Could you please tell me if there is any important difference between a drought and a dry spell ? (perhaps in length, intensity...)

A drought sounds a lot more serious.

A drought is a prolonged period of abnormally low rainfall leading to a shortage of water, which can be disastrous.

A dry spell is a short period of low rainfall, usually not more than a month.
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Many thanks to both of you, it's quite clear now.
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