![]() ![]() Typical undular bore patterns might display one or two rows of clouds the above example shows more than 10 waves of clouds, which is unusually many. The undular bore is the result of the clash of the two air masses. © NASAĪs can be seen in figure 5, the dry air from the African continent pushes up the moist air located over the Atlantic. This type of atmospheric disturbance is also known as an undular bore.įigure 5: On 2 July, 2013, the SEVIRI instrument aboard Meteosat-10 captured this view of an undular bore propagating westward off the coast of western Africa. Gravity waves can also appear as a solitary wave and propagate as a disturbance that is characterized by a sudden and relatively permanent change in the height of a horizontal fluid interface. Moderate to dense fog bows formed as a result of the vertical motion of the fog layer.įigure 4: METOP-A AVHRR Natural Colours RGB and AMV wind barbs of the 900-1000 hPa layer retrieved from SEVIRI. When it reached the Danish Bornholm Island (maximum elevation 162 meters) its movement was partly blocked by the island, and an upstream wave developed due to this blocking. Figure 4 shows thin fog advected over the Baltic Sea. If the background flow is not too strong, gravity waves will propagate against the wind. ![]() This is the case when an obstacle in the flow causes the stable air to oscillate around its state of equilibrium. On rare occasions low level gravity waves can move against the background flow. Clouds form in areas of upward wave motion and evaporate in phases of downward motion.įigure 3: Met-10 HRV and GFS analysis comparison, 26 June 2016, 12:00 UTC ![]() Repeated again and again, this process creates gravity waves. As it plummets toward the surface, the air gets warmer than the surrounding air and is pushed up again by buoyancy as a result. When moist air goes up, it cools, and then gravity pushes it down again. When the cold and humid air is pushed up again to higher altitudes, it may reach its dew point and the moisture condenses into droplets, forming clouds. This counter-force drags the cold air down and away from hydrostatic stability. The force acting on the warm air when lifted to higher levels by the underlying cold air is answered by a counter-force (the force of inertia) of the warm air acting on the cold air layer. As it glides up, the warm air causes the underlying cold air to oscillate similar to a wind blowing over water. When a warm and dry air mass encounters humid air, it is lifted and will glide up on top of the cold air mass ( figure 3). Such gravity waves can interfere with each other producing a distinctive crisscross pattern. When this dry air encounters moister and more stable air masses over the water, the clash can yield distinctive and beautiful wave patterns in the clouds which form at the transition of the two air masses. Fast-moving, warm air masses often sweep from east to west across African deserts, sending out surges of dry air over the Atlantic. Low Level Gravity Wavesįigure 2: On 26 June, 2016, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite acquired this natural-color image of cloud gravity waves off the coast of Angola and Namibia (click on the image to get the loop).© NASAįigure 2 shows gravity waves over the Atlantic Ocean. They are visible in satellite images only if the air mass in which they propagate is near saturation, so that humidity condenses as soon as the air column is lifted. ![]() Gravity waves propagate over hundreds of kilometers under favorable meteorological conditions. Gravity waves are also found near the tropopause on top of cirrus anvils belonging to large convective systems and tropical cyclones. They are particularly common over the ocean because of its uniform background. While lee waves are stationary waves related to surface elevations, gravity waves propagate horizontally through the troposphere. They are generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore the hydrostatic equilibrium.įigure 1: The atmosphere is in hydrostatic equilibrium when the pressure gradient force drawing air upwards from higher pressures near the surface to lower pressures aloft is balanced by the force of gravity pulling air back down to the surface. Gravity waves are a very common phenomenon in the atmosphere. ![]()
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