When a property owner can show that a strict application of the terms of this ordinance relating to the use, construction or alteration of buildings or structures or the use of land will impose upon him practical difficulties or particular hardship, the board may consider and allow variations of the strict application of the terms of this ordinance if the variations are in harmony with the general purpose and intent of this ordinance, and the board is satisfied, under the evidence heard by it, that a granting of the variation will not merely serve as a convenience to the applicant but will alleviate some demonstrable hardship or difficulty so great as to warrant a variation from the comprehensive plan by this ordinance created.