ฟ้าหลัว จริงๆแล้วคืออะไร?

เราคงเคยได้ยินกันบ่อยๆเวลาดูข่าวพยากรณ์อากาศว่า "อากาศร้อน มีฟ้าหลัวในตอนกลางวัน" จริงๆแล้วฟ้าหลัว มันเกิดจากอะไร?... หรือจริงๆแล้วใครบ้างไม่รู้ด้วยซ้ำว่าฟ้าหลัว มันหน้าตายังไง?
ฟ้าหลัว หมายถึงเวลาที่เรามองขึ้นไปบนท้องฟ้าแล้วฟ้าดูขุ่นๆ มัวๆ มองไม่ทะลุไปเห็นอะไรบนฟ้ามากนัก อาจจะเหมือนมีฝ้าสีน้ำตาล จนไปถึงสีฟ้าขุ่นๆ ทั้งที่ก็ไม่ได้มีเมฆฝนหรือหมอกให้เห็น ส่วนมากจะเกิดในหน้าร้อน หรือเวลามีอากาศร้อนจัด ซึ่งฟ้าหลัวนี้ไม่ใช่เมฆหรือหมอกครับ แต่เป็นฝุ่นละอองขนาดเล็กมากๆ จะเข้าใจที่มาของฟ้าหลัว ต้องเล่าย้อนไปเรื่องความกดอากาศกันก่อนครับ
หากจำกันได้ Perspective เคยเล่าเรื่องหย่อมความกดอากาศต่ำ ความกดอากาศสูงกันไปแล้ว(ลองอ่านกันได้ที่นี่ครับ) เวลาอากาศร้อน อากาศจะขยายตัว เมื่ออากาศขยายตัวก็จะเบา และลอยขึ้น เกิดเป็นหย่อมความกดอากาศต่ำ(เพราะอากาศเบา ออกแรงกดลงบนพื้นน้อยลง) ทีนี้เมื่ออากาศร้อน ฝุ่นผงบนผิวดินก็แห้ง ไม่มีความชื้นมาทำให้จับเป็นก้อน หรือยึดติดกับพื้นผิวต่างๆไว้ บวกกับธรรมชาติของอากาศร้อนที่จะลอยขึ้นแล้ว ฝุ่นละอองเหล่านี้ก็โดนอากาศร้อนหอบลอยขึ้นไปฟุ้งกระจายอยู่ในอากาศจำนวนมาก และเมื่อฝุ่นจำนวนมากล่องลอยอยู่ในอากาศ ก็จะไปขัดขวางทัศนวิสัย ทำให้เรามองอะไรไม่ค่อยชัด ฝ้าๆมัวๆไปหมด เกิดเป็นปรากฏการณ์ทางอากาศที่เรียกว่า "ฟ้าหลัว" นั่นเองครับ
ในหน้าหนาว หรือหน้าฝน ไม่ค่อยมีฟ้าหลัว ก็เพราะในหน้าหนาวนั้นความกดอากาศค่อนข้างสูง เพราะอากาศเย็นจะค่อนข้างหนัก ไม่ลอยขึ้นแบบอากาศร้อน ก็จะพาฝุ่นละอองในอากาศร่วงลงสู่พื้น ไม่ค่อยเหลือลอยอยู่ในอากาศเท่าไหร่ ส่วนในหน้าฝนนั้นยิ่งแล้วใหญ่ ฝนจะชะเอาฝุ่นละอองลงมาจนหมด ฟ้าหลังฝนจึงมักจะใสแจ๋วนั่นเองครับ
By ษัษฐา ศุขสวัสดิ ณ อยุธยา

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