Dear all numpy users, I have 10 90X720 arrays. let's say they are in a list 'a' with each element a 90X720 numpy masked array. then I create a new empty ndarray: data data=np.empty([10,90,720]) ##first I store all the 10 ndarray in a 10X90X720 array: for i,d in enumerate(a): data[i]=a data.shape=(10, 90, 720) then I use data_av=np.ma.average(data, axis=0) to get the average. The strange thing is, I don't have any 'nan' in all the 10 90X720 array, but I have nan value in the final data_av. how does this come? In [26]: np.nonzero(np.isnan(data_av)) Out[26]: (array([ 0, 0, 2, 2, 3, 5, 5, 6, 6, 6, 6, 7, 8, 8, 8, 9, 10, 10, 10, 11, 11, 12, 13, 13, 14, 17, 17, 19, 22, 22, 44, 63, 64, 64, 67, 68, 71, 72, 73, 76, 77, 77, 78, 79, 80, 80, 81, 82, 82, 84, 85, 85, 86, 86, 87, 87, 88, 89, 89, 89]), array([159, 541, 497, 548, 90, 97, 170, 244, 267, 587, 590, 150, 126, 168, 477, 240, 271, 277, 588, 99, 179, 528, 52, 256, 230, 109, 190, 617, 377, 389, 707, 539, 193, 361, 262, 465, 100, 232, 206, 90, 87, 93, 522, 229, 200, 482, 325, 195, 239, 228, 159, 194, thanks, Chao -- *********************************************************************************** Chao YUE Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL) UMR 1572 CEA-CNRS-UVSQ Batiment 712 - Pe 119 91191 GIF Sur YVETTE Cedex Tel: (33) 01 69 08 29 02; Fax:01.69.08.77.16 ************************************************************************************