Like most of the Hawaiian Islands, Kauai has only two seasons:
"summer" between May and October and "winter" between October and April.

Depending upon locale:

Average temperature ranges from 71.3 (°F) to 79.1 (°F) at Lihue airport.
Average annual rainfall ranges from 35 inches at Poipu Beach to 444 inches at the summit of Waialeale (one of the wettest spots in the United States).

Monthly Averages

The monthly averages according to the information at The Kauai Marathon 2009:

J

F

M

A

M

J

J

A

S

O

N

D

High Temp f/c

78/26

78/26

78/26

80/26

81/27

83/28

84/29

85/29

85/29

83/28

81/27

79/26

Low Temp f/c

65/18

65/18

67/19

69/20

70/21

72/22

74/23

74/23

74/23

72/22

70/21

67/20

Rain in/mm

6/150

3/85

4/106

4/89

3/80

2/43

2/54

2/45

2/60

4/112

5/138

5/131

Winds mph/kmh

12/19

12/19

13/22

15/23

13/21

14/22

15/23

13/22

12/20

12/19

13/21

12/20