Add new value to row based on content of previous row

I'm looking to do what I thought would be a simple task. I have a large dataset that is created on various if conditions in python. Such as:

    for index, row in df.iterrows():
        if int(row['Fog_Event']) >= 4:
            df.at[index, 'Fog_Event_determine'] = 'Fog'
        elif int(row['Fog_Event']) == 3:
            df.at[index, 'Fog_Event_determine'] = 'Dust'
        elif int(row['Fog_Event']) == 2:
            df.at[index, 'Fog_Event_determine'] = 'Dust'
        else:
            df.at[index, 'Fog_Event_determine'] = 'Background'
            continue

These work perfectly to do what I want them to do, but there are some issues with the final analysis of the data. To fix the issue I need to add a running threshold value that is based on the results of the previous row. So if Row 1 >=4: then I want row 2 to be +1.

I tried this:

df['Running_threshold'] = 0

for index, row in df.iterrows():
    if int(row['Fog_Event']) >= 4:
        df.loc[index[+1], 'Running_threshold'] = 1
    else:
        continue

But this only adds a 1 to the second row of the index, which makes sense upon looking on it. How can I ask python to add a +1 to every row after the condition ['Fog_Event']) >= 4 is met?

Thank you.

Solution

do if logic using np.where() as it's more efficient than looping
first set Running_threshold to zero or one based on previous row condition
the cumsum() it to get running total as you noted

df = pd.DataFrame({"Fog_Event":np.random.randint(0, 10,20)})

df = df.assign(Fog_Event_Determine=np.where(df.Fog_Event>=4, "fog", np.where(df.Fog_Event>=2, "dust", "background"))
        , Running_threshold=np.where(df.Fog_Event.shift()>=4,1,0)
).assign(Running_threshold=lambda dfa: dfa.Running_threshold.cumsum())

output

 Fog_Event Fog_Event_Determine  Running_threshold
         9                 fog                  0
         3                dust                  1
         2                dust                  1
         9                 fog                  1
         7                 fog                  2
         0          background                  3
         4                 fog                  3
         7                 fog                  4
         6                 fog                  5
         9                 fog                  6
         1          background                  7
         6                 fog                  7
         7                 fog                  8
         8                 fog                  9
         6                 fog                 10
         9                 fog                 11
         6                 fog                 12
         2                dust                 13
         7                 fog                 13
         8                 fog                 14