Take Care of Your Health
Stats indicate that excessive heat is the number one weather-related killer in the United States; on average, excessive heat claims more lives each year than floods, lightning, tornadoes and hurricanes combined, according to the National Oceanic and Atmospheric Administration (NOAA). If you or someone traveling with you shows signs of heat cramps, exhaustion or heat stroke, the AARP offers Heat Emergency Tips. The AARP notes that infants, children and the elderly are at particularly high risk, and recommends staying in an air-conditioned place during the hottest part of the day (2 - 5 p.m.).
Avoiding sunburn is essential as well, as it can significantly reduce the skin's ability to shed excess heat -- so don't forget the sunscreen.
I am not a health expert, so I will offer a few straightforward tips. Stay hydrated and strong by drinking water regularly, keeping yourself from getting sunburned and taking all the usual precautions against heat-related problems. Eat lighter meals, as the fuel in your belly heats up your body, and also can commandeer precious water from your system. If you see signs of heat-related illness, act sooner rather than later, as things get bad very quickly.
A few useful tactics: wear a wet hat or damp evaporation scarf; hold your wrists under cold water to bring down your core temperature; and go lightly on alcohol and perhaps caffeine -- the latter can be a diuretic and add to dehydration.
Consider Travel Insurance
Typically, if you pay for the "Cancel for Any Reason" option offered by most travel insurance companies, you can recover the cost of your trip if a heat wave looks likely to make your chosen trip unbearable (a fossil-hunting expedition in Morocco or a biking trip in Italy, for example). There may be restrictions on how soon before your trip you can purchase this option; check the fine print first any time you are considering travel insurance.
Lightweight, light-colored clothing is much preferable to heavier, darker clothing. Also, opt for loose-fitting garments that allow air to circulate against the skin. Note that clothing that exposes your skin is not always the best option; many health experts recommend very light long-sleeve shirts over T-shirts, for example. This may require you do choose and pack different clothing than your usual favorites and go-to travel clothing; it's worth the time and even a trip to the store to stay cool during extreme weather at your destination. Think about it: Do you have the right clothes to survive a week in the desert? If not, consider that you are headed for desert-like conditions, in many cases with merciless, heat-absorbing cement as the terrain.
Different "types" of heat may require different types of clothing; very hot but dry conditions mainly demand that you protect yourself from the sun, where in very hot, humid conditions you need to wear clothes that will let your body cool off.
There is no excuse for not knowing what the weather will be like where you are headed; there are now fairly reliable 10-day forecasts for almost every square foot of the planet. You should also check weather at your local airport, and all stops along the way. Take your pick of Weather.com, Wunderground.com, WeatherBug.com, Weather.gov, etc., and know before you go.
Arrive Early at the Airport
With cement and tarmac everywhere, jet engines and taxicabs exuding intense fumes, parking lot buses that show up both full and hot, and heavy luggage being hauled by overheating bodies all around, airports can be merciless environments during extreme heat. I recommend that you check airport maps before you head to the airport to figure out how hard or easy it is to get around, and plan accordingly. If there are air-conditioned monorails, maybe your whole family can hack a trip to the parking lot; if not, and you are facing a ride on a full shuttle bus, maybe you get there a little early, drop everyone and your luggage off at the terminal, and have just one person run the car to the parking lot, without luggage.
Check Ahead for Air-Conditioning
You will want to know whether your lodging and daytime attractions have air-conditioning. While most such establishments in the U.S. have A.C., this is not necessarily so everywhere in Europe, and certainly not in parts of Asia, Central/South America, Africa, the Caribbean or other regions of the world. Call ahead or check the websites of museums, restaurants, hotels, tourist attractions and the like.
Wake Up Running
The morning hours are going to be the most conducive to moving around, and you would do well to focus your attention on maximizing the hours before, say, 11 a.m. A major bonus to this tactic is that most attractions are far less crowded in the morning hours. Check the websites of places you would like to visit for early opening hours, do any walking tours or park visits as early as possible, and have a half-day's work or amusement done before most folks leave their hotel.
Give Me the Night
At night it's a different world, as the old song goes; late afternoons and evenings will still be hotter than most mornings, but the declining sun, open attractions and sometimes evening breezes can make these hours among your most productive and enjoyable. Check for evening discounts and extended hours to get even more out of the twilight hours each day.
Take a Siesta
On several trips over the years, I came to appreciate the practice of a daily siesta during the heat of the day. It required that everyone worked fairly late, but the trade-off was almost always well worth it. A great schedule is to tour and cavort from something like 8 - 11:30 a.m., then take about four hours' downtime to eat some lunch, read or doze in the A.C., then get back at it in the late afternoon.
I have sometimes found attitudes of travelers and locals alike to be much better in the mornings and evenings rather than in the heat of mid-afternoon, and many destinations look no better than they do in the soft light at either end of the day.
Cluster Your Activities by Location
Dragging yourself all over the place can be brutal enough without doing so in the high sun hours during a heat wave. If you plan out your days to cluster activities by location or neighborhood, you can avoid long, hot (and time-consuming) walks or subway and bus rides from one place to another.
Have Some Ice Cream
You laugh, but foods like ice cream bring the cold way down into your belly, helping to lower your core temperature. Yeah, a cold drink will do the job, but doesn't compare to gelato on a hot day in Rome.