Quick Review: Rising Air and Clouds

Transcription

1 Quick Review: Rising Air and Clouds Remember that clouds form when air rises. Specifically - As air rises, it expands and cools... - And as it cools, its RH increases... - And once its RH hits 100% (saturation), a cloud starts to form. Rising air cools. If it cools enough, we'll reach saturation (RH = 100%), allowing clouds to form.

2 Quick Review: Rising Air and Clouds Remember that clouds form when air rises. Specifically - As air rises, it expands and cools... - And as it cools, its RH increases... - And once its RH hits 100% (saturation), a cloud starts to form. But this raises some important questions. For example: Why is the air rising in the first place?...or more precisely Is the air rising on its own? Or did it have to be pushed upwards?

3 Quick Review: Rising Air and Clouds Remember that clouds form when air rises. Specifically - As air rises, it expands and cools... - And as it cools, its RH increases... - And once its RH hits 100% (saturation), a cloud starts to form. But this raises some important questions. For example: Why is the air rising in the first place?...or more precisely Is the air rising on its own? Or did it have to be pushed upwards? To answer these questions, we'll need the concept of stability

4 Stability The concept of stability addresses the following problem: - Suppose I take a small mass of air from the environment and bump it upwards some distance Will this air mass keep rising on its own? Or will it fall back to its original position? small air mass displaced upwards

5 Stability The concept of stability addresses the following problem: - Suppose I take a small mass of air from the environment and bump it upwards some distance Will this air mass keep rising on its own? Or will it fall back to its original position? If the air mass falls back down, we say the atmosphere is stable, since it's resistant to small disturbances. But if it keeps rising, then we say the atmosphere is unstable, in that a small initial disturbance leads to a permanent change.

6 Temperature and Stability But remember: When comparing air masses at a given height, colder air is more dense and tends to sink, while warmer air is less dense, and tends to rise...which means we can rephrase our stability question as Is our displaced air mass warmer or colder than the air surrounding it (at the same height)? warmer or colder than surroundings?

7 Temperature and Stability But remember: When comparing air masses at a given height, colder air is more dense and tends to sink, while warmer air is less dense, and tends to rise...which means we can rephrase our stability question as Is our displaced air mass warmer or colder than the air surrounding it (at the same height)? If it's colder, then it sinks back down, which means the atmosphere must be stable. But if it's warmer, then it keeps rising, which means the atmosphere must be unstable.

8 The Stability of Dry and Moist Air When we're talking about clouds, we say an air mass is dry if the air is unsaturated (no droplets), while an air mass that's saturated is referred to as moist Dry air is clear, while moist air is cloudy! the air up here is moist down here the air is dry

9 The Stability of Dry and Moist Air Now, from before, we know that as air rises (or ascends), it expands and cools. But, as it turns out... Dry air that's ascending cools faster than moist air that's ascending! Why?

10 The Stability of Dry and Moist Air Now, from before, we know that as air rises (or ascends), it expands and cools. But, as it turns out... Dry air that's ascending cools faster than moist air that's ascending! Why? Well, the reason is that moist air is saturated, meaning that as it ascends and cools, some of the vapor condenses. And since condensation is a warming process, it partially offsets the cooling due to ascent.

11 The Stability of Dry and Moist Air Now, from before, we know that as air rises (or ascends), it expands and cools. But, as it turns out... Dry air that's ascending cools faster than moist air that's ascending! Why? Well, the reason is that moist air is saturated, meaning that as it ascends and cools, some of the vapor condenses. And since condensation is a warming process, it partially offsets the cooling due to ascent. Ok, fair enough. But now let's put some numbers on it...

12 It turns out that for dry (or really unsaturated) air, the rate of cooling is always the same An ascending dry (unsaturated) air mass always cools at a rate of 10 o C per km of height. 2 km 1 km 0 km starting temperature 30 o C

13 It turns out that for dry (or really unsaturated) air, the rate of cooling is always the same An ascending dry (unsaturated) air mass always cools at a rate of 10 o C per km of height. 2 km 1 km 20 o C cools 10 o C / km height 0 km starting temperature 30 o C

14 It turns out that for dry (or really unsaturated) air, the rate of cooling is always the same An ascending dry (unsaturated) air mass always cools at a rate of 10 o C per km of height. 2 km 10 o C cools 10 o C / km more 1 km 20 o C cools 10 o C / km height 0 km starting temperature 30 o C

15 It turns out that for dry (or really unsaturated) air, the rate of cooling is always the same An ascending dry (unsaturated) air mass always cools at a rate of 10 o C per km of height. This rate of cooling has a name: It's called the dry lapse rate.

