Threshold numpy array, find windows

Here's one way:

# Create a mask
In [42]: mask = (a[1] >= 4)
# find indice of start and end of the threshold 
In [43]: ind = np.where(np.diff(mask))[0]
# add 1 to starting indices
In [44]: ind[::2] += 1
# find and reshape the result
In [45]: result = a[0][ind].reshape(-1, 2)

In [46]: result
Out[46]: 
array([[52, 54],
       [59, 61]])