Understanding the data

Units

Radar reflection (Z) is usually expressed logarithmically which results in the dBZ unit. SMHI converts radar reflection to rainfall intensity (R in mm/h) according to the following formula:

Z=10log10(200R1.5)

Reflectivities below 5 dBZ are usually not counted as precipitation (these could for example be birds or insects), while reflectivities higher than 70 dBZ are less likely in the Nordic climate. The following is a table containing some values converted from dBZ to mm/h.

Z5101520253035
R0.060.140.290.631.362.926.30
 
Z40455055606570
R1429631362926301357

Images

The radar images are delivered as transparent PNG files and as 8-bit GeoTiff.
Both GeoTiff and PNG images are in SWEREF99 TM projection with corner coordinates:
bottom left : 5983984 126648
top right : 7771252 1075693

GeoTiff Images

The GeoTiff data files contain values between 0-255.
0 = No echo. The measuring point is inside the radar coverage area, but no echo is reported.
255 = No data. The measuring point is outside the radar coverage area.
Other values in the file may be translated to dBZ by applying an offset and a gain. In these files the offset = -30 and gain = 0.4. The value of the dBZ = pixel value * Gain + Offset

PNG images

The RGB values in the PNG images correspond to the following dBZ values:

RGB dBZ
rgb(54,54,54)-29
rgb(55,55,55)-28
rgb(56,56,56)-27
rgb(57,57,57)-26
rgb(58,58,58)-25
rgb(59,59,59)-24
rgb(60,60,60)-23
rgb(61,61,61)-22
rgb(62,62,62)-21
rgb(63,63,63)-20
rgb(64,64,64)-19
rgb(65,65,65)-18
rgb(66,66,66)-17
rgb(67,67,67)-16
rgb(68,68,68)-15
rgb(69,69,69)-14
rgb(70,70,70)-13
rgb(71,71,71)-12
rgb(72,72,72)-11
rgb(73,73,73)-10
rgb(74,74,74)-9
rgb(75,75,75)-8
rgb(76,76,76)-7
rgb(87,87,87)-6
rgb(97,97,97)-5
rgb(108,108,108)-4
rgb(118,118,118)-3
rgb(129,129,129)-2
rgb(139,139,139)-1
rgb(150,150,150)0
rgb(160,160,160)1
rgb(171,171,171)2
rgb(181,181,181)3
rgb(192,192,192)4
rgb(0,50,255)5
rgb(0,70,255)6
rgb(0,90,255)7
rgb(0,110,255)8
rgb(0,130,255)9
rgb(0,150,255)10
rgb(0,170,255)11
rgb(0,128,0)12
rgb(0,138,0)13
rgb(0,148,0)14
rgb(0,158,0)15
rgb(0,163,0)16
rgb(0,168,0)17
rgb(0,173,0)18
rgb(0,178,0)19
rgb(10,208,10)20
rgb(10,218,10)21
rgb(10,228,10)22
rgb(10,238,10)23
rgb(10,248,10)24
rgb(255,255,15)25
rgb(255,246,15)26
rgb(255,238,15)27
rgb(255,229,15)28
rgb(255,220,15)29
rgb(255,200,0)30
rgb(255,180,0)31
rgb(255,160,0)32
rgb(255,140,0)33
rgb(255,120,0)34
rgb(255,35,35)35
rgb(255,15,15)36
rgb(255,0,0)37
rgb(235,0,0)38
rgb(215,0,0)39
rgb(195,0,0)40
rgb(175,0,0)41
rgb(155,0,0)42
rgb(135,0,0)43
rgb(115,0,0)44
rgb(175,0,175)45
rgb(184,0,184)46
rgb(193,0,193)47
rgb(202,0,202)48
rgb(211,0,211)49
rgb(219,0,219)50
rgb(228,0,228)51
rgb(237,0,237)52
rgb(246,0,246)53
rgb(255,0,255)54
rgb(0,255,255)55
rgb(13,255,255)56
rgb(26,255,255)57
rgb(39,255,255)58
rgb(51,255,255)59
rgb(64,255,255)60
rgb(77,255,255)61
rgb(90,255,255)62
rgb(102,255,255)63
rgb(115,255,255)64
rgb(128,255,255)65
rgb(141,255,255)66
rgb(154,255,255)67
rgb(166,255,255)68
rgb(179,255,255)69
rgb(192,255,255)70