16 For moist air, the rate of cooling depends on the conditions. But a typical rate would be between 4 and 8 o C / km. For our purposes, we'll say an ascending moist (or saturated) air mass cools at roughly 6 o C / km. 2 km 1 km 0 km starting temperature 30 o C

17 For moist air, the rate of cooling depends on the conditions. But a typical rate would be between 4 and 8 o C / km. For our purposes, we'll say an ascending moist (or saturated) air mass cools at roughly 6 o C / km. 2 km 1 km 24 o C cools 6 o C / km height 0 km starting temperature 30 o C

18 For moist air, the rate of cooling depends on the conditions. But a typical rate would be between 4 and 8 o C / km. For our purposes, we'll say an ascending moist (or saturated) air mass cools at roughly 6 o C / km. 2 km 18 o C cools 6 o C / km more 1 km 24 o C cools 6 o C / km height 0 km starting temperature 30 o C

19 Now, for the sake of argument, suppose we have two air masses ascending: one that's dry, and one that's moist. We then have three possible scenarios...

20 First possibility: If the rising dry and moist air masses are both colder than the surroundings, then we say the atmosphere is absolutely stable (or just stable). Environment Dry Air Mass Moist Air Mass 1 km 32 o C 25 o C 29 o C 0 km 35 o C 35 o C 35 o C For a stable atmosphere, the air mass always falls back down, regardless of whether it's dry or moist.

21 Second possibility: If the ascending air masses are both warmer than the surroundings, then we say the atmosphere is absolutely unstable. Environment Dry Air Mass Moist Air Mass 1 km 23 o C 25 o C 29 o C 0 km 35 o C 35 o C 35 o C For an absolutely unstable atmosphere, the air mass always keeps rising, regardless of whether it's dry or moist.

22 Third possibility: If the dry air mass is colder than the surroundings, but the moist air mass is warmer, then we say the atmosphere is conditionally unstable. Environment Dry Air Mass Moist Air Mass 1 km 27 o C 25 o C 29 o C 0 km 35 o C 35 o C 35 o C For a conditionally unstable atmosphere, the air mass keeps rising, but only if it's saturated (i.e., cloudy).

23 In the big scheme of things, absolutely unstable conditions are relatively uncommon. However, stable and conditionally unstable conditions happen all the time.

24 Stability and Cumulus Clouds Cumulus clouds are clouds that form in conditionally unstable environments, and they consist of rising plumes of warm, saturated air. each tuft is a rising plume of warm air

25 Stability and Cumulus Clouds Cumulus clouds are clouds that form in conditionally unstable environments, and they consist of rising plumes of warm, saturated air.

26 Stability and Cumulus Clouds Cumulus clouds are clouds that form in conditionally unstable environments, and they consist of rising plumes of warm, saturated air.

27 But remember: In a conditionally unstable environment, the lifted air mass will only keep rising if it's saturated. - What this typically means is that dry air must first be forced upward by some trigger mechanism. Once the lifted air cools to saturation, it then begins rising on its own. we need a trigger mechanism, to lift the air while it's dry

28 But remember: In a conditionally unstable environment, the lifted air mass will only keep rising if it's saturated. - What this typically means is that dry air must first be forced upward by some trigger mechanism. Once the lifted air cools to saturation, it then begins rising on its own. - This trigger mechanism could be any number of things: a front, a dry line, a warm thermal, a wave disturbance, low-level convergence, etc.

29 conditionally unstable environment

30 we need a trigger mechanism to lift the air to saturation (in this case a helicopter???) conditionally unstable environment

31 conditionally unstable environment we need a trigger mechanism to lift the air to saturation (in this case a helicopter???) at cloud base, the air is still colder than the environment

32 but once it becomes saturated, the air mass becomes warmer than the environment and rises on its own conditionally unstable environment we need a trigger mechanism to lift the air to saturation (in this case a helicopter???) at cloud base, the air is still colder than the environment

33 So when it's all said and done, what we end up with is a rising plume of warm air. conditionally unstable environment

34 Stability and Stratus Clouds Finally, recall that for stable conditions, a displaced air mass will never ascend on its own. In this case, any clouds that form must result from forced ascent--- i.e., there must be some process pushing the air upwards. Forced ascent typically results in stratus, since you're likely lifting a whole layer of air at once. Convergence into a low center, resulting in forced ascent