# Difference between Mountain Breeze/Katabatic Winds and Valley Breeze/Anabatic Winds

I am studying Meteorology for my CPL exams, and taking EASA's 2020 Learning Objectives as a guideline for the topics I have to study.

In the topic of local winds, there are subtopics for Anabatic & Katabatic Winds and Mountain & Valley Winds (050.02.04.01.01 and 050.02.04.01.02 in the screenshot below).

The book I am primarily using does not mention anything about Mountain & Valley winds and I have searched the internet and the only thing closer to the Mountain and Valley Winds I could find were the Mountain and Valley Breezes, which I am assuming refer to the same phenomenon.

The problem, however is that some resources say that Mountain Breeze and Katabatic Winds are the same:

...however, some online resources categorically say that they are different, but no further explanation is provided except that Anabatic and Katabatic Winds are on much larger scale than Valley and Mountain Winds:

The origin of Anabatic and Valley winds are similar:

Slope of hill warms up due to more direct insolation to mountain side than the valley floor, leading to air on the slope being warmer and less dense than the air in the valley. Air on the mountain slope rises up and cold air in the valley replaces the rising air

Similarly, the formation mechanism for Katabatic Winds and Mountain Winds are also similar:

At night the ground will cool more quickly and becomes colder than the air above. Air in contact with the ground cools. This now denser air slides downslope and moves into the valleys.

So my question is What makes Anabatic and Katabatic Winds differ from Valley and Mountain Winds? Since their formation mechanisms are the same, what makes Anabatic and Katabatic larger and stronger?

P.S. I realize that the resources that I have shared are not very reliable and can contain inaccurate information. However, I couldn't find any reliable resource which dealt with and compared both phenomenons

## 3 Answers

Under the right conditions-- i.e. with the right large-scale meteorological forcing-- the wind can blow up or down a slope -- causing an associated cooling or heating effect-- independent of any diurnal cycle. E.g. the famous downslope "Santa Ana" winds, or "Chinook" winds. Note that in these cases, the air is being warmed by its descent, so much that a person in the valley below will feel an increase in temperature when the wind arrives.

In the case of diurnal mountain/valley breezes or winds, the air is warmed by the sun (or cooled by radiation due to lack of sunlight) and then moves up or down the slope accordingly. If you stood in a valley in the mountains in the evening and felt the temperature of the evening downslope breeze (or wind), you'd likely feel a drop in temperature, not a rise, because the air entering the valley from above would be air that had lost heat through radiation somewhere higher up on the slope, and then moved down the slope. Even though the air would experience some amount of katabatic warming during its descent down the slope, it would still likely feel like a cool wind, displacing warmer air from the valley. In the case of diurnal breezes or winds where there is no larger-scale meteorological forcing, if the descending air experienced katabatic warming such that it was no longer cooler and and denser than the air it was encountering on it its downslope journey, it would lose its impetus to continue downslope, and the breeze or wind would tend to come to an end or at least disperse in altitude so that it was no longer concentrated near the ground. If there is larger-scale meteorological forcing, the downslope winds can continue even in conditions where they are causing the temperature in the valley below to increase, not decrease. They can also continue day and night, independent of the diurnal cycle.

But the highlighted content in your source is confusing, because it tends to suggest that predictable daily upslope/ downslope (mountain/ valley) effects should not be described as anabatic/ katabatic phenomena. Fundamentally, "anabatic" and "katabatic" just mean "upslope" and "downslope". They don't pertain to why the air is moving up or down the slope.

I do not agree with the answer by quiet flyer. I especially do not agree with the conclusion "Fundamentally, "anabatic" and "katabatic" just mean "upslope" and "downslope". They don't pertain to why the air is moving up or down the slope."

Anabatic and katabatic winds are thermally induced winds, (with katabatic sometimes also used for other downslope winds) with the mechanisms similar to a land/see breeze, but also involving a sloping terrain. The classical explanation involves the Bjerknes circulation theorem and can be found in textbooks.

