"You should have been here last week/month/year!" is the knee-jerk reaction I've come to expect - and dread - in every season of the year, somewhere around the globe. When I spent a large chunk of January turning rusty on the Costa del Sol, it was "freak weather"; when I had to flee south of Marrakech in December as torrential floods had wiped out several roads, part of the railway line and hundreds of mud brick houses, they claimed it was unbelievable. Then there were the winter wash-outs in Tahiti, or the time I spent during an Australian summer being cling-filmed in dust by the willy-willies - the violent tropical storms that can hit the north coast. Sure, Down Under is the place to escape our winter, but only in some places Down Under. Even Melbourne's climate can change as much as four times in the course of one day, and Sydney is regularly dowsed, while summer monsoons can bring down rainfall in buckets on the hot and steamy Barrier Reef holiday playgrounds.

In order to be a really slick traveller, one who knows to avoid Mal in June, Mali in August and Mah in December, you would need to carry around the weather charts in the Directory; but there are a few rules that have helped me decide whether to pack my swimsuit and beach mat or brolly and thermals.

For a start, any temperature above about 27C in humid Turkish-bath zones or 30C in dry climes precludes vigorous exploration and activity for most, while anything below 17C means lots of walkabouts and sightseeing but no beach.

The sad sight of winter holiday-makers huddled over Trivial Pursuits in all too many rainswept and deserted Mediterranean resorts has taught me to distrust the Med's winter weather. Sizzling or drizzling, it's always a gamble; and there's a particular cloud that arrives in southern Europe around the third week in October (a month later in Cyprus, Lebanon, North Africa and other southern Mediterranean resorts) that sends a message warning beach-dwellers that their time is up.

Cold, hard statistics reveal that Gibraltar or Haifa have nearly twice as much rainfall as London in January, which in turn is drier than the Algarve, Monaco and even Cyprus and Malta, not to mention Istanbul in February.

Even in that so-called winter sun Mecca, Tenerife (on the same latitude as Delhi and parts of Florida), a daily cloud arrives at 2 pm at the lush northern resort of Puerto Cruz, while on the other side of the island the sun shines evenly all day on the bare slopes and concrete jungle of Playa de las Americas - which is why the north is lush and the south barren. Eilat, Israel's winter offering, can also be grey and grizzling in January, though at least you can get away from the weather by exploring the resort's technicolour wonders below sea level.

But if it's winter sunshine you're after, you'll have to head at least as far south as Luxor, while the cheapest guaranteed hot stuff can be found in Gambia, followed by high probabilities in Goa and the Kenyan coast. Alas, the Caribbean is at its most pricey and exclusive during our winter.

Wherever you are travelling in desert conditions, you'll find surprisingly large diurnal variations with cool, even cold, snaps at night and at dawn. But on and around the equator you'll need to overdose on anti-perspirant as it is nearly always hot and humid. There are so few seasonal variations there that you find yourself dreaming wistfully of spring flowers in the Lake District, the autumn wine harvest in Provence or fall in New England. And just to remind you of the vagaries of weather, The Times carried a picture of New England under deep snow at the beginning of April this year.

>From trial and error in the tropics, I've at least learned that December to February are the most tolerable months in which to tackle Bangkok, but that in the same months it is so steamy and wet in Singapore you won't ever get your smalls dry unless you have a hotel room with air conditioning.

Weather in the Far East, generally, can present tricky problems, even within the same or neighbouring countries. Backpackers who have boned up on their weather charts will know that the May rains signal the end of the tourist season in Phuket and that they must migrate to Koh Samui, moving on from there to Bali for their fun in July, August and September. Malaysia, and even relatively small islands such as Sri Lanka, have considerable climatic variations - bi-annual heavy rainfalls often happen at night, but winds can make the sea rough and dangerous for bathing during the day. And remember that in both of these countries - and others - where you can move from steamy lowlands to cool highlands in a few hours, you need to pack sweaters for evenings alongside your sun hat for midday.

The rains, both long and short, fall somewhere in Africa throughout the year: from December to February in Zimbabwe and southern Africa, March to May in Kenya and Tanzania; May to August in Cape Town. From November to February, just when holiday-makers are being lured to the Seychelles and Mauritius by the eternal sunshine on the glossy brochure covers, these Indian Ocean playgrounds are getting their biggest annual soaking - whereas they are both ideal escapes during our summer months, when the Med resorts have become blowsy with tourists.

East Africa's safari season runs during the dry months from December to April, when the grasses are low for better game spotting, and the animals will seek out and congregate around water-holes rather than dispersing through the bush. But even here, alas, the rains don't always watch the calendar, and timing can often be a matter of luck. It took me several false starts before I caught the beginning of the migration in Serengeti - but the brief moments before dawn waiting for the sun to rise over the flat endless plains, surrounded by the shadowy silhouettes of millions of wildebeest on their starting blocks, are heart-stopping memories that will last a lifetime.

Even if you get your timing wrong, there are other compensations: some countries are almost reborn with changes of weather, none more so than Africa. Unexpected early rains in Botswana once turned my dry and dusty campsite into a Garden of Eden overnight as we awoke to a Jungle Book fantasy of monkeys chattering in the trees, impala playing in the bush and dragonflies and butterflies dazzling us with their pirouettes.

I witnessed as one of the driest places on earth, the Namib Desert, was almost transformed into an Irish meadow by rain after several years of drought, so heaven knows what the unique species that have adapted to this burning environment must have made of it. The desert onyx has developed a brain irrigated by a special network of veins that enables it to withstand fiery temperatures, while the dune beetle does handstands each morning to let droplets of Atlantic fog roll down into its mouth. More soberly, in such communities, you learn to realise that the weather is a matter of death or life.

Moving across from Africa to the Atlantic, anyone who believes that America's weather is a breeze may be in for some shocks. The wettest place I've been is Hawaii, where I was stuck for days waiting for a helicopter trip to photograph the volcanoes, of which I got no glimpse. The hottest I've felt was in the humid concrete canyons of New York, though body temperatures plummeted to sub-Arctic levels in the icy air conditioning of the Big Apple's restaurants, hotels and even cabs.

It was Mark Twain who said "The coldest winter I ever spent was summer in San Francisco." British holiday-makers should take note of this, before flocking in droves to visit Florida when it's almost guaranteed to be humid and wet, the Everglades' mosquitoes are at their most bloodthirsty, and there's a real likelihood of hurricanes as well.

After 20 years of travelling from Alaska to Zanzibar, writing hundreds of travel articles and answering thousands of questions on radio and television phone-ins, I've learned to identify the hardy perennials: "How much?", "What are the beaches like?", "Is it safe and can you drink the water in...?"

I express my opinion on a few general rules, pull out some statistics and weather charts, and then get myself off the hook with warnings that little complications such as the meltemi, the mistral or the sirocco winds can make a mockery of the most reliable Mediterranean temperature statistics; that you can get a deep suntan north of the Arctic Circle, or catch pneumonia in the desert - in other words, that the world's weather can be astonishingly unpredictable.

What would we have to talk about if it wasn't?