凝(ning)露就是(shi)水(shui)分子在(zai)被(bei)試(shi)(shi)(shi)驗樣(yang)品(pin)上(shang)(shang)吸附的(de)(de)一種現(xian)象(xiang),但(dan)它是(shi)在(zai)試(shi)(shi)(shi)驗溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)變(bian)化時才(cai)會(hui)產生。在(zai)溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)上(shang)(shang)升(sheng)(sheng)(sheng)時,被(bei)試(shi)(shi)(shi)驗樣(yang)品(pin)表(biao)面溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)低于(yu)周圍(wei)空氣露點溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)時,水(shui)蒸氣變(bian)回在(zai)被(bei)試(shi)(shi)(shi)驗樣(yang)品(pin)表(biao)面凝(ning)結(jie)成液(ye)體形成水(shui)珠。在(zai)交變(bian)濕(shi)熱(re)試(shi)(shi)(shi)驗溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)上(shang)(shang)升(sheng)(sheng)(sheng)階(jie)段,由(you)于(yu)被(bei)試(shi)(shi)(shi)驗樣(yang)品(pin)的(de)(de)熱(re)慣性,所以它的(de)(de)溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)上(shang)(shang)升(sheng)(sheng)(sheng)會(hui)滯后于(yu)試(shi)(shi)(shi)驗箱內的(de)(de)空氣溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)。由(you)此就會(hui)在(zai)被(bei)試(shi)(shi)(shi)驗樣(yang)品(pin)表(biao)面產生凝(ning)露現(xian)象(xiang)。這種表(biao)面凝(ning)露量的(de)(de)多少取決于(yu)被(bei)試(shi)(shi)(shi)驗樣(yang)品(pin)的(de)(de)熱(re)容量的(de)(de)大小(xiao)以及空氣溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)上(shang)(shang)升(sheng)(sheng)(sheng)的(de)(de)速度(du)(du)(du)(du)(du)(du)和溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)上(shang)(shang)升(sheng)(sheng)(sheng)時的(de)(de)相對濕(shi)度(du)(du)(du)(du)(du)(du)。在(zai)溫(wen)(wen)(wen)(wen)濕(shi)度(du)(du)(du)(du)(du)(du)試(shi)(shi)(shi)驗的(de)(de)降溫(wen)(wen)(wen)(wen)階(jie)段,封閉外殼的(de)(de)內壁比內部空氣的(de)(de)溫(wen)(wen)(wen)(wen)度(du)(du)(du)(du)(du)(du)下降更快(kuai),這時也會(hui)出現(xian)凝(ning)露現(xian)象(xiang)。