I'm posting a different option that will read the table just once. It might be faster but I can't assure that. I included an option with ISNULL which will work for 2 options and an option with COALESCE which will work with multiple options but need to be casted to retain the datatype.

SELECT t.CalendarDate,

ISNULL(MAX( CASE WHEN t.UOM = 'F' THEN t.Temperature END), MAX( CASE WHEN t.UOM = 'C' THEN (t.Temperature * 1.8000) + 32.00 END)) AS Fahrenheit,

CAST( COALESCE(MAX( CASE WHEN t.UOM = 'F' THEN t.Temperature END), MAX( CASE WHEN t.UOM = 'C' THEN (t.Temperature * 1.8000) + 32.00 END)) AS decimal(10,2)) AS Fahrenheit2

FROM @Temperature t

GROUP BY t.CalendarDate

ORDER BY t.CalendarDate