Severe weather is the generic term for any dangerous weather phenomenon with the potential to cause damage, serious social disruption, or loss of human life. Severe weather can occur anywhere in the world, and there are different types of it, which can depend on geography, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects of severe weather, as are thunderstorms, tornadoes, waterspouts, and cyclones. Regional and seasonal severe weather phenomena include blizzards, snowstorms, ice storms, and dust storms.

Travellers are strongly advised to be aware of any risk of severe weather affecting their area as they may affect any travel plans. Attractions may be closed, transportation will likely be impeded, and even your life may be at risk!

General precaution

“ Climate is what we expect. Weather is what we get. ”

—Attributed to Mark Twain

The best advice for travellers is to first research the climate of your destination; if any risk of severe weather is the norm for the time period that you travel, you are advised to prepare for unpredictability regarding the weather. The predicted weather on your travel plan may only come out at the earliest 2 weeks before your travel and becomes more accurate only as the day gets closer, so you should plan ahead while at the same time be flexible, especially on where to go and what to wear.

An example of a weather forecast in a newspaper or on the internet (with temperatures in Fahrenheit)

As weather can change somewhat on a day-to-day or even hour-to-hour basis, check the weather forecast for your destination before and during your travel to be aware of any forthcoming weather threats on your destination, using either local TV and radio or your smartphone app. While television weather forecasts may be shown in different languages, their weather symbols (those that indicate for example sunny, rainy, or partly cloudy weather) are usually comprehensible. There are also radio stations which just transmit weather forecasts; Canada and USA reserve seven frequencies (162.4 to 162.55MHz in 0.025MHz steps) for weather radio, there's also one channel reserved on marine VHF radio for marine weather.

Most of the world uses Celsius and millimeter as their temperature and precipitation intensity, respectively, while the USA uses Fahrenheit and inches. A few countries with close ties to the U.S. (Bahamas, Cayman Islands, Liberia, Palau, The Federated States of Micronesia and Marshall Islands) also use Fahrenheit. See metric and imperial equivalents for details and conversions between these measurement systems. One inch of precipitation equals to 25mm (2.5cm) of liquid in the rain gauge, higher amounts mean that more rain is falling and severe weather may be happening. Many weather forecasts also use the overly complicated unit "liters per square meter", which they seem to think sounds more intuitive than "millimeter", but the two units are exactly the same with the former just being a roundabout way to express the latter. Marine and aeronautical forecasts are likely to use nautical miles per hour (knots) and will report conditions (such as rough seas for mariners) to suit their specific target audience.

Temperature ranges and conversions

	←frigid		freezing		cold		cool		mild		warm		hot		swelter		cooked→
°C	-40	-18	-10	-7	0	4	7	10	13	15	18	21	24	26	30	32	35	38
°F	-40	0	14	20	32	40	45	50	55	60	65	70	75	80	85	90	95	100

If a severe weather does exist in the area that you wish to travel, you are highly suggested to have a change of travel plan; if a blizzard is occurring at the ski resort that you wished to go, you should go to an alternative resort, or perhaps go to the beaches or downtown if the weather there is better. If it does or will happen to where you are right now, obey all commands and warnings from local authorities; do not risk yourself by disregarding any threats.

Getting in and out of the area will be difficult because of severe weather: roads may be closed, flights or cruises may be delayed or even cancelled. Trains are sometimes the last thing still running, but when overhead wires have been hit and cut by trees and no Diesel locomotive is available, there is little the railway can do. Check your transportation provider regarding any alternative arrangements, compensation or rebooking. Keep your composure when interacting with them as weather is always beyond their control and they would have got the same complaints from other travellers that are stuck with you. Contact any people you will meet or the hotels you'll stay at your destination or your workplace and someone at your departure point to give updates on your expected arrival time and alternative arrangements.

Geography

See also: Biomes and ecosystems

The world can be divided between climate zones, with local variations due to landforms.

Coastal regions have modest temperature variations, but tend to have more wind and precipitation than inland regions.

Mountains and highlands are usually colder than lower lands, and can have more wind and precipitation.

Cyclone

Hurricane Katrina making landfall in Louisiana (2005)

Main article: Cyclones

A cyclone, also known as a hurricane (in the Americas) or typhoon (in Asia and Oceania), is an organized rotating precipitation system packed with damaging winds and heavy rain. Effects include but are not limited to: windstorm, very heavy rain which can lead to widespread flooding and mudslides, thunderstorms, and high waves. They occur mainly in tropical and subtropical regions, but there are also extratropical cyclones that occur far from the equator, often as remnants of a tropical cyclone.

Cyclone wind speeds vary from tropical storm winds to category 5 storms, which can have winds above 170 miles per hour (76 m/s). Generally, cyclones lose energy once they hit land, but they can still cause great damage along coastlines and, in some cases, inland areas. Cyclones form in the oceans and usually travel west until they reach land.

