1/28/2024 0 Comments Air parcel![]() We will use the dew point temperature of the air in the parcel to keep track of the water vapor content. The starting temperature and water vapor content of the parcel is taken to be the measured conditions at ground level.Rules for moving air parcels up and down in the atmosphere ![]() ![]() We will now take a more quantitative look at the process of cloud formation by tracking what happens to the temperature and water contained in parcels of air as they are raised and lowered in the atmosphere. If the air cools to its dew point temperature (in other words if it reaches saturation with respect to water vapor), condensation is forced and some of the water vapor in the air condenses into liquid water droplets. As it cools, the air's capacity for water vapor (its saturation mixing ratio) decreases. This is called positive energy and means the atmosphere at that level is 'unstable'.Moving Air Parcels Up and Down in the Atmosphere Brief Review of Cloud FomationĬlouds typically form where air is rising upward in the atmosphere. If the temperature of the rising parcel remains higher than the surrounding atmosphere (despite its cooling), the parcel, being less dense than the surrounding environment, will continue to rise. This is called negative energy and means the atmosphere at that level is 'stable'. If the temperature of the rising parcel decreases to less than the surrounding atmosphere (due to its cooling) the parcel will become denser than the surrounding environment and gravity will slow, or even reverse, the rise. As an unsaturated parcel rises it will cool at the fixed rate of 9.8☌ per 1,000 meters (5.5☏/1,000 feet). The reason for looking at parcels is to help determine the stability of the atmosphere. However, the 'parcel theory', while an over-simplification of real-world processes in the atmosphere, is a good way of thinking about how the atmosphere produces the weather. The released heat that is added to the atmosphere slows the rate of cooling.īecause of many different influences on a parcel of rising air most, if not all, of the assumptions will not be 100% true at all times. This is because the process of water vapor condensing into a liquid releases heat. Any saturated parcel (parcel with 100% relative humidity) cools at a slower rate.Any parcel that is unsaturated (relative humidity less than 100%) will cool (or lapses) at a rate of 9.8☌ per 1,000 meters (5.5☏/1,000 feet) until the relative humidity becomes 100% (the air becomes saturated).We also assume there is no outside source of heating added to the parcel.Therefore, stability/instability is based upon the vertical temperature profile of the atmosphere.We generally assume the ratio of moist air to dry air in the parcel remains constant as it rises (or sinks) in the atmosphere. In both cases the parcel's rate of cooling remains fixed. The theory behind the "parcel" has several assumptions. A parcel is a bubble of air of no definite size that we generally assume it retains its shape and general characteristics as it rises or sinks in the atmosphere. In meteorology, we often treat 'pockets of air' in a similar way to ballooning. A front represents the boundary where cooler, more dense air undercuts less dense, warmer air forcing it up into the atmosphere forming the storms. This is why thunderstorms often form along weather fronts. The cooler air undercuts the warmer, less dense air trapped inside the balloon causing it to lift. Outside of the balloon, the cooler, denser air is pulled down by gravity. A flame is used to heat the air inside of the balloon making it less dense. As the denser air reaches the earth's surface it spreads and undercuts the less dense air which, in turn, forces the less dense air into motion causing it to rise. Gravity's role is its pull of cooler, denser air toward the earth's surface.
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