The AMS glossary:

anabatic wind
In mountain meteorology, an upslope wind driven by heating (usually daytime insolation) at the slope surface under fair-weather conditions. ...
In general usage, this term does not suffer from the multiplicity of meanings that katabatic wind does.

katabatic wind

1. Most widely used in mountain meteorology to denote a downslope flow driven by cooling at the slope surface during periods of light larger-scale winds; the nocturnal component of the along-slope wind systems.

2. Occasionally used in a more general sense to describe cold air flowing down a slope or incline on any of a variety of scales, including phenomena such as the bora, in addition to thermally forced flows as described above.

Still in the second general sense it is still a cold heavy air. Not just any downslope wind. The bora gets its strength from the stable stratification and from the heavy cold air rushing down after overcoming the mountain ridge, albeit it was forced to overcome it dynamically.

The mountain and valley and slope winds specifically:

These are thermally induced winds in a specific environment. They can be explained by density differences arising in the horizontal direction due to mountain slopes being heated during the day and cooled during night. This causes a density difference in comparison with the air at the same altitude far from the mountain. The physical mechanism is similar to the mechanism causing the land and the sea breeze and the vorticity generation can be explained by the Bjerknes circulation theorem. (An interesting historical perspective at Thorpe et al., BAMS 2003.

The mechanism is quite general, but the nomenclature distinguishes:

1. the land and the sea breeze - mostly horizontal across the shore. The density difference arises due to higher sensible heat fluxes over land.

2. the slope winds - along sloped surface. The density difference arises due to the slope surface being heated and cooled by radiation (solar shortwave and own thermal longwave)

3. the mountain and the valley winds - or the mountain-valley wind systems These happen on a larger scale, not just along a single slope but on the scale of a whole mountain range. The air heated close to the surface by solar radiation rises from the valleys during the day and the air cooled by the surface infra-red radiation descends from the mountains during the night.

The main difference between the terms anabatic/katabatic vs. mountain/valley is the specificity of the term. The mountain and valley winds are more specific terms describing specific examples of anabatic and katabatic winds.

• Interesting answer, and likely the best one. I was going in part off the actual derivation of the words. But does not your answer imply that the some of the source material in the question itself was incorrect? For example the original source material quoted by the OP implied that anabatic and katabatic winds were a different phenomenon that mountain/valley winds. Another source quoted by the OP stated that anabatic and katabatic winds were "larger and stronger" than mountain/ valley winds. I take it you would disagree with all this? Might be worth incorporating into answer. Apr 4 at 18:55
• @quietflyer I should probably address this. Anabatic and katabatic are more general terms than mountain/valley winds. Apr 4 at 18:58

When I was younger I used to wonder why they even bothered to draw "dry fronts" on the map out in Texas and why they were so important.

Just as cold air is denser than warm air, dry air is denser than humid air (at the same temperature).

The misconception here seems to be description of local and larger weather phenomena, behaving the same way for different reasons.

Locally, the ground absorbs heat from the sun$$^1$$ better than air during the day, resulting in upslope winds during the day. At night, the ground radiates its heat and becomes colder than the air (except for a layer near it), resulting in a downslope flow of wind. Because this is a local phenomena, there are not any great changes in humidity. Anabatic and katabatic winds are strictly driven by temperature differences.

On a larger scale, winds driven up over higher terrain have moisture wrung out of them by forming clouds and rain. This "dry" air can descend into valleys by the exact same mechanism: higher density. Being of larger scale, winds such as the Chinook can reach hurricane strength, whereas local winds are more like "breezes".

According to this reference downslope winds created by differences in humidity are not "true" katabatic winds. (Even though temperature and humidity differences both cause wind, and combinations of cold/dry and warm/humid produce the strongest effects). Curiously, the California's Santa Ana wind is considered a "katabatic" wind, but, coming off the desert, one might wonder where its moisture went?

$$^1$$ much of the solar radiation reaching the earth is in the Infrared wavelengths, along with visible light