Write SQL query to solve given problem: What was the average temperature difference between store number 18 and 19 on 16 September 2022?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT SUM(CASE WHEN T1.store_nbr = 18 THEN T2.tavg ELSE 0 END) - SUM(CASE WHEN T1.store_nbr = 19 THEN T2.tavg ELSE 0 END) FROM relation AS T1 INNER JOIN weather AS T2 ON T1.station_nbr = T2.station_nbr WHERE T2.`date` = '2012-09-16'