- Climate is what we expect. Weather is what we get. – Attributed to Mark Twain
Severe weather is the generic term for any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather vary depending on latitude, altitude, 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 storm.
The UN Office for Disaster Risk Reduction has much information on these and other natural disasters.
A cyclone, also known as a hurricane (in North America) or typhoon (in Asia and Oceania), is an organized rotating precipitation system packed with damaging winds and heavy rain. Unlike a tornado which usually inflicts damage only along a narrow path, a cyclone is generally spread out over a broad area; they can be hundreds of miles wide. Effects include but are not limited to: windstorm, very heavy rain which can lead to widespread flooding and mudslides, thunderstorms, and high waves.
This section covers only the dangers of cyclones for travellers on land. However, cyclones are also a major hazard at sea; see our article Cruising on small craft for basic advice and consult specialist publications for more detailed advice.
It is never advised to travel in an area where a cyclone is expected to occur because of its potentially devastating aftermath. If you are caught when a cyclone hits, you are strongly advised to seek shelter in a steady enough building and stay indoors until it is all over.
Cyclones form most often in tropical or subtropical regions, typically over water where high sea temperatures encourage their development. In these areas the buildings are often designed to withstand high winds; for example they may have strong shutters which can be closed when a storm is expected.
There is also a special type of storm called an extratropical cyclone which can happen outside the tropical regions or during the fall and winter months; these can cause great damage when they hit areas that are not prepared for them. For example, Hurricane Hazel in 1954 caused very extensive damage in Toronto when it reached there, even though by then it was a far weaker storm than when it made landfall in the Carolinas.
Areas where cyclones are common often have good warning systems, and visitors should know about those systems; see our destination articles for details. On the map, "RSMC" indicates a Regional Specialized Meteorological Center, the people responsible for such warnings.
The two regions with the greatest hazard — because they are more heavily populated — are the Western Pacific typhoon belt (zone 3 on the map) and the Atlantic hurricane belt (zone 1). However, other regions also get cyclones; see the map. In fact there is some risk anywhere that is near a large body of warm water.
There are several methods of rating the severity of cyclones; a widely-used one is the Saffir-Simpson scale which rates them based on sustained wind speeds. Ratings are 1 (some damage), 2 (extensive damage), 3 (devastating damage), 4 or 5 (catastrophic damage). Even a Category 1 storm should be taken seriously; it can easily kill you with flying debris or ruin a holiday by grounding planes, taking out power lines and so on.
For Category 4 or 5 storms, the US government site linked above says "Most of the area will be uninhabitable for weeks or months." This means the problems are enormous when such a storm hits a densely-populated region; you may have millions of people in an area that is basically uninhabitable.
The two worst cyclones in history both had sustained winds well over 300 km/h (near 200 mph, 85 m/s) at their worst; some suggest adding a Category 6 to describe them. Typhoon Haiyan hit the Philippines in late 2013, causing over 6,000 deaths and major damage across a wide area with estimated total damage around US$2 billion. Hurricane Patricia hit the West coast of Mexico in 2015 and caused much less damage; it had lost some of its force before making landfall, it hit a sparsely-settled region, and most of its fury quickly dissipated in mountainous terrain. Even so, it caused torrential rain over a wide area, about a dozen deaths, and damage estimated at $280 million.
The defining phenomenon in thunderstorms are lightning and the associated thunder, but dangers include heavy rain, strong gusty wind and possibly hail. Most lightning strikes from inside or between clouds, but some hit the ground, causing fires, electrical damage etc.
Thunderstorms result from rapid upward movement of warm, moist air, which form very high (even more than 20 km) cumulonimbus clouds. They are common in tropical regions during the rainy season, but can occur even in the Arctic.
You can estimate the distance to the lightning by knowing that while you see the lightning instantly, the sound travels a kilometre in three seconds. This is not too much help though: you can usually hear the thunder only within a few kilometres, which means you are too close to be safe.
Lightning tends to hit the highest object towering from the ground, so avoid hill tops and open areas and do not seek shelter under a high or lone tree. Get indoors if possible. Cars with enough steel (many modern ones are mostly plastic) form Faraday cages and are thus safe, but avoid driving. Avoid anything connected to power lines, phone lines etc., as electricity induced by a lightning strike can travel far along these. Swimming or taking a shower is dangerous for the same reason.
During a storm, unplug your electronic devices to avoid risk of having them fried by a lightning-induced power surge. If you plan to spend a lot of time in a high-risk area, get a surge protector. These are reasonably cheap devices that go between your computer or other valuable device and the wall socket; they do not give perfect protection, but they do greatly reduce risk.
In a semi-open area the safest position is at a distance about half the height from a tree or other high object – if the lightning strikes here, it will hit that object instead of you. The strong sound, electricity spread in the ground and pieces flying from the hit object are still dangerous. Parts of the lightning can even bounce off to people in the vicinity. Keep your feet together so that electricity in the ground does not travel between them through you. Spread out, so that if somebody gets hurt, the others are unharmed and ready to help.
Fog and smog
Surprisingly, fog is the most lethal kind of weather in some parts of the world, mostly because of traffic accidents. When driving a car in fog, limit the speed. If fog is very thick, you might need to wait it out. 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. With a GPS, remember it does not show other vessels, even a radar shows only some of them.
Smog is the combination of air pollution and fog that can occur in big cities, especially those with poor environmental protection, and also sometimes in suburban or rural areas near heavily-polluting factories, electric plants and refineries. Specific geographic features and environmental conditions, such as inversion (a warm layer of air sitting atop a cold layer of air and thus "blocking" all air beneath it from rising) may make certain places especially prone to smog. Smog is potentially deadly, especially for those with breathing problems. Some cities have taken drastic measures to combat smog in recent years, such as outlawing coal powered heating or drastically reducing the number of cars allowed to drive (even-numbered license plates one day and odd-numbered plates the other day, for example). Indeed, several cities once notorious for smog have now greatly reduced or all but eliminated it, but in places like China, Mexico City or Los Angeles, smog is still a major problem.
Intense rain with insufficient areas to absorb the water can result in flash floods. In the US the National Insurance Flood Program defines it as “rapid flooding of low-lying areas in less than six hours, which is caused by intense rainfall”.
Flash floods often occur where the soil is too dry to quickly start absorbing water, where the topography forces the water into narrow valleys, and where wetlands have been drained and streams walled. In some areas quite moderate rain much upstream can cause a flash flood with little warning.
Like in the case of a tsunami, immediately try to get to high ground when there is a flash flood warning. Try to keep a flotation device near you. When the flash flood strikes, don’t try to walk or swim — the current is stronger than you and already 6 inches (15 cm) of rapidly moving water can knock you down (not to mention items from trash cans to branches that may be carried by the water).
Driving is also a bad idea; two feet (60 cm) is enough to sweep your vehicle away, even less is enough to make steering impossible and probably also to stall the engine. You’re actually more of a sitting duck if you’re in a car and the water is rising. Open the car door if you can, if the water level is high enough to block the door, you'll have to roll down a window or break it to escape.
In areas where flash flood can be expected, avoid camping near streams and rivers (including wadis, dry riverbeds), which obviously are where most of the water will go, causing flooding. Be especially careful with canyons, from where it is difficult to escape and where flash floods often really are worth their name.
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 nice sunny day at -10°C may turn into a night at -25°C, 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. 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 -10°C will feel like -20°C.
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.