The Collaborative International Dictionary of English v.0.48:

Mournful \Mourn"ful\, a.
   Full of sorrow; expressing, or intended to express, sorrow;
   mourning; grieving; sad; also, causing sorrow; saddening;
   grievous; as, a mournful person; mournful looks, tones, loss.
   -- Mourn"ful*ly, adv. -- Mourn"ful*ness, n.
   [1913 Webster]

   Syn: Sorrowful; lugubrious; sad; doleful; heavy; afflictive;
        grievous; calamitous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mournfulness
    n 1: a state of gloomy sorrow [syn: mournfulness,
         sorrowfulness, ruthfulness]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "mournfulness":
   aggrievedness, bitterness, bleakness, cheerlessness,
   comfortlessness, depression, discomfort, dismalness, distress,
   distressfulness, dolorousness, dreariness, funerealness, grief,
   grievousness, joylessness, lamentability, lamentation,
   lugubriousness, pain, painfulness, pathos, pitiability,
   pitiableness, pitifulness, plaintiveness, plangency, poignancy,
   regrettableness, ruefulness, sadness, sharpness, sorrowfulness,
   tearfulness, woebegoneness, woefulness