Falguni (Arjuna) was invisible under cover of the clouds, but Karna was visible on account of the clouds being dispersed from him by the sun.
मेघच्छायोपगूढस्तु ततोऽदृश्यत फाल्गुनः। सूर्यातपपरिक्षिप्तः कर्णोऽपि समदृश्यत॥