However, after just a few hours, a tropical cyclone over land will begin to weaken rapidly - not because of friction - but because the storm lacks the the moisture and heat sources that the ocean provided. This depletion of moisture and heat hurts the tropical cyclone's ability to produce thunderstorms near the storm center. Without this convection,the storm rapidly fills.
(Source:
http://www.aoml.noaa.gov/hrd/tcfaq/C2.html)
登陸之後颱風減弱既主要原因係冇水氣同熱能,呢樣嘢都解釋埋點解夏季、環流比較大既颱風減弱會比較慢(因為可以通過西南季風維持水氣通道),而秋冬季、環流比較小既颱風減弱會比較快。
事實上颱風可以係登陸之後加強,例如今年年初澳洲有一個叫Kelvin既颱風,就係登陸之後發展咗風眼結構。呢種現象叫棕海效應,就係透過濕土黎維持水氣同熱能。
所以可以話絕大多數既颱風登陸都會減弱,但減弱速度就有非常大既分別。