Hot weather

The Earth's axis is tilted by 23 degrees in relation to the ecliptic, the plane of its orbit around the Sun, and this causes the temperate seasons of winter, spring, summer, and autumn. When your part of Earth is tilted toward the Sun, you get summer; when it is tilted away, winter.

A region's weather is typically defined, among other factors, by latitude relative to where the sun is directly overhead, with regions closer to where it is overhead receiving the most intense heat. The latitude at which the sun is overhead "moves" from 23° north of the equator in June to 23° south of the equator in December. Therefore, a city at 30° north of the equator is only 7° north of direct sunlight in summer, causing it to receive high temperatures, but is a significant 53° north of direct sunlight in winter, causing temperatures to decline until the sun "moves" north again in spring. In the tropics, which are within 23 degrees of the equator, there is one day each year when the sun is directly overhead, and the sun is close to being overhead for most if not all of the year, making low temperatures impossible.

In general, the closer a place is to the equator, the hotter it will be, and the less chance exists of cold temperatures during winter. For example, Singapore is only one degree off the equator so the angle there (relative to overhead sunshine) ranges from 22° one way to 24° the other, causing the climate to be uniformly hot all year; and the record high and low temperatures are 36 °C (97 °F) and 19.4 °C (66.9 °F) respectively. Near the Equator, extremely high temperatures are rare, unlike what you may encounter in desert environments such as Sahara, the southwestern U.S. or central Australia (cf the Death Valley), but the humidity can make lower temperatures quite unbearable.

Other factors also come into play. Large bodies of water tend to moderate the temperature of nearby places (causing a "temperate" climate) while high altitude makes a place colder, even in the tropics. Note, however, that these do not reduce the strength of the sun which may be hazardous even when the air is relatively cool. In fact, altitude increases the solar risks since there is less atmosphere above to protect you.

Meanwhile, temperate regions with continental climates, far from oceans, experience great variations in temperature from summer to winter. In Edmonton at 53°N, for example, the sun is 76° from vertical at noon in mid-winter, so winters are quite severe; but in mid-summer it is only 30° off vertical, so summers can be quite hot. In Northern and Central Asia, the world's most continental climates, the difference between winter and summer extremes can be almost 90 °C (194 °F).

Regions described as "tropical," "desert," "arid" or "semi-arid," "subtropical" and "Mediterranean" should be taken as possible indicators of "hot" summers. "Subtropical" refers to a climate that is tropical during summer and temperate during winter.

See also: Sunburn and sun protection

Choosing the right clothing can help a lot. Points worth considering include:

• "When in Rome, do as the Romans do."
• Light colours absorb much less sun than dark ones, so they are usually cooler.
• Loose clothing allows air to circulate much better than tight clothing. This both cools directly and helps perspiration evaporate faster for additional cooling. It may also keep biting insects away from your skin.
• A large hat protects your head. This can be important for anyone, but especially for balding men. A wide-brim is also recommended to reduce glare, but avoid cheap plastic 'sun visors' which have little value.
• A parasol reduces exposure to the sun. In many tropical climates, an umbrella is needed for rain and showers as well.
• Depending on a region's climate, you may need multiple layers of clothing, for example a jacket to put on in the cooler evenings.
• In humid areas, lighter shoes like sandals may be more common, as they help keep feet cool and dry. Note that sandals do not protect against sunburn and insects.

Of course it is wise not to violate local taboos; for example a woman should not wear shorts or a miniskirt in some places, no matter how hot it gets. Also, clothes that make you stand out as a tourist may make you a target for scams, pickpockets and in some places even muggers. See the linked articles and the "respect" sections of country, region and city articles for details.

Heat waves are abnormally (and sometimes unexpectedly) high temperatures that can range from a few days to an extended period in some instances. They, along with high heat that is persistent in a location, can cause discomfort and health risk, even to those who thought they were prepared.

In cooler climates, a general definition of a heat wave is when temperatures of 27 °C (81 °F) or above are occurring, but for those from temperate climates with mild temperatures, don't be misled by this classification of a heat wave! In various climate types, temperatures of 27 °C (81 °F) are mild, when compared to high temperatures experienced regularly throughout the summer months. If you find 80 °F (27 °C) a hot temperature, chances are you'll find 90 °F (32 °C) intolerable, and 100 °F (38 °C) absolutely unbearable. Many subtropical climates receive temperatures exceeding 27 °C (81 °F) in autumn or spring, or even on occasion year-round.

The effect of a heat wave can vary dramatically depending on humidity, which affects the temperature that one feels on one's skin. A person experiencing a dry heat wave may begin to feel discomfort when it is 32 °C (90 °F), but the same temperature and 75% humidity would feel like 43 °C (109 °F)! Humidity is not felt equally by everyone, but prepare for the worst, since when temperatures reach the upper 90's Fahrenheit and humid, no-one can cope who is not accustomed to these extremes.

