Long, sunny days and warm tropical nights are an essential part of cruising.  So cruise ships tend to follow nice weather from one hemisphere to the next.  When the weather starts to change for the worse, they relocate to a sunnier spot.

These voyages in which a ship moves from one region of the world to another are called repositioning cruises.  And because they are less popular than other cruises, they are often terrific cruise bargains.

Most take place in the spring or fall and last anywhere from one to three weeks.

Repositioning cruises usually have a lower per diem or daily rate than conventional cruises.  And recently, those rates have been as low as $45 per person, per day. 

When you consider that the cruise fare includes transportation, lodging, meals, and entertainment, repositioning cruises are one of the best travel deals out there.

Because these cruises aren’t usually booked to capacity, you’ll stand a better chance of getting a cabin upgrade.  And you’ll also find shorter lines at the buffet, and have an easier time getting reservations in the specialty restaurants or spa.

Most people take cruises to visit the glamorous port cities that they’ve always dreamed about.  But many are surprised to discover how much they enjoy their days at sea.

While it’s certainly exciting to go ashore and see the sights, it’s also hard work.  To maximize your time in port, you usually have to get up and out early.  If you’re taking a shore excursion, you’ll be on a pretty tight schedule.  And you often won’t return to the ship until late afternoon.

Sea days are just the opposite, and most repositioning cruises have several.  You can sleep in, have a leisurely breakfast in your cabin or in one of the restaurants, and then spend the day relaxing.  There are plenty of activities on board.  But you won’t feel like you missed out if you just loaf around and do nothing.

Another benefit to repositioning cruises is that they often call at ports you wouldn’t ordinarily get to visit.  Places like the Canary Islands, the Cape Verde Islands, Greenland, Tonga, and Samoa are often part of the cruise itinerary.

Because repositioning cruises are less attractive to people who want to call at lots of ports, some cruise lines provide cultural programs and seminars or a special theme such as golf or big band music to make them more appealing.

From late August to early October, most of the ships that have been sailing Scandinavia, the British Isles, and the North Sea leave the area and head for warmer destinations.

Those that sail from Scandinavia often make a couple of port calls in the British Isles before entering the North Atlantic.  Then, they frequently stop in Iceland and Greenland before visiting Newfoundland and Nova Scotia.

These cruises are between eleven and eighteen days in length, with five or six sea days.  And most end in New York or Boston. 

Princess, Holland America, Royal Caribbean, Crystal, and Celebrity all offer repositioning cruises that cross the North Atlantic.

Two European cruise lines -- Costa and MSC Cruises -- reposition their ships from the North Sea to the Mediterranean.   They call at several ports in Portugal and Spain before disembarking passengers in Italy.

The days get shorter and the Gulf of Alaska starts to get pretty rough by the middle of September.  So most of the ships that have spent the summer cruising there reposition.

The Merchant Marine Act of 1920, which is also known as the Jones Act, prohibits cruises that begin in U.S. ports from ending in U.S. ports, unless another country is visited in between.  For this reason, Alaska repositioning cruises that end in a U.S. city, usually depart from Vancouver, British Columbia.

As a rule, repositioning cruises are more flexible than other cruises, and you can often embark or disembark in more than one place.  This is especially true of Alaska repositioning cruises where you can board in Vancouver and disembark in Seattle, San Francisco, or Los Angeles.

If you have the time and the inclination, you can even stay aboard and transit the Panama Canal.  These voyages usually make a few stops in Mexico and the Caribbean before disembarking passengers in Fort Lauderdale.

The trans-canal cruises are usually around sixteen days, and because that’s a popular destination, they aren’t usually discounted as much.  If you’d like to see some of Costa Rica, Guatemala, Columbia, and Panama, these are your best bet.

Holland America, Princess, Royal Caribbean, and Norwegian all offer these cruises.

Cruisers who love sea days can make a bee-line from London to New York about Cunard’s Queen Mary 2.   Christened in 2002, the QM2 offers the best of both worlds -- old-school glamour and state-of-the-art amenities, including a 20,000-square-foot Canyon Ranch Spa.  This is one of the few crossings with no stops.

Several other cruises leave Mediterranean ports bound for Fort Lauderdale, the Caribbean, or South America.  These often call at the Canary Islands and Funchal, Madeira. 

MSC, Seabourn, Holland America, Windstar, and Sea Dream Yacht Club offer trans-Atlantic voyages in the fall. 

If time and money are no object, Queen Mary 2 will take you pretty much wherever you want to go.  In early January, she sail from New York to Dubai, calling in Lisbon, Rome, Cairo, and Muscat en route. 

You can disembark in Dubai or continue on to Singapore, visiting Mumbai, India, Phuket, Thailand, and Kuala Lumpur, Malaysia along the way.  Still have time on your hands?  Continue on to Hong Kong, seeing Bangkok and Viet Nam. 

Not done yet?  Stay aboard another 23 days and call at Japan, Guam, Papua New Guinea, the Whitsunday Islands, and New Zealand before arriving in Sydney.  From there, QM2 stops at Adelaide, Perth, and Mauritius before arriving in Durban, South Africa.

To close the loop, stay aboard and visit Rio de Janeiro, Barbados, and Fort Lauderdale before disembarking in New York 108 days later.

Because you’ll be going to one city and returning from another, you’ll need what’s called an open-jaw air ticket.  These can be expensive, so figure out your airfare before you book your cruise.  Otherwise, that great cruise bargain may not be such a good deal after all. 

Most cruises line can help you arrange your air, and several of them include airfare -- and sometimes even an overnight -- in the package price.