Africa
In January, in Northern Africa it is winter: it's a good time to visit the Sahara, which is mild and sunny during the day, even though it can get cold at night. South of the Sahara, the weather is hot; around the Equator it's hot as in the rest of the year, but you can go at high alitudes, and where it does not rain too much. South of the Equator it's summer, but you can go where the weather is not too hot.
Where to go: Abidjan, Accra, Addis Ababa, Bamako, Cape Town, Conakry, Dakar, Cairo, Johannesburg, Kampala, Lagos, Monrovia, Ndjamena, Nouakchott
Asia
In January, the dead of winter, it's very cold in central and northern Asia. In South Asia it's warm or even hot, and the north-east monsoon prevails, bringing fine weather, except on the slopes that are directly exposed to the winds coming from the sea.
Where to go: Bangkok, Colombo, Dhaka, Doha, Dubai, Ho Chi Minh City, Karachi, Kuwait, Mecca, Manila, Mumbai, Muscat, New Delhi, Phnom Penh, Singapore, Taipei, Vientiane, Yangon
North and Central America
In North America, it's midwinter, and the climate is cold in most of Canada and north-central United States. In Florida, the temperature is pleasant, although sometimes it can get cold, at least in the north-central part. In Central America it's hot and sunny, except the slopes exposed to the northeast trade winds, which are rainy.
Where to go: Guatemala City, Havana, Mexico City, Miami, San José, San Salvador, Santo Domingo, Tegucigalpa
South America
In January, it's summer in the Southern Hemisphere, which means that it's hot and there are widespread rains at tropical latitudes. The northernmost Andean areas are, however, in the least rainy season, while the Pacific Coast is sunny and not too hot.
Where to go: Bogotá, Caracas, Lima
Europe
January is the central month of winter. The north and east of Europe are freezing cold, and also Central Europe can sometimes be below freezing and under the snow. In Northern Europe the day is very short and the sun sets very soon. The countries having a Mediterranean climate are milder, but may experience waves of bad weather with rain and wind.
Oceania
It's summer in the Southern Hemisphere, which means that at tropical latitudes it's hot, it rains and tropical cyclones can pass. In Australia, in the south the temperatures are generally acceptable, but sometimes waves of scorching heat can occur. On the contrary, it's the best season in New Zealand.
Where to go: Auckland, Wellington