However, don't assume that dry heat is tolerable. Arid or semi-arid climates in the equatorial to lower temperate regions can reach extremely high temperatures in the triple digits Fahrenheit, feeling just as hot as a tropical, humid location. The dry heat combined with intense sun can cause extreme heat on surfaces, such as sidewalks, especially when clouds are not present, such as in a Mediterranean climate in summer. Mediterranean and desert climates tend to have the hottest times in high pressure, low humidity "dry seasons", making the chance of a temperature-reducing rainstorm unlikely to impossible, giving the sun "free reign" to increase temperatures steadily over the course of the day.

What is a heat wave and what isn't a heat wave largely depends on where you are. What would be extreme heat during summer in the United Kingdom would be the usual high temperature in many parts of Northern Africa. In some places that normally receive high summer temperatures, such as parts of California, heat waves can increase the temperature, to highs above 44 °C (111 °F).

A danger during heat waves, besides the heat during the daytime, is the heat during nighttime, since without outdoor temperatures cooling sufficiently at night, buildings without air conditioning can become extremely hot, not providing any relief from the high temperatures outside. This is typical in climates with high humidity, since the humidity prevents temperatures from falling significantly. Hot desert climates such as that of Phoenix have significantly lower temperatures at night compared to those experienced during the day, but during summer, temperatures simply cannot drop far enough to provide relief, making air conditioning more or less a necessity.

• Avoid strenuous activities outside, especially during the afternoon when the temperature is the hottest. In some regions it's normal to take a break in the early afternoon (the siesta), resuming any activity later when things are cooler.
• Drink lots of fluid as that will cool you down while at the same time replacing the fluids in your body exerted by the heat. See dehydration for more detail.
• It is a good idea to stay indoors or go to a park and sit under a tree to avoid the sun.
• Prolonged exposure to heat can lead to heat exhaustion. Children and elderly people are especially prone to heat-related illness, from cramps to heatstroke. You should at regular intervals check how they feel, make sure they drink enough, try to keep them cool, and go for emergency treatment if symptoms show up. You should also not leave them unattended in a hot location, such as a parked car or unshaded queue line.
• Do not leave children or pets in your car, as the inside car temperature quickly can become dangerous.

Even in comparatively healthy travelers, heat stress should not be ignored: your body has to work harder to cool down. Activities that feel normal in cooler temperatures can become a major effort in conditions of high heat, and even the healthiest traveller isn't immune to heat stroke. Excessive heat also affects how you sleep.

Extreme heat is an issue especially for those with certain medical conditions, including asthma, diabetes and kidney disease.

Heatstroke and heat exhaustion are life-threatening conditions, which occur when the body can no longer control its temperature; the body temperature rises and the victim becomes extremely ill.

High ambient temperature is a major factor in this, of course. Humidity is also very important; when the humidity is high, water evaporates more slowly and the body's normal method of cooling itself by sweating is much less effective. Any physical exertion adds to the problem. Dehydration can contribute to the problem by making you sweat less, so you should always drink lots of water when it is hot. You should also make sure you get enough electrolytes (minerals): you can get some by eating nutritious food, but sport drinks and the like can be useful in a pinch.

Some medications, including several common antidepressants, tend to increase risk of dehydration and therefore of all heat-related problems. If you are on any long-term medication and expect to undergo heat stress, ask your doctor about this.

In certain areas, including expanses of sand or surfaced roads, mirages can easily form under the hot conditions. These are a form of heat haze or shimmer, that can give a distant object the appearance of being viewed on a water like surface. The object often looks expanded vertically, so that e.g. a low hill can look like a tower or town. A mirage can also make it appear you are seeing a distant lake or shore, when no such body of water exists.

These are an optical illusion; there is no water, the apparent reflection is just light bending as it passes between layers of hot air and layers of marginally cooler air (which differ in density). Do not be fooled!

Main article: Wildfires

In areas where there's been prolonged heat and an associated drought, the risk of wildfires is higher. In some places that are wildfire-prone, such as California, signs are used in places such as national parks to show the chance of a wildfire, based on factors such as heat and precipitation. Hot, dry summers contribute to the possibility of wildfires because the lack of rain encourages a wildfire once it begins to spread.

Along with the risk of heat contributing to the possibility of a wildfire, wildfires themselves can cause temperatures to rise in nearby communities.

When the heat breaks in humid climates there is a risk of thunderstorms, hail and flash floods. While in tropical rainforest climates, rain tends to be a more year-round feature, in many semitropical regions the infamous "monsoon" or "wet season" takes place as hot, dry temperatures give way to high precipitation. It can be wise to avoid visiting semitropical or subtropical regions at these times, since they can become a battleground between extreme humidity and heat at times and extreme precipitation on the other.

• Arid region safety
• Cold weather
• Sunburn and sun protection