from fossil fuel-fired power plants, releasing more pollutants and greenhouse gases, accelerating climate change, worsening heat waves, causing more energy draw for air conditioning, et cetera, ad nauseam. This is known as the positive feedback loop.

When the heat settles in for an extended period, you shouldn’t need to set the thermostat lower than 80 degrees. Most air conditioners act as dehumidifiers, as well, which will significantly lower the humidity in your home, making the heat a bit more tolerable. By adjusting the thermostat on your A/C unit, you can significantly reduce your energy consumption.

Though extreme heat has a host of impacts to our society, it seems our society is exerting an even greater influence on weather patterns both local and global. To mangle an oft-heard expression in these parts: It’s not the heat; it’s the humanity.