Africa

In October it's autumn in North Africa, and low pressure systems begin to affect the Mediterranean, the Atlantic coast of Morocco and the Canary Islands, particularly in the second half of the month. In the Red Sea, the temperature begins to be acceptable, especially in the second half of the month. In Northern Tanzania and in Kenya, the short rains season begins. In the Southern Hemisphere it's spring, and the heat begins to be remarkable in many tropical areas.
Where to go: Addis Ababa, Antananarivo, Asmara, Cape Town, Johannesburg, Marrakesh, Maseru, Santa Cruz De Tenerife (first half), Tunis

Asia

In October it's autumn and it gets cold in the northernmost areas; the Indian monsoon begins to retreat, and the cooler north-east monsoon takes his place, but it brings rainfall in the exposed slopes. You can go in temperate areas, including the capitals of China and Korea.
Where to go: Amman, Ashgabat, Baku, Beijing, Beirut, Bishkek, Damascus, Dushanbe, Jerusalem, Guangzhou (second half), Hong Kong (second half), Kathmandu (second half), Nicosia (second half), Sana'a, Seoul, Shanghai, Tashkent, Tbilisi (first half), Tehran, Thimphu (second half)

North and Central America

Autumn advances in Canada and the United States. In the Gulf of Mexico and in Central America, it's still hot, muggy and rainy, with some risk of hurricanes. You can visit the intermediate areas, which are more temperate.
Where to go: Los Angeles, New York (first half), San Francisco (first half), Washington (first half)

South America

In October, it's springtime in the Southern Hemisphere. It's the dry season in Guyana and in North-eastern Brazil, even though it's hot.
Where to go: Buenos Aires, Cayenne, Georgetown, Montevideo, Paramaribo, Rio de Janeiro, Salvador, Santiago

Europe

The temperatures are still pleasant in Southern Europe, sometimes even hot, but we can not exclude the first waves of bad weather, although they are more likely in the second half of the month. The weather is by now cold and dull in Northern Europe, where during the month the first snowfalls may occur.
Where to go: Athens (first half), Valletta (first half), Lisbon (first half), Madrid (first half), Rome (first half)

Oceania

It's spring in the Southern Hemisphere, and it's generally a pleasant season in South Australia, and also in the east coast.
Where to go: Adelaide, Melbourne, Perth, Sydney