Thunderstorm

Main article: Thunderstorms

Thunderstorms are storms with lightning and the associated thunder, but dangers include heavy rain, strong gusty wind and possibly hail. While most lightning strikes from inside or between clouds, some does hit the ground and causes a wide range of effects from electric damage to fires and death of persons hit.

Tree struck by lightning

A sometimes dangerous occurrence in a thunderstorm is hail. Hail occurs when solid balls of ice (hailstones) fall to the ground during a thunderstorm; while hail is not usually dangerous due to its small size, hailstones can sometimes reach the size of golf balls, and if they reach this size they can shatter car windshields and knock people unconscious. When there is a threat of hail, get yourself and all of your belongings indoors as soon as possible, and do not go near windows or openings while inside that indoor space.

Tornado

Main article: Tornado safety

In some places, particularly in the Great Plains region of the United States, you can get tornadoes in a thunderstorm. Tornadoes are extreme winds that spin in a small area, and blow everything near them inward and upward. Tornadoes typically come with little warning, and they are powerful enough to destroy houses.

Tornadoes are easy to spot during the day but harder to spot during the night. However, if you notice a funnel reaching downward from a dark cloud or you see objects flying, you are probably about to see a tornado. Also, hail often occurs before a tornado. If you know there is a high chance of tornadoes but it is nighttime and you can't see any, follow the weather forecasts on TV or radio. Sometimes, sirens will give warning of a tornado in the vicinity, in which case you need to seek shelter immediately. The best places to be when inside are in interior rooms (such as hallways or windowless basements) with as few doors as possible and no windows and as few large pieces of furniture as possible, such as dressers, bookshelves, or credenzas, as these may fall on you. Note that no room is entirely safe as tornadoes have the power to rip off roofs and in some cases floors. If outside, get inside as quickly as possible, driving to the nearest viable option if necessary, avoid trees, loose items (like farming equipment), bridges/overpasses, and – most importantly – the tornado itself. Due to the heavy rains that occur during a thunderstorm, and therefore when tornadoes are around, getting away from a tornado by driving can be difficult. If outside when debris and objects get thrown around, get down in the nearest ditch or gully, covering your head with your hands; if in a car, park the car, remain seated, and keep all parts of you below the windows with your head covered. If you are outside and you see a tornado, and you cannot see it moving, it is probably headed towards you. If you insist on trying to drive away from the tornado, drive in a direction perpendicular to the path of the storm.

In areas not known for tornados, there may be minor versions of the same, capable of lifting a bouncing castle or a light vehicle (and fell trees). For these, going indoors should be enough, but as warning systems aren't in place, you might want to avoid certain activities or areas when the weather type and cloud configurations suggest suitable conditions.

Fog

Driving a car in the middle of dense fog on the stretch of road in South Yorkshire, England. Scary, huh?

Surprisingly, fog is the most lethal kind of weather in some parts of the world. The deaths are usually caused by traffic accidents that become much more likely when visibility is reduced. In the same way, fog can be lethal in places where you must find your way or watch your step, for instance mountain environments. Fog is most common in the early morning just before sunrise, though there are places where fog is most common during other parts of the day. When driving a car in fog, go slow. If fog is very thick, you might need to wait it out. Sometimes there is fog only in valleys or by bodies of water; when driving downhill visibility can be abruptly reduced unless you are alert.

With a boat, mooring in a safe place is the best option, but if that is not possible you have to quickly note your position and keep track of your movement, especially if you do not have a GPS. Remember your fog signals, avoid shipping lanes and choose a route that is safe in these circumstances. GPS does not show other vessels; even a radar shows only some of them.

There are variants of fog that are extra nasty. Smog is smoke plus fog, generally found in areas with heavy industry or where coal is used for domestic heating as in parts of northern China. It can be particularly bad if there is an inversion in the atmosphere, for example around Los Angeles cold air coming in off the Pacific sometimes forms a layer that seals off the air below it so that smoke cannot escape. Vog is volcanic smog, water droplets with dissolved gasses from an eruption, sometimes with dust as well. The main gas is sulphur dioxide which combines with water to form a strong acid, so vog is seriously irritating to eyes and throat; with heavy exposure or for vulnerable people, it can even be fatal.

Floods

See also: Flash floods

Sometimes, large amounts of rain occur within short amounts of time, causing floods that rise very quickly. These can be extremely dangerous and are called flash floods. As a general rule, if weather forecasts predict very heavy rains in the near future, or a flash flood has already started, get to high ground as soon as possible. Also, avoid rivers or camping in narrow canyons during the times of the year when heavy rainstorms or thunderstorms are likely, since these can flood very quickly.

There are risks involved also in more slow flooding. You have some time to prepare, but if the roads around you are flooded you may not be able to leave. Driving in flood water is as dangerous as in the case of a flash flood, and the shores of rivers may collapse. There will be debris in the water, so wading or boating can be dangerous. The water is probably also unhealthy, as result of flooded fields and sewage systems.

