更快的温暖口袋变得比围绕它们的较冷的空气密度低，并且它们开始上升。这些上升的柱子或空气流被称为“热量”。随着空气上升，热量和水分被向上（垂直）输送到大气中。表面加热越强，对流延伸到大气中越强大。 （这就是为什么对流在炎热的夏天下午特别活跃的原因。）在我们深入研究大气对流之前，让我们看一个你可能更熟悉的例子 – 一壶沸腾的水。当水沸腾时，锅底部的热水上升到表面，导致热水泡沫，有时蒸汽在表面上。除了空气（流体）取代水之外，空气中的对流也是如此。随着对流的继续，空气在达到较低的空气压力时会冷却，并可能达到其中的水蒸气凝结并形成（你猜对了）顶部积云的程度！如果空气含有大量水分并且非常热，它将继续垂直生长，并将成为高耸的积云或积雨云。在这个主要的对流过程完成之后，可能会发生许多情况，每个情景形成不同的天气类型。 “对流”一词经常被添加到他们的名字中，因为对流“跳跃开始”他们的发展。如果对流云有足够的云滴，它们将产生对流降水。与非对流降水相反（当空气被力提升时产生），对流降水需要不稳定，或空气继续自行上升的能力。它与闪电，雷声和大雨阵阵有关。 （非对流降水事件的降雨率较低，但持续时间更长，降雨更稳定。）积云，高耸的积云，积雨云和高积云都是可见的对流形式。它们也都是“潮湿”对流的例子（对流，其中上升空气中的多余水蒸气凝结形成云）。在没有云形成的情况下发生的对流被称为“干”对流。 （干对流的例子包括在空气干燥的晴天发生的对流，或者在加热强度足以形成云之前的早期发生的对流。）所有通过对流的上升空气必须平衡其他地方下沉的空气量。当加热的空气上升时，来自其他地方的空气流入以替换它。我们觉得这种空气平衡运动如风。对流风的例子包括foehns和海风。除了产生上述天气事件外，对流还有另一个目的 – 它可以去除地球表面的多余热量。没有它，已经计算出地球上的平均表面气温大约在125°F左右而不是当前可居住的59°F。只有当温暖的上升空气袋已经冷却到与周围空气相同的温度时它会停止上升吗？
Convection is a term you’ll hear quite often in meteorology. In weather, it describes the vertical transport of heat and moisture in the atmosphere, usually from a warmer area (the surface) to a cooler one (aloft). As the ground’s temperature warms, it heats the layer of air directly above it through conduction (the transfer of heat from one substance to another). Because barren surfaces like sand, rocks, and pavement become warmer faster than ground covered by water or vegetation, air at and near the surface heats unevenly. As a result, some pockets warm faster than others.
The faster warming pockets become less dense than the cooler air that surrounds them and they begin to rise. These rising columns or currents of air are called “thermals.” As the air rises, heat and moisture are transported upward (vertically) into the atmosphere. The stronger the surface heating, the stronger and higher up into the atmosphere the convection extends. (This is why convection is especially active on hot summer afternoons.) Before we delve into atmospheric convection, let’s look at an example you may be more familiar with—a boiling pot of water. When water boils, hot water in the bottom of the pot rises to the surface, leading to bubbles of heated water and sometimes steam on the surface. It’s the same with convection in the air except air (a fluid) replaces the water. As convection continues, the air cools as it reaches lower air pressures and may reach the point where the water vapor within it condenses and forms (you guessed it) a cumulus cloud at its top! If the air contains a lot of moisture and is quite hot, it will continue to grow vertically and will become a towering cumulus or a cumulonimbus. After this main process of convection is complete, there are a number of scenarios that could happen, each which forms a different weather type. The term “convective” is often added to their name since convection “jumps starts” their development. If convective clouds have enough cloud droplets they’ll produce convective precipitation. In contrast to non-convective precipitation (which results when air is lifted by force), convective precipitation requires instability, or the ability for air to continue rising on its own. It is associated with lightning, thunder, and bursts of heavy rain. (Non-convective precipitation events have less intense rain rates but last longer and produce a steadier rainfall.) Cumulus, towering cumulus, Cumulonimbus, and Altocumulus Castellanus clouds are all visible forms of convection. They are also all examples of “moist” convection (convection where the excess water vapor in the rising air condenses to form a cloud). Convection that occurs without cloud formation is called “dry” convection. (Examples of dry convection include convection that occurs on sunny days when air is dry, or convection that occurs early on in the day before the heating is strong enough to form clouds.) All of the rising air through convection must be balanced by an equal amount of sinking air elsewhere. As the heated air rises, air from elsewhere flows in to replace it. We feel this balancing movement of air as wind. Examples of convective winds include foehns and sea breezes. Besides creating the above-mentioned weather events, convection serves another purpose — it removes excess heat from the earth’s surface. Without it, it has been calculated that the average surface air temperature on earth would be somewhere around 125° F rather than the current liveable 59° F. Only when the pocket of warm, rising air has cooled to the same temperature of the surrounding air will it stop rising.