2024 L6 Atmospheric Pressure and Wind (1).pptx

Meteorology and Climate Change (SL23603) Prepared by: Lecture 6: Dr. Madihah Jafar Sidik Atmospheric Pressure and Borneo Marine Research Institute Wind Universiti Malaysia Sabah  Sub-topics Pressure gradient force...

Meteorology and Climate Change (SL23603) Prepared by: Lecture 6: Dr. Madihah Jafar Sidik Atmospheric Pressure and Borneo Marine Research Institute Wind Universiti Malaysia Sabah  Sub-topics Pressure gradient force Friction Force Upper air vs. surface winds Cyclones and anti-cyclones Questions: 1. Why does the wind blow? 2. How can one tell wind direction by looking a weather chart  Why Does Wind Blow? Gases move from high- pressure areas to low- pressure areas. the bigger the difference between the pressures, the faster the air will move from the high to the low pressure. That rush of air is the wind we experience.  Wind is a part of weather we experience all the time, but why does it actually happen? The air will be still one day, and the next, powerful gusts of wind can knock down trees. What is going on here? The answer is: The main cause of wind is a little surprising. It’s actually temperature. More specifically, it’s differences in temperature between different areas. How would temperature differences make the wind blow? The gases that make up our atmosphere do interesting things as the temperatures change. When gases warm up, the atoms and molecules move faster, The sun warms up the air, but it does so spread out, and rise. unevenly. That’s why steam coming off a pot of Because the sun hits different parts of boiling water always the Earth at different angles, and goes upward. because Earth has oceans, mountains, and other features, some places are When air is colder, the gases get slower and warmer than others. closer together. Colder air sinks. Because of this, we get pockets of warm air and cold air. Different temperatures lead to different pressures Since gases behave differently at different temperatures, that means you also get pockets with high pressure and pockets You might think that the warm air with low pressure. would lead to a higher pressure area, but actually the opposite is – In areas of high true. pressure, the gases in the air are more Because warm air rises, it leaves crowded. behind an area of low pressure behind it. – In low pressure zones, the gases are a little more Here comes the wind! Now we’re getting to the part where wind happens. Gases move from high- pressure areas to low-pressure areas. The bigger the difference between the pressures, the faster the air will move from the high to the low pressure. That rush of air is But why does the air move at all? You might be wondering why the air would move from high pressure to low pressure in the first place. This is something that happens in nature all the time: things always try to even out. It’s called diffusion. Even people do it! When people get onto a bus, do they all sit on the same side of the bus first? Do strangers sit next to each other when there are plenty of open seats? No way. People want to spread out as much as possible. Next time you feel the wind blow, think about where it’s going, and what temperatures and pressures are causing it to do that.  In the atmosphere, the wind blows in an attempt to equalize imbalance in air pressure. Does this mean that the wind always blows directly from high  low pressure? – Not really: air movement of air is controlled not only by pressure differences but by other forces such as Coriolis. We will be able to tell how the wind blow in particular region by examining surface and upper-air charts. Atmospheric Pressure Air pressure: mass of air above a given level – Higher altitude (fewer molecules above us) – Lower altitude (more molecules above us) Atmospheric pressure always decreases with increasing height. One way to change air pressure is simply move up or down in the atmosphere.  What causes the air pressure to change in the horizontal?  Why does the air pressure change at the surface? A simple atmospheric model: A column of air, extending well up into atmosphere. Assumption: – The molecules are not crowded close to the surface. Unlike the real atmosphere, the air density remains constant from up to the top of the column – The width of the column does not change with height – The air is unable to freely move into and out of the column. – The air pressure at the surface is related to the number of molecules above. If the air temperature in the column does not change, the added air would make the column would make: Column more dense Increase the surface air pressure Atmospheric Pressure Same air temperature Same no. of molecules Same surface pressure Figure 1: Two air columns are located at the same elevation and have identical surface air pressure.  It takes a shorter column of cold air to exert the same pressure as a taller or warm air. Aloft: – cold air associated with low pressure – Warm air with high pressure Atmospheric pressure decreases more rapidly with elevation of cold air. In the warmer, less dense air, the pressure does not decrease rapidly with height (fewer molecules in the same vertical distance  Move up the warm column until the letter H. Move up the cold column the same distance until the letter L. – More molecules above the letter H on the warm column > above the letter L in the cold column. The fact that the number of molecules above any level is a measure of the atmospheric pressure leads to important concept: “Warm air aloft is normally associated with high atmospheric pressure, and cold air aloft is associated with low atmospheric pressure” The horizontal difference in temperature creates a horizontal difference in pressure. The pressure difference established a force (called the pressure gradient force)  that causes the air move from higher toward lower pressure. In summary: heating or cooling a column of air ca establish horizontal variations in pressure that cause air to move. The net accumulation of air above the surface causes air pressure to rise, whereas a decrease amount of air above the surface causes the surface air pressure to fall. Measuring Air Pressure Barometer – measure pressure changes in milibar (mb) A common pressure unit used in aviation and on television, radio weather broadcasts in inches of mecury (Hg). At sea level, standard atmospheric pressure is: 1013.25 mb = 1013.25 hPa = 29.92 in.Hg Near the Earth surface, atmospheric pressure decreases on the average by about 10 mb for every 100 m increase in elevation (~1 in of mecury for