天空在西方繪畫史的重要性於浪漫時期的英國達到高峰，而此時期的天空描繪又以J. M. W. Turner、John Constable和David Cox三人最為傑出。Cox比Turner和Constable年輕，又曾受Turner影響，且從未進入皇家學院（Royal Academy）學習，故其在天空方面的成就並未受到應有的重視。
本文以Cox的天空為主題，探討的問題主要有三：一、探究Cox天空的創作背景及他重視天空題材的原因。得知Cox極熱衷於當時流行的戶外速寫及水彩畫，且受當時美學思潮的影響。他終生酷愛速寫旅遊，尤其是選擇荒涼空曠的郊區，故能感受到一大片天空變幻之美。二、探索Cox天空風格之發展及其轉變之因。藉由分析Cox的論述及其畫作中的天空，得知Cox極重視天空的角色，且其天空風格的發展是循序漸進。早期由簡單清新，漸傾向注意細節的多樣變化。1830年以後風雨交加的景象增多，且漸轉向高遠而模糊的意境。這與1836年始用蘇格蘭粗糙紙、1840年向W. J. Muller學油畫、1841年脫離教職專注創作有關。而1850年以後風格更模糊自在，達崇高意境的極致。這與他1850年視力轉弱及握筆不穩、1853年跌倒中風視力更差有關。三、探討Cox、Turner、Constable三畫家天空之異同。經比較得知Cox的天空不像Constable的天空那麼注意雲的型態變化及立體感，亦不像Turner的天空那麼重視光線和色彩，而是簡單廣闊，並強調風雨的感覺，且常增添生動的飛鳥。
The role of the sky in western painting worked up to a climax during the Romantic era, especially in the first half of the 19th century British landscapes. Cox, with Turner and Constable, were the three British painters most famous for depicting the sky, and each of them had his own style of skies, but Cox was the one has been ignored so long. In order to construct a more comprehensive understanding about Cox' s skies, this study attempts to explore three major points: the reasons why Cox paid so much attention to depict the sky; the development of Cox' s skies and the causes of its transform at each stage; the comparisons of Cox' s skies with those of Turner' s and Constable' s.
Firstly, after exploring the artistic background of the Romantic era and Cox' s experience of learning, we find out that his devotion to outdoor sketching and? ?watercolour revealed the most relevant to his great interest in depicting the sky. Secondly, through an analysis of Cox' s treatises and his important works in a chronological order, we can roughly divide Cox' s skies into three stages, and the style of it developed gradually from simplicity to variety, then to vagueness, more wind and rain, finally to the sublime. Thirdly, by a comparison among Cox' s skies with those of Constable' s and Turner' s, we understand that Cox' s skies are much simpler and broader than those by the other two masters, contain more feeling about wind and rain, also usually include some energetic flying birds.