After heavy rain there is also a risk of mudslides. These are like avalanches, but the soil itself getting unstable when wet enough. Near volcanoes there may be a risk of lahars, a particularly dangerous type of mudslide created when a deposit of volcanic ash gets wet. These can be deadly since they often occur with little warning and can move quickly over long distances.

Heatwave

Main article: Hot weather

See Arid region safety for places with persistently hot and dry climate.

A sign along freeway warning drivers of extreme heat in Los Angeles.

Heatwaves are abnormally high temperatures for a period of a couple of days. They can cause discomfort and health risk, even to those that are seemingly prepared.

Though a general definition of heatwave is when temperatures that are about 27°C (80°F) or above, its effect can be dramatically different depending on humidity, which affects the temperature that one feels on its skin, and the average temperatures for a particular place. A person experiencing a dry heatwave may begin to feel discomfort when it is 32°C (90°F), but at the same temperature and 75% humidity, 90°F will feel like 43°C (109°F)!

Avoid strenuous activities outside, especially during the afternoon when the temperature is the hottest. 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. Do not wear any dark clothing, as it absorbs heat and will make your body lose fluid even quicker. It is a good idea to stay much of the time somewhere indoors where cooling systems are usually found, or go to a park and sit under a tree to avoid the sun; if you choose to enjoy the sun instead, wear a sunscreen or preferably lightweight sport fabrics, sunglasses, and a wide-brimmed hat to protect your head. Do not leave children and pets in your car as the inside car temperature can rise quicker than the outside, and can soon be outright dangerous. Children and elderly people are especially prone to heat related illness, from cramps to heatstroke; you should regularly check how they feel, and go for emergency treatment as soon as the symptoms show up.

Cold weather

Main article: Cold weather

See also: Ice safety, Snow safety, Winter driving

Polar nights in Finnish Lapland can be extremely cold.

Cold weather is an issue in winter sports, in wintertime in temperate areas, when visiting high mountains – even by driving over mountain passes – and year round in the Arctic and Antarctic. Cold weather is part of everyday life for the residents of these areas and for visits in the cities the cold is rarely a threat. Some preparations, like having adequate clothes, will make your visit much more comfortable, will let you stay outside for longer times, and may allow you to enjoy even quite severe circumstances. When venturing out on the countryside or out in the wilderness, or even driving along less busy roads, neglecting basic precautions can mean risking your life.

In places like Finnish Lapland, a sunny day at -10°C may turn into -25°C (+14 to -13°F) at night, which means much more clothing will be needed. In mountainous areas even more extreme changes are common. This means you should be aware of possible changes at least when returning to your base will take substantial time (do not count on taxis if you are returning when everybody else is). If there is wind, the cold may find its way through your clothing and windchill will add to the cold, such that at 10 m/s (22,5 mph), -10°C (+15°F) will feel like -20°C (-5°F).

In temperatures near or below freezing, snowfall is usually possible. Snowfall or blowing snow can severely limit the visibility, like dense fog or worse. Snow and ice can hide dangers, such as clefts in the rock, and make a lake look like a field. While frozen lakes and rivers are often used for transport, weak ice is a severe risk. Snowfall can also severely hamper orientation. Without anything to guide them, humans have a natural tendency to walk in circles as their strides are slightly longer on one side. In heavy snow, this can make you totally lost and you'd be surprised to hear how many people died of exhaustion or exposure just a few kilometers from safety. A compass will help avoid this, just make sure you know the direction to a safe area you cannot miss (near the poles, note the difference between magnetic and geographic north). A map can of course allow more flexible options. A GPS may feel even better, but take care about it not dying from exhausted batteries, moisture or cold – a compass still makes a good backup.

When driving in a white-out (blizzard-like conditions with minuscule visibility), drive slower than the posted speed limit, but not too slow as to cause vehicles behind you hit you. In most places the roads will have some form of marker to follow. In daytime, use full headlights to make you visible for meeting traffic, in dark or dusk fog lights or low beam are much better, as the falling snow is not what you want to see. Do not pull over and park your car on the shoulder unless it is your last resort, as it can be hit by other traffic or snowploughs. Instead drive until you find a safer exit, such as to a small village. If you get stuck or park somewhere with no people, do not try to walk to safety, but stay in your vehicle for shelter and wait for help to arrive. The best option for white-out conditions is to not go out at all, but if you are caught in this scenario the best chances of survival are much like that of high fog.

Air pollution

Main article: Air pollution

Air pollution, including smog, is a nuisance in some large cities, worsened when air is dry and stagnant. Visitors with asthma or other medical conditions might be affected even by mild air pollution, and when it gets severe, it's unhealthy for everyone. In case of severe smog or an existing medical condition, it is unwise to get out and travel; if you have to, wear an N95 mask. Look out for weather advisories, and for long-term stays, consider buying an air filter.

See also

• Arid region safety
• Earthquake safety
• Motion sickness
• Outdoor life
• Sunburn and sun protection
• Tornado safety
• Wildfires
• Weather records