each 1000-ft rise. See Level Pressure Map Surface and Upper-Air charts Surface map shows areas of high and low pressure and arrows that indicate wind direction. ‘H’ on the map indicates the center of high pressure, which is called anti- cyclones. ‘L’ represents center of low pressure, also known as depressions, mid- latitude cyclone, or extratropical cyclone (they form in mid latitude, outside of the tropics) In Northern Hemisphere: the winds blow counterclockwise and inward towards the low Clockwise and outward from the center of high Surface weather map Low air pressure and high air pressure Upper air-map It shows a constant pressure chart. It is constructed to show height variations along a constant pressure (isobaric surface). This particular isobaric map shows height variations at a pressure level of 500 mb (which is about 5600 m above sea level. The solid dark lines on the map are contour lines – lines that connect points of equal elevation above sea level. Although contour lines are height lines, they illustrate pressure much like isobars do. Consequently, contour lines of low height represent a region of higher pressure. Surface Upper-air map map  Surface Upper-air map map Contour lines typically decrease in value from south  north. The dashed red lines, which are isotherms – lines equal temperature. Observed that cooler air is generally to the north and warmer air to the south [cold air aloft is associated with low pressure, warm air aloft with high pressure]. The contour lines are not straight, however, they bend and turn indicating: – ridges (elongated high) where the air is warmer, – trough (elongated low) where the air is colder The arrows on the 500-mb map show the wind direction. Unlike the surface winds that flow across the isobars, the winds on 500-mb flow parallel to the contour lines in wavy west to east direction. Surface and upper-air charts are valuable tools for meteorologists. Surface maps describe where the center of high and low pressure are found, as well as winds and weather associated with these systems. Upper-air maps are extremely important in forecasting the weather. It also determine the movement of surface pressure systems and determine whether these surface system will intensify or weakens. Theories of wind blowing Our understanding of why the wind blow can be applied with several fundamental laws of motion. Newton’s law of motion: – 1st law: an object at rest will remain at rest and an object in motion will remain in motion (as long as no force is exerted on the object) – 2nd law: the force exerted on an object equals its mass times to acceleration produced. [F=ma]. Because more than one force may act upon an object, Newton’s 2nd law always refers to the net or total force that results. An object will always accelerate in the direction of the total force  Therefore, to determine in which direction the wind will blow, we must identify and examine all the for all the forces that affect the horizontal movement of air. These forces include: – Pressure gradient force – Coriolis force – Centripetal force – Friction ** however, only Pressure Gradient force and Coriolis Force the influence the wind We will first study the forces that influence the flow of air aloft. Then we will see which forces modify winds near the ground.  Horizontal differences in atmospheric pressure cause air to move and, hence wind to blow.  The greater the pressure difference, the stronger the force, and the faster the air moves Pressure gradient force A difference in pressure across a surface. In general, a pressure is a force per unit area, across a surface. A difference in pressure across a surface then implies a difference in force, which can result in an acceleration according to Newton's second law, if there is no additional force to balance it. The isobars show how horizontal pressure is changing. Pressure gradient = difference in pressure/distance When differences in horizontal air pressure exist, there is a net force acting on the air, called Pressure Gradient Force (PGF) PGF is directed from higher toward lower pressure at right angles to the isobars. The magnitude of the force is directly related to the pressure gradient. The PGF is the force that causes the wind to blow.  If the pressure gradient force were the only force acting upon air, we would always find winds blowing directly from higher toward lower pressure. However, the moment air starts to move, it is deflected in its part by Coriolis force. The Coriolis Force describes an apparent force that is due to rotation of the Earth. All free-moving objects (such as ocean currents, aircraft, artillery projectiles, and air molecules) seem to deflect from a straight-line path because the earth rotates under them. The Coriolis Force causes the wind deflect to the – right of its intended path (Northern Hemisphere) – Left of its intended path (Southern Hemisphere)  If the Earth not rotating, the path of the satellite would be observed to move directly from north  south, parallel to the Earth’s meridian lines. However, the Earth does rotate, carrying us and meridians eastward with it. – Because of this rotation, in the Northern Hemisphere we see the satellite moving southwest instead of due south: it seems to veer off its path and move toward its right. – In Southern Hemisphere, the earth’s direction of rotation is clock-wise as view from above the South Pole: consequently, a satellite move northwest (veer to the left of its path).  As the wind speed increases, the Coriolis force increases: the stronger the wind, the greater the deflection. Additionally, the Coriolis force increases from all wind speeds from a value of zero at the equator to a maximum at the poles. The amount of deviation is greatest at the pole and nonexistent at the equator.  In summary, to an observer on the earth, objects moving in any direction (north, south, east, west) are deflected to the right of their intended path (Northern hemisphere) and to the left (Southern Hemisphere). The amount of deflection depends upon: Summa – The rotation of the Earth – The latitude ry – The object speed In addition, the Coriolis force acts at right angles to the wind, only influencing wind direction and never wind speed. The Coriolis force is also minimal on small- scale wind, such as those that blow inland along the coasts in summer. Here, the Coriolis force might be strong because of high winds, but the force cannot produce much deflection over the relatively short distances. Only where winds blow over vast regions is the effect significant. Thank you