{"id":7829,"date":"2024-04-05T23:55:40","date_gmt":"2024-04-05T15:55:40","guid":{"rendered":"https:\/\/tommykwan.com\/blog\/?p=7829"},"modified":"2025-07-20T20:09:38","modified_gmt":"2025-07-20T12:09:38","slug":"when-the-air-is-stable","status":"publish","type":"post","link":"https:\/\/tommykwan.com\/blog\/aviation\/when-the-air-is-stable\/","title":{"rendered":"When the air is stable?"},"content":{"rendered":"

Ref: https:\/\/skybrary.aero\/articles\/lapse-rate<\/a><\/p>\n

\u5230\u5e95\u54a9\u60c5\u6cc1\u4e0bparcel of air \u53ebstable?<\/p>\n

To answer this question, we need understand few concept:<\/p>\n

    \n
  1. \u54a9\u4fc2Lapse rate<\/li>\n
  2. \u54a9\u4fc2\u74b0\u5883\u4e0a\u9762\u65e2Lapse rate<\/li>\n
  3. \u54a9\u4fc2pacel of air \u65e2Lapse rate<\/li>\n<\/ol>\n

     <\/p>\n

    Lapse Rate<\/strong> is the rate at which temperature changes with height in the\u00a0Atmosphere<\/a>.<\/p>\n

    In my language, how many degree Celsius drop when vertical distance increase for a fixed number.<\/p>\n

     <\/p>\n

    2 Types of lapse rate<\/span><\/p>\n

      \n
    1. Lapse rate for environment: ELR<\/li>\n
    2. Lapse rate for parcel of air: SALR, DALR<\/li>\n<\/ol>\n

      ELR\u5168\u540d\u53eb\u00a0(Environmental Lapse Rate) is the actual rate at which the ambient temperature changes with height.<\/p>\n

      My language, the change of environmental pressure with increasing altitude<\/p>\n

      Saturated<\/strong> Adiabatic Lapse Rate (S<\/strong>ALR) is therefore the rate at which saturated air cools with height and is, at low levels and latitudes, 1.5\u00b0C per thousand feet. At higher altitudes and latitudes, where there is generally less water content in the air, and therefore less latent heat to release, the SALR is closer to 3\u00b0C per thousand feet.<\/p>\n

      My language, when the parcel of air is saturated, the lapse rate is a variable, and named as SALR.<\/p>\n

      Dry Adiabatic<\/strong> Lapse Rate (DALR) is for unsaturated<\/strong> air (i.e RH < 100%<\/strong>), when the parcel of air reaches the Dew Point<\/a> and becomes saturated, water vapour condenses, latent heat is released during the condensation process, which warms the air, and the lapse rate reduces. (fixed lapse rate 3\u00b0C per thousand feet<\/strong>)<\/p>\n

      No matter the parcel of air is saturated or not, once the parcel of air cool faster than the environment, for same amount of vertical distance increasing, the parcel of air will cooler than the surround, which has no tendency to “lift” anymore, and so it is stable. This happen when the DALR\/SALR > ELR (the air is stable)<\/strong><\/span><\/p>\n

      \u63db\u53e5\u8a71, \u5982\u679cELR > DALR > SALR …\u5c31\u6703\u4fc2absolute unstable~<\/p><\/blockquote>\n

       <\/p>\n

      https:\/\/youtu.be\/AosYZ-2DrBY?si=zeQ2Ejed767tybA2<\/a><\/p>\n