Many travelers plot their trips around the weather, heading to destinations when the sun's supposed to be shining.
But in this El Nino year, there has been no telling what the weather will be in parts of the United States and beyond.
El Nino-spawned storms already have brought major flooding to parts of California and deadly tornadoes to Florida.
In some areas of Peru and Bolivia, heavy rains linked to El Nino have caused flooding and a breakdown in sanitary conditions that have been blamed for an increase in cholera and other diseases.
On the other side of the Pacific Ocean, in Indonesia, El Nino's undermining of weather patterns has made the monsoon rains less intense and forest fires are burning again, sparking fears and memories of the choking, smoky haze that cloaked much of Southeast Asia last summer and fall.
Obviously disastrous for residents affected by flooding and other El Nino offshoots, the weather conditions also have dampened tourism in some places, including California, where some travel businesses have complained of a drop in customers in recent weeks.
El Nino, the naturally occurring culprit behind all this, is a massive Pacific Ocean current that can occur every few years.
Under normal conditions in the Pacific, warm water flows toward Asia and cooler water rises along the coast of North and South America. With an El Nino current, the trend reverses and the warmer water flows toward the Americas. That affects rainfall, trade winds and even the high-altitude jet stream.
As it has this year, El Nino's effects can include a dramatic increase in rain across the southern United States and South America and droughts in Indonesia and Australia.
Like most weather, it's difficult to predict how long El Nino's effects will last or how intense they will be. But here's a look at what El Nino has caused in recent weeks and how it can affect travelers.
Sunny California it hasn't been in recent weeks, with torrential rainfall (and some mud slides and flooding) in the Sacramento Valley, the Bay Area and down the coast to Los Angeles and San Diego.
Though the state was drying out recently, tourists have stayed away this winter. The economic impacts haven't been quantified yet, but Jack Kyser, chief economist for the Economic Development Corp. of Los Angeles, told the Associated Press: "People have lost money, that is the bottom line. And they're losing business that they can't make up."
One hard-hit area is Big Sur on the central California coast south of Monterey. Seventy miles of Highway 1, the scenic coast-hugging highway, has been closed by flooding and mud slides that have damaged the road from about five miles south of Carmel to just north of Hearst Castle. The stretch of highway likely will remain closed until late April. (Hearst Castle, a major tourist destination, is still accessible from the south.)
There is a silver lining in the rain clouds, though, including lots of snow in the mountains for skiers and snowboarders and an unusually profuse bloom of desert wildflowers in wilderness parks such as Death Valley, Joshua Tree and Anza-Borrego (east of San Diego), sparking tourism there.
In neighboring Arizona, where rainfall also has been much heavier than usual this winter in the deserts (with heavy snow in the mountains) there has been concern about a recurrence of hantavirus.
The pulmonary disease is transmitted to humans via deer mice (through their droppings). The rodents thrive in rainy years when the deserts and arid uplands blossom with vegetation and insects on which the deer mice depend.
The last outbreak of hantavirus infections in humans in the Four Corners area of the Southwest occurred in 1993 after a rainy El Nino winter. A case of hantavirus in Arizona's Apache County was reported in January; health authorities are watching the situation.
In central Florida, more than a half-dozen tornadoes linked to weather patterns changed by El Nino devastated some communities and killed about 40 people last month.
Tourism wasn't directly impacted by the twisters -- but Universal Studios in Orlando has postponed the April opening of its thrill ride Twister, which re-creates natural-disaster scenes from the movie of the same name.
In Indonesia, thousands of bush and forest fires burned out of control on the islands of Borneo and Sumatra last summer and fall, cloaking those islands and parts of Malaysia, Singapore and even southern Thailand in choking smog.
Airports closed, respiratory infections were widespread among residents, schools and businesses were hampered, and tourists stayed home.
Indonesian forests long have been cleared by burning by timber companies (who want to create tree plantations) and farmers, but the El Nino-caused drought last year let the fires rage out of control. Some environmental groups recently have estimated last year's crisis caused more than $1 billion in damage in Southeast Asia, according to reports by the British Broadcasting Corp.
Fires are burning again in parts of the island of Borneo following El Nino-caused unusually dry conditions, not the usual drenching monsoon rains. Although the fires are limited now and with only localized haze, there's still some fear of a repeat of last year's massive pall of smog and authorities in Indonesia, Singapore and Malaysia are concerned.
The South American nation of Peru has suffered severe weather linked to El Nino, including heavy rains, floods and mud slides, particularly along the coast. Some roads have been washed out.
The Peruvian government expects the El Nino-caused climatic conditions to continue until June, said the U.S. State Department in a recently issued travel advisory.
The State Department added: "Although most tourist travel has not been affected by the severe weather, its effects should be considered in planning travel in Peru during this period."
There also has been an increase in cases of cholera, malaria and dysentery which are blamed on a breakdown in sanitation, caused by heavy rains and flooding.
Bolivia also has had unusually heavy rains, especially in the Andean region, and there have been reports in Bolivian media of an increase in cholera cases particularly in the city of El Alto, Bolivia's fourth-biggest city.
Two of the best sources for information on international travel and health conditions, including El Nino's impacts, are the U.S. State Department's travel-advisory service and the federal Centers for Disease Control and Prevention.
For the State Department's travel information on countries around the world, phone the automated line: (202) 647-5225 or check its Web site: http://travel.state.gov (click on travel warnings).
For the Centers for Disease Control, phone its International Travelers' hot line at (888) 232-3228 or check its Web site: http://www.cdc.gov (click on travelers' health).
A particularly useful Internet site for updates on fire/haze conditions in Southeast Asia is the British Broadcasting Corp.'s Web page: http://news.bbc.co.uk/hi/english/world/asia-pacific
To check on road (and storm) status in California, phone the state tourism office at (800) 862-2543 or check its Web site: http://gocalif.ca.gov/