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An alternate airport is required when the weather conditions at the departure airport are below the authorized landing minimums. This is a crucial safety measure that ensures the aircraft has a location to divert to in case conditions deteriorate and prevent a safe landing at the intended departure airport. Regulatory agencies, such as the FAA, establish these minimums to mitigate risks associated with poor visibility, low cloud cover, or other adverse weather conditions that could impact a safe landing.

When the weather at the departure airport does not meet the minimum standards for approaches and landings, it creates significant risk for the flight operations. Therefore, flight planning must include an alternative route to a different airport that can accommodate the aircraft should conditions worsen.

In contrast, having marginal VFR weather at the destination or above minimums at the departure airport does not inherently necessitate an alternate airport. Instead, they may indicate that while caution is warranted, the conditions do not require diverting to an alternate location under FAA regulations, assuming that the conditions remain safe for departure.