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Scalar and vector fields
Published in A.V. Durrant, Vectors in Physics and Engineering, 2019
The barometric pressure in the atmosphere is a scalar field commonly shown on weather maps by isobars. An isobar is a line joining points that have the same atmospheric pressure (Fig 4.2a) and is labelled by that pressure. Isobars are usually drawn at fixed pressure intervals, ten millibars in Fig 4.2a. The isobars show regions of high and low pressure indicating dry and wet conditions respectively. Regions where the pressure gradient is large and the winds strong are shown by the isobars being close together. Isobars are examples of contour lines, or contour curves. Another familiar example is land elevation above sea level shown on land maps by means of elevation contour lines. A contour line joins points of the same land elevation and is labelled by that value; adjacent lines are usually separated by some fixed elevation interval, 50 m in Fig 4.2b. The contours enable you to identify summits, valleys, ridges, saddle points, steepness, etc.
Waves and offshore engineering
Published in P. Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan, Hydraulic Structures, 2017
P. Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan
There is a certain interaction between the wind and the isobar spacing as given in the meteorological charts. In meteorological practice, isobars are spaced at 4 mbar in the UK and at 3 mbar in the USA. The wind direction is parallel to the lines of isobars but is modified by friction over the water surface. The pressure distribution normal to the isobars is determined by the Coriolis force resulting from the Earth’s rotation and the centripetal force due to the curvature of the moving air masses. The resultant wind is called the gradient wind. When the isobars are parallel and straight, only the Coriolis force is important and the wind is called geostrophic wind. The equation governing the motion of the geostrophic wind at a particular point is
Atmosphere
Published in Wayne T. Davis, Joshua S. Fu, Thad Godish, Air Quality, 2021
Wayne T. Davis, Joshua S. Fu, Thad Godish
Pressure differences in the lower atmosphere occur along a gradient in the horizontal dimension. The force associated with these differences is called the pressure gradient force. The nature of PGFs can be seen from weather maps on which lines of equal pressure (isobars) are plotted (Figure 1.9). The spacing of isobars describes pressure gradients. Isobars relatively close together indicate a strong or steep pressure gradient with high associated winds. If these isobars were contour intervals, they would describe steep or mountainous terrain. Isobars relatively far apart are characterized by small pressure gradients and light winds. The PGF is directed perpendicular to isobars from high to low pressure.
Practical aspects of meteorology and oceanography for mariners: A guide for the perplexed
Published in Cogent Engineering, 2018
L. C. Aroucha, H. O. Duarte, E. L. Droguett, D. R. A. Veleda
This understanding allows mariners to predict what they might expect when shiphandling (Calazans, 2011). For example, when the weather report or synoptic chart mentions the presence of instability line, it means that in this region the mariner will find bad weather condition, with the possibility of intense rain and strong winds due to convective activities from direct circulation originated from instability (Lobo & Soares, 2007). In fact, the observation of surface meteorological charts that indicate horizontal gradients of pressure (i.e. synoptic charts) is very important for navigation. The wind tends to blow almost parallel to isobars (i.e. lines of constant pressure with 4hPa spacing between them) and the closer the isobars the stronger the wind. However, near surface, in the friction layer, the turbulent friction that the Earth exerts on the air slows the wind down (Chiang & Zebiak, 2000). This slowing wind reduces the Coriolis force, and the pressure gradient force becomes more dominant, so the total wind deflects slightly towards lower pressure.