As hard as it is to believe, UV rays are stronger on cloudy days.
Remarkably, UV rays on cloudy days are stronger compared to sunny days.
この場合、次のような言い方ができますよ。
ーAs hard as it is to believe, UV rays are stronger on cloudy days.
「意外なことに、紫外線は曇りの日のほうが強い」
as hard as it is to believe で「意外なことに」
ーRemarkably, UV rays on cloudy days are stronger compared to sunny days.
「意外なことに、曇りの日の紫外線は晴れの日に比べ、より強い」
remarkably で「意外なことに」を表現することもできます。
ご参考まで!