Magnitude of the electric field is 
Explanation:
In this problem, the length of the cloud (horizontally) is a few kilometers, while here we want to evaluate the electric field it produces just a few meters below the cloud.
Therefore, we can approximate the cloud as being an infinite sheet with charge on it.
The electric field produced by an infinite sheet of charge is perpendicular to the sheet and it is given by

where
is the surface charge density
is the vacuum permittivity
For the cloud, we have

Therefore, the electric field produced by the cloud is

Where the negative sign means the field points towards the cloud.
Therefore, the magnitude of the field is

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