Technically, '0' is neither +ve or -ve. If anyone could be bothered then it should go like this. -3, -2, -1, 0, +1, +2, +3. Normal notation though leaves out the +.
there is no such thing as -0. numbers such as 1,2,3 etc mean "1, 2 or 3 more than 0" and -1,-2,-3 mean "1,2 or 3 less than zero". So no, you cannot have -0. It is likely to be, as already stated, a rounding of numbers thing so the actual temp was probably -0.4, -0.3 etc (not -0.5 cos then it would have been rounded up to -1)