![]() If we remove the fronts from the chart and increase the number of isobars by drawing them at 1 hPa spacing, we get an increased level of detail about the pressure field, as below:Ĭhart at same time as previous, but fronts removed and intermediate isobars added (in grey) In this chart the spacing between the isobars is 4 hPa. coming from the southwest, and therefore cold). This is why the air around an anticyclone (High pressure area) rotates anticlockwise in our hemisphere. A previous blog-post describes the anticyclonic outflow of anticyclones, together with the descent that is characteristic of Highs.Īt the time of the chart above, NZ was mostly affected by a southwesterly air-stream (i.e. In the Earth’s atmosphere, when the air accelerates from high to low pressure, the rotation of the Earth deflects the out-flowing air towards the left in our southern hemisphere (the opposite deflection occurs in the northern hemisphere). The air ejects from the region of much higher pressure within the tyre to the region of lower air pressure outside the tyre. ![]() Let me explain by describing what happens when you release the valve on a tyre. These lines give a good idea of what the broad-scale winds are. The thin solid lines are isobars, which means lines connecting places with equal air pressure (note the “bar” in “isobar”). Mean Sea Level analysis, 6am 3 July (18UTC 2 July) 2014 In 1930 Shaw sent his Christmas greetings to the then director of the NZ Meteorological Service, Dr Edward Kidson. Shaw also created an important thermodynamic diagram that is still used extensively today in modern meteorological services worldwide. The millibar was introduced by the British meteorologist Sir Napier Shaw in the early 1900s. One hPa = one millibar = one thousandth of a “bar”. The hectoPascal is the modern replacement unit for the millibar: At the Earth’s surface the air pressure of the atmosphere is usually within the range 980 to 1030 hPa. But for the range of air pressures that occur naturally in the atmosphere we normally use hectoPascals (hPa, hundreds of Pascals) rather than kPa. For typical values of air pressure in a tyre, it’s best to measure pressure in units of kilopascals (kPa, thousands of Pascals). An example of where air has high pressure is the inside of an inflated tyre. Pressure is measured in Pascals, named after the French mathematician and physicist Blaise Pascal (who also devised the famous “Pascal’s triangle”). Pressure is the force applied over a unit of area, so it can be increased by having more force acting over a smaller area. The hectoPascal and Air Pressure In meteorology, the quantity pressure is an important driver of physical processes in the atmosphere. Written by Chris Webster Metservice Meteorologist
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