I have created a panda DataFrame with columns as: "Radius", "Zenith", "Tem". Now I want to retrieve all the temperature values based on the zenith values in the DataFrame. Next thing is to fetch the max and min value of "Tem" for each value of the zenith angle. 

I have written the below code block but it is throwing me **ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().**

```
theta = np.around(np.arange(0,90,90/151),6)
a = np.ndarray(shape=(1,2), dtype=float, order='C')

for i in theta:
    while (final_data[final_data['zenith'] == i]):
        T = final_data[['Tem']].values
        T_max = np.max(T)
        T_min = np.min(T)
        T_range = np.row_stack((a,[[T_max,T_min]])) 

```