Amazon Climate

Officially, being south of the equator, Brazilian seasons are:

Spring - 22 September to 21 December; 
Summer - 22 December to 21 March;
Autumn - 22 March to 21 June; and
Winter - 22 June to 21 September.

However since Manaus is only 3 degrees south, it’s fairly moot - and in any case, the two things you really need to know is how hot and wet it’s going to be (or not) and how high or low the river’s going to be. Below is a table with averages for Manaus, with extremes highlighted. You'll see  that there is no highlighting in the temperatures table, as the reality is that the average temperature is fairly constant throughout the year. You should note however that in fact it will feel much hotter throughout the dry season (August - October in particular), especially in the city centre. 

Rainfall	J	F	M	A	M	J	J	A	S	O	N	D
Total (mm)	260	288	314	300	256	114	88	58	83	126	183	217
Sun & RH	J	F	M	A	M	J	J	A	S	O	N	D
Sunshine (h)	114	88	99	112	149	185	214	225	156	171	141	131
Rel.Humidity %	86	87	88	87	87	83	80	77	77	79	81	85
Temperatures	J	F	M	A	M	J	J	A	S	O	N	D
Maximum	31	30	31	31	31	31	31	33	33	33	32	31
Minimum	23	23	23	23	23	23	23	23	24	24	24	24
Average	26	26	26	26	26	26	27	27	28	28	27	27
River levels	J	F	M	A	M	J	J	A	S	O	N	D
Metres amsl	22.3	23.4	23.6	25.2	27	28	27.7	26	21.7	18.2	18.6	20.4

You can see that there’s no great variation in temperature, and the temperatures don’t look so extreme - but how you’ll feel on the day is just as much a function of the high levels of humidity and how much sunshine there is, as the actual temperature itself. With this in mind, you can probably see that the months of July - December are hot and sunny, while those of January - June are hot and wet.   As far as river levels are concerned, low water occurs in October, and high water in June (in 2009 the river reached it’s highest on record at 29.77m a.m.s.l.).

The practical impact of this is that at lower water you will have more access to jungle and beaches, while at high water you will see more flooded forest (igapo). Fisherpeople will note that fishing is best at low water, and can be frustratingly difficult at high water, when most self-respecting fish have disappeared into the flooded forest.   Finally, we should note that the most dramatic weather is often in the period September - December, with hot sunny spells interspersed with crazy thunderstorms.