Think in Manaus, for Amazon tours. The easiest way to get a quick and satisfactory experience of the Amazon begins with a flight to Manaus. If you want a good tropical hotel, or a tropical resort, think in this city.
Manaus, itself, is an oddity, reflecting unreasonable dreams of industrialization of Brazil. The city has grown, during part of the twentieth century, as a Free Trade Zone and as an important electronic industry centre of Brazil. But it is also much more, nowadays. It is at the very heart of the Amazon, and its outskirts plunge deeply in the jungle. Manaus is just a small and outside point in the immense surrounding rain forest, and an excellent gateway to visit the river and the forest.
Don't worry about the organization of local Amazon tours before arriving to Brazil. Once in Manaus, you can easily arrange plenty of trips, through your hotel and local agencies: visits to treks of the rainforest, to caboclo settlements, to water lilies, to the meeting of the waters (a huge natural phenomena, developed for more than six miles, just a few miles outside Manaus, showing the birth of the official named Amazon river through the confluence of the waters of the Negro river - with its darker and much heavier waters - with the quicker and silver waters of Solimoes river), etc.
Though humid and wet, the Amazon - as Brazil, in its whole - is not as hot as often imagined. There are very hot days in the dryer season, that can turn your Amazon tours less agreeable, but average temperature ranges from 23-33? C (73-92? F). Near the river, at night, the temperatures can even drop significantly. There are cold nights, on the wetter places.
The average daytime temperature in the rainy season is in the mid-80? F (30? C), with a fall of 10 to 20? F (7 to 12? C) at night. In the dry season the average temperatures are 5? F (2-3? C) higher. Those are, of course, only references for central regions as Manaus.
Though precipitation varies significantly within the Amazon (near the Andean region, in the eastern zones of Amazon, precipitation doubles Manaus region) it is usual to consider two distinct periods: the rainy season, and the dry season.
In the central zone of the Amazon (Manaus) the rainy season begins in December and lasts till mid-May. The dry season involves the other months: June-November.
For other non-central regions the calendar should be anticipated (southern regions) or postponed (northern regions) one to two months.
All these standards are, anyway, just averages or references. Recent alterations in global climate and phenomena like El Nino disrupt usual patterns.
When to go
The high periods, with more tourists, are July-September and December-January. These periods are within the low water season (July-February), and have significant advantages for your Amazon tours: less rain (sub-period before December), less mosquitoes, easier observation of aquatic life in lakes, better time for fishing or swimming.
But there are some disadvantages also: temperatures are higher (specially before December), some waterways are more clogged with vegetation, limiting access to wildlife zones; the forest life is less visible except for aquatic life in isolated lakes (the best period for viewing wild life, in Manaus region, is March-June; this is also a cooler period, and a nicer one for your Amazon tours: waterside plants such as acacias, morning glories, or cecropias, flourish at these months).
Some people prefer transitional periods (October and November; and May and June, in the region of Manaus), where you may have some advantages of the rainy and the dryer periods. But there aren't true rules on that issue. For most people, the more determinant factor is the rain. December to May period can have many rainy days, and that can spoil many planned Amazon tours.
Do you need a guide? What are the risks?
How risky is travelling in the Amazon?
The answer is easy: as risky as you wish. You can't face your Amazon tours as if you where in Central Park. You must follow some basic rules, and not despise or minimize your guide tips and advices. Some regions of Amazon are still truly wild. Just be careful and tactful and you will not incur in any special risk.