氣壓的基本知識（二）
（供進階閱讀）
撰文：孔繁耀
- 氣壓和溫度有什麼關係？
我們知道大氣壓力是水平表面（每單位面積計）所承受來自其上面空氣的重量。在一般相同情況下，冷空氣柱因其密度較高，會比暖空氣柱重，因而產生氣壓較大
- 氣壓和重力有什麼關係？
我們有重量是因為地球在我們身上有一種吸力，稱為“重力”。重力在不同太空星體均有很大分別。月球上的重力比地球小，故此太空人在月球能跳得更高。重力在地球各地也有不同，然而其變化幅度則小很多。因此在一般相同情況下，氣壓（上空空氣柱的重量）在地球各地均會因重力稍為不同而有所變化。
- 什麼是標準大氣？
要比較不同地方的氣壓讀數，一個方便方法是把它們換算至同一高度，例如海平面。氣象界使用“標準大氣”把各地的氣壓讀數轉換成海平面氣壓。標準大氣是國際公認的一個溫度、氣壓和密度的假設垂直分佈。
- 什麼是QFE、QNH和QFF？
航空界因應不同的運作需要，使用特定名詞如QFE、QNH和QFF等來形容氣壓。QFE是測量站（或機場）水平高度的氣壓。QNH是海平面氣壓，根據國際民航組織所訂立的標準大氣將QFE轉換而成。QFF是根據實際大氣溫度情況，把QFE轉換而成海平面氣壓。
- 什麼是等壓線？它有什麼用途？
氣象人員把各地氣象站同一時間所量度到之氣壓值（根據溫度等作出適當的修正後）填上天氣圖上，然後將氣壓數值相同的地點用線連接起來，便成為等壓線。等壓線一般以2或4百帕斯卡分隔。等壓線能有效地顯示地面氣壓的分布狀況。
等壓線也可用來推斷風的情況。在北半球，若你的左面氣壓較高（較低），則風大致會是迎面（背後）吹來。而在一般相同情況下，相鄰等壓線愈緊密（愈疏），風力也愈強（愈弱）。
- 氣壓有什麼基本系統？
認識及理解地面氣壓的特徵對天氣預報非常重要，天氣圖上的氣壓特徵通常有下列幾個類別:
氣旋或低氣壓 
當地區的大氣壓力比其外圍低時，這個系統便稱為低氣壓或氣旋。在北（南）半球，低氣壓附近的空氣循逆時針（順時針）方向運行（從上面往下望）。接近地球表面，摩擦力使空氣稍微向內越過等壓線。進入低壓區的空氣只能往上升，因此低氣壓地區的天氣通常比較不穩定（如有雲、雨或驟雨等）
反氣旋或高氣壓 
當地區的大氣壓力比其外圍高時，便稱之為反氣旋或高氣壓。在北（南）半球，反氣旋附近的氣流是順時針（逆時針）方向運行的。接近地球表面，摩擦力使空氣稍微向外越過等壓線。一般來說，高氣壓地區內的天氣比較穩定及晴朗。
高壓脊 
當高氣壓呈延長狀，稱為高壓脊。在天氣圖上，它有時會伴隨著一條脊軸線。脊軸線上的氣壓會較其兩旁氣壓為高。
低壓槽 
當低氣壓呈延長狀，稱為低壓槽。在天氣圖上，它會伴隨著一條槽線。槽線上的氣壓會較
Introduction to Air Pressure (Part
II)
(for the advanced reader)
How is the air pressure related to temperature?
How is the air pressure related to gravity?
What is standard atmosphere?
What are QFE, QNH and QFF?
What is an isobar? What is its use?
What are the common pressure patterns?
回到目錄Written by: HUNG Fan-yiu
- How is the air pressure related to
temperature?
We know that air pressure is the weight of the column of air above a
horizontal surface of unit area. Everything else being equal, a cold air column,
because of its higher air density, is heavier than a warm air column and thus
produces a higher air pressure.
- How is the air pressure related to gravity?
We have a weight because the Earth exerts an attractive force on us. We call
this "gravity". It tends to vary quite a lot on astronomical bodies. This is why
an astronaut can jump higher on the Moon, where the gravity is smaller, than on
the Earth. Gravity also varies at different parts of the Earth, though to a much
smaller extent. For this reason, everything else being equal, the air pressure
(which is the weight of air column above a place) can vary from place to place
on the Earth because of the slight changes in gravity.
- What is standard atmosphere?
To enable comparison of air pressure at different places, it is convenient to
convert the air pressure to a common level, e.g. the sea level. The
meteorological community makes use of the "standard atmosphere" to convert
pressure readings at different places to the sea level. The standard atmosphere
is a hypothetical profile of atmospheric temperature, pressure and density
agreed internationally.
- What are QFE, QNH and QFF?
In aviation, specify terms such as QFE, QNH and QFF are used to describe the
air pressure according to different applications. QFE is the pressure at the
station (or aerodrome) level. QNH is the pressure at mean sea level, reduced
from QFE by applying corrections according to the International Civil Aviation
Organisation's standard atmosphere. QFF is the pressure corrected to mean sea
level, taking into account the actual temperature conditions.
- What is an isobar? What is its use?
Pressure readings measured at different weather stations (after making
corrections according to temperature etc.) at the same time are plotted on a
weather chart. Meteorologists then draw lines to show places with the same
pressure. These lines are called isobars. Isobars are usually drawn at 2 or 4
hPa intervals. Isobars are useful in revealing the surface pressure pattern.
Isobars can also be used to
deduce wind conditions. In the northern hemisphere, if the air pressure is
higher (lower) on your left, then the wind will generally blow towards you (from
your back). Also, everything else being equal, winds tend to blow harder
(gentler) where isobars are closer together (further apart).
- What are the common pressure patterns?
The ability to recognize and understand pressure features on the surface is
important in weather forecasting. Pressure features commonly observed on a
weather chart are described below:-
Depression / Cyclone / Area of low pressure 
When the air pressure in a region is lower than its
surrounding, the feature is called a depression, a cyclone or an area of low
pressure. In the northern (southern) hemisphere, the air around a depression
moves in a counterclockwise (clockwise) direction (looking from above). Near the
Earth's surface, however, friction tends to cause the air to move slightly
inward across the isobars. As it moves into an area of low pressure, the air has
nowhere to go but up. Hence, a low pressure area is usually associated with
unstable weather (e.g. clouds, rain or showers).
Anticyclone / Area of high pressure 
When the air pressure in a region is higher than
its surrounding, it is called an anticyclone or an area of high pressure. In the
northern (southern) hemisphere, the air around an anticyclone moves in a
clockwise (counterclockwise) direction. Near the Earth's surface, however,
friction tends to cause the air to move slightly outward across the isobars. In
an area of high pressure the air is generally stable and is usually associated
with fine weather.
Ridge of high pressure 
A ridge of high pressure is an elongated area of
high pressure. On the weather chart, it sometimes has associated with it a ridge
line. The pressure there is higher than that at neighbouring points on either
side of the ridge line.
Trough of low pressure 
A trough of low pressure is an extended area of low
pressure. On the weather chart, it has associated with it a trough line. The
pressure there is lower than that at neighbouring points on either side of the
trough line.
- http://www.hko.gov.hk/education/edu01met/wxobs/adv_pressure2e.htm
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