I'm sitting here in the A/C, looking outside at the grass which is going to get mowed for the last time in a long time this afternoon. It's sure looking brown out there. We missed out on the heavy rain from Tuesday night...I got about .10" at my house while many areas about 5-10 miles north of me saw 2-2.5". So I'll probably end up getting the sprinkler out tomorrow and giving the yard at least a little drink of water. I'm not big on watering, and when the heat/humidity hit last summer at about this time the grass got fried and between me not watering and some sort of fungus that hit so many yards in the area it wasn't a pretty picture out there.

Temperatures are near 90 now and with dewpoints that are in the middle 70s+ the heat index is near 103-105 in spots...and again this will be the rule for the next 7 days or so. The latest GFS shows perhaps a weak front edging into the N MO area next weekend before falling apart and moving back to the north, essentially keeping us in the heat for the next 12+ days.

So with all that said...what exactly is a heat wave...you might be surprised that there really is now concrete definition for specific parts of the country...let's start with the definition from the Glossary Of Meteorology.

heat wave—(Also called hot wave, warm wave.) A period of abnormally and uncomfortably hot and usually humid weather. To be a heat wave such a period should last at least one day, but conventionally it lasts from several days to several weeks. 

In 1900, A. T. Burrows more rigidly defined a “hot wave” as a spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90°F. More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region. In the eastern United States, heat waves generally build up with southerly winds on the western flank of an anticyclone centered over the southeastern states, the air being warmed by passage over a land surface heated by the sun. See also hot wind.

Now going by that definition, every summer day in these parts would be considered a heat wave. So that definition really won't cut it for the KC area at least.

From wikipedia...here is the definition courtesy of the World Meteorological Society....


The definition recommended by the World Meteorological Organization is when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 Celsius degrees (9 Fahrenheit degrees), the normal period being 1961–1990.^[3]
I like that one a bit better because you can quantify it a bit since it connects a definitive number in relation to average...so by using this definition, and checking averages...it would need to be 98+ degrees 5+ days in a row for us to fit that criteria...but certainly a brief check of the FOX 4 Weather Almanac would show that we certainly had some decent heat waves over the past 5+ years!

Also from wikipedia comes the various meanings o various parts of the country...

In the United States, definitions also vary by region; however, a heat wave is usually defined as a period of at least two or more days of excessively hot weather.^[5] In the Northeast, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds 90 °F (32 °C), but not always as this is ties in with humidity levels to determine a heat index threshold.^[6] The same does not apply to drier climates. A heat storm is a Californian term for an extended heat wave. Heat storms occur when the temperature reaches 100 °F (38 °C) for three or more consecutive days over a wide area (tens of thousands of square miles). The National Weather Service issues heat advisories and excessive heat warnings when unusual periods of hot weather are expected.
This also has merit...a heat wave in Phoenix, considering their average high is in the 107 would have be to well above that number...however a heat wave in N Michigan where the average high in Marquette now is 77 degrees is something else...again though there is no qualifier number.

The folks at NOAA really don't have a quantitative definition. You would think they would...I google searched and found some generic stuff...then even read through a conference report where they asked that very question...here is a brief recap of one of their group reports...this is from 1996.

Click on that to make it more readable...basically they decided that further review was necessary to define what a heat wave was.

So really there isn't one quantitative definition that is widely used for the definition of a heat wave...while I like that 9+ degree above average thought...you would have to quantify the time period that it can be used...temperatures 10-15 degrees above average in April are usually a welcome thing...even into the beginning of May...perhaps something that starts on June 1st...along the lines of 10+ degrees above average for at least a set period of time would work. NOAA though does have a point...suppose the temperature is 87 but the dewpoint are closer to 75...then you have the whole heat index thing to worry and contend with. It's valid thought as well, while last mid to late July wasn't crazy hot, the dewpoint were near to above 80 giving us a heat index of close to 110 to 115 for various days...it was awful. OK so then let's run with using a Heat Index based criteria...well then that sort of eliminates the Desert SW from a lot of the criteria where a temperature of 115 and low dewpoints, as is common, actually reduces the heat index compared to the temperature!

It's a frustrating thing for me since I'm into the numbers and trying to quantify things.

Regardless, stay cool and remember all the heat safety thoughts that are out there...

Joe