<p>I remember reading a long time ago in a meteorology book that certain dust storms were caused by a rapid descent of a large air mass. As the air is pressed against the ground, it is forced to spread outward at a high velocity, whipping up dirt along the way. A similar effect can be viewed with water. </p>

<p>If you were to pour a stream of water onto the ground, you could observe that the beads of water near the contact site are spreading outward more quickly than the velocity of the stream, before drag forces slow them down. </p>

<p>However, that was years ago, I don't remember the book or the name of the phenomena. </p>

CC BY-SA 4.0
2018-06-29T14:43:11.143Z
In the case of the dust storm, you may have heard of the word 'haboob,' of which a downburst is a part of.
BarocliniCplusplus
CC BY-SA 4.0
2018-06-29T19:37:34.747Z
Also called microburst.   Big problem for aircraft, as they are invisible.  Several bad crashes have been attributed to a plane on final landing approach getting hit with one (abrupt increase in descent speed) or having one hit the ground behind them (sudden tail wind stalls the wing)

Sometimes you can see the effect in wheat fields where downbursts have flattened parts of the field.
Sherwood Botsford
CC BY-SA 4.0
2018-07-02T14:32:29.760Z
The phrase "more quickly than the velocity of the stream" sounds almost like you could be saying that hitting the ground speeds up the fluid more than it was moving the moment before it hit the ground.  That shouldn't be true physically.  Both certainly gain a lot of velocity as they descend (due to the density of the [cold] air for wind and gravity acceleration for the stream for water), but they only transfer their vertical motion into horizontal motion when hitting the ground, rather than somehow gaining additional speed at the ground.
JeopardyTempest
CC BY-SA 4.0
2018-07-02T17:13:09.850Z
@JeopardyTempest Well I read that it is actually true physicall in the weather book I was referring to when it described a microburst which is why I'm so adamant about it. I think it makes sense because if the fluid particles are losing potential energy, they gaining kinetic energy, and the maximum kinetic energy they gain is just at ground level. Then, when they diffuse into a thinner layer, the conservation of momentum would say these lower-mass pockets of air travel faster with the same energy.
John Joe
CC BY-SA 4.0
2018-07-02T17:31:09.430Z
Definitely agree they will have gained the most kinetic energy when they are just getting to ground level, yes [well, assuming for wind that the air density remains greater than the surrounding air as it descends (and warms), which I don't believe will always be true, but we can consider a very strong cold pool, where it should be].
JeopardyTempest
CC BY-SA 4.0
2018-07-02T17:53:55.187Z
But don't think spreading out/thinning the layer should change speeds, as each part of the mass keeps its own part of the momentum... total mass isn't changing, so total velocity shouldn't change (since momentum = $m*v$) unless some outside force causes acceleration. $\frac{1}{2}(m_1+m_2)v^2 = \frac{1}{2}m_1v^2 + \frac{1}{2}m_2v^2$, so energy separation doesn't equate to velocity change. Comets/cars don't speed up if they break apart. Certainly up for being shown wrong, it certainly happens plenty! But thinking this may be a mistake in understanding or language somewhere.
JeopardyTempest
CC BY-SA 4.0
2018-07-02T17:59:47.230Z
(Or perhaps there's some other confusion, such as trying to use Bernoulli's Equation, which I don't believe is applicable since there's no pressure change [no force change upon the water])?
JeopardyTempest
CC BY-SA 4.0
2018-07-02T18:01:14.010Z
But anyways, it's an aside to your question, and perhaps if you're still uncertain with that aspect, we can continue in a [chat](https://chat.stackexchange.com/?tab=site&host=earthscience.stackexchange.com) or you can ask a separate question :)
JeopardyTempest
<meteorology><storms>
Is there a conventional name for when wind speed increases as a result of a large descending air mass?
John Joe
# Is there a conventional name for when wind speed increases as a result of a large descending air mass?

(Asked by: username_0 on 2018-06-29)

I remember reading a long time ago in a meteorology book that certain dust storms were caused by a rapid descent of a large air mass. As the air is pressed against the ground, it is forced to spread outward at a high velocity, whipping up dirt along the way. A similar effect can be viewed with water.

If you were to pour a stream of water onto the ground, you could observe that the beads of water near the contact site are spreading outward more quickly than the velocity of the stream, before drag forces slow them down.

However, that was years ago, I don't remember the book or the name of the phenomena.

**Tags:** meteorology, storms

> **Comments:**
> **username_1**: In the case of the dust storm, you may have heard of the word 'haboob,' of which a downburst is a part of.  
> **username_2**: Also called microburst.   Big problem for aircraft, as they are invisible.  Several bad crashes have been attributed to a plane on final landing approach getting hit with one (abrupt increase in descent speed) or having one hit the ground behind them (sudden tail wind stalls the wing)  Sometimes you can see the effect in wheat fields where downbursts have flattened parts of the field.  
> **username_3**: The phrase "more quickly than the velocity of the stream" sounds almost like you could be saying that hitting the ground speeds up the fluid more than it was moving the moment before it hit the ground.  That shouldn't be true physically.  Both certainly gain a lot of velocity as they descend (due to the density of the [cold] air for wind and gravity acceleration for the stream for water), but they only transfer their vertical motion into horizontal motion when hitting the ground, rather than somehow gaining additional speed at the ground.  
> **username_0**: @username_3 Well I read that it is actually true physicall in the weather book I was referring to when it described a microburst which is why I'm so adamant about it. I think it makes sense because if the fluid particles are losing potential energy, they gaining kinetic energy, and the maximum kinetic energy they gain is just at ground level. Then, when they diffuse into a thinner layer, the conservation of momentum would say these lower-mass pockets of air travel faster with the same energy.  
> **username_3**: Definitely agree they will have gained the most kinetic energy when they are just getting to ground level, yes [well, assuming for wind that the air density remains greater than the surrounding air as it descends (and warms), which I don't believe will always be true, but we can consider a very strong cold pool, where it should be].  
> **username_3**: But don't think spreading out/thinning the layer should change speeds, as each part of the mass keeps its own part of the momentum... total mass isn't changing, so total velocity shouldn't change (since momentum = $m*v$) unless some outside force causes acceleration. $\frac{1}{2}(m_1+m_2)v^2 = \frac{1}{2}m_1v^2 + \frac{1}{2}m_2v^2$, so energy separation doesn't equate to velocity change. Comets/cars don't speed up if they break apart. Certainly up for being shown wrong, it certainly happens plenty! But thinking this may be a mistake in understanding or language somewhere.  
> **username_3**: (Or perhaps there's some other confusion, such as trying to use Bernoulli's Equation, which I don't believe is applicable since there's no pressure change [no force change upon the water])?  
> **username_3**: But anyways, it's an aside to your question, and perhaps if you're still uncertain with that aspect, we can continue in a [chat](https://chat.stackexchange.com/?tab=site&host=earthscience.stackexchange.com) or you can ask a separate question :)  

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## 0 answers

