There are many different kinds of measures of convective instability. We will go into much more detail about the initiation of convection (storm initiation) in one of the future tutorials. Forcing mechanisms include fronts (warm front, cold front) and outflow boundaries (small-scale cold fronts produced by individual thunderstorms), surface wind convergences, orographic lift, and upper-level forcing. Other processes may help the air to rise and convection to begin, even if the convective temperature is not reached. If this temperature is not reached, convection will not begin. The temperature at which this process begins is called convective temperature. Recap: when warm, moist air near the ground is heated by the Sun it may become warm enough to start lifting convectively. This creates an area of low pressure, which encourages vertical motion, which in turn helps moist warm air rise before reaching convective temperature. Upper-level forcing happens when winds at high altitudes (5-10 km) diverge, i.e. There is another mechanism that can assist in initiating convection without a convergence of surface winds or orographic lift. The convergence zone also acts as a frontal boundary if two air masses are of significantly different temperatures and/or humidities.Īdditionally, convergence zones are typically areas of strong moisture pooling increased moisture increases convective instability, lowering the lifted condensation level and consequently also the level of free convection.Īnother way of initiating convection is orographic lift: air moves horizontally over flat terrain but is forced upwards when it encounters higher terrain (hills, mountains). The air in the convergence zone is forced upwards. The convergence of surface winds may also lead to the initiation of convection. Drylines form near mountain ranges, and under the right conditions, also initiate convection. Along with this type of front, air temperature does not change much, but humidity (moisture content of the air) changes a lot. There is a third, much more rare type of front, called a dryline. Interesting fact: in Tornado alley, thunderstorms on warm fronts are prolific producers of tornadoes In some cases, on more robust warm fronts, convection does occur and forms rain showers and, more rarely, thunderstorms. Typically, the wedge angle is very low and the warm front produces little or no convection. Warm air is pushed over the cold air, forming a wedge of cold air between the ground and the warm air aloft. In some cases, thunderstorms also form on the warm front. More often than not, a warm front produces relatively shallow convection and mostly moderate rain showers. The cold air forms a wedge, which is slowly pushed away by the warm air. As the warm, moist air is more buoyant than cold air, it is pushed up and over the cold air. On a warm front, warm air pushes on the colder air ahead of it. This upward motion helps initiate convection. The cold front pushes forward like a bulldozer or a wedge, forcing the warm, moist air ahead of it upwards. The cold front can be a large (synoptic) scale front of the boundary of cool air flowing out of an already existing thunderstorm (outflow boundary). The case is reversed along a warm front: the warmer (less dense) air is forced over the wedge of cold air, which is being slowly pushed forward by the warm air (see figures below). A cold front effectively bulldozes the warm air ahead of it, forcing it upwards. The height at which moisture in the rising air condenses is now called the lifted condensation level (LCL)Ī typical way of forcing air parcels upwards is along a boundary, such as a cold front or warm front. There needs to be another lifting mechanism that pushes the air to a height where it becomes buoyant and begins to rise. There are other ways of getting air parcels at the surface to begin lifting, even if the air near the surface is stable (it is below convective temperature). The height at which moisture in the rising air condenses is called the convective condensation level (CCL).Īir parcels at the surface buoyantly lift and require no other mechanisms to start lifting. When the convective temperature is reached on a clear, hot, and humid day, the atmosphere becomes widespread explosive development of thunderstorms begins. The temperature at which free convection begins is called the convective temperature. On a warm day, convection begins when the air close to the surface warms enough to become buoyant and start rising.
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