Confused about temperatures on hills and valleys

[Sandy Loam]

I have never quite figured this out.  If you live here in the northern Florida, it is beneficial for the tropical gardener to live on a hill top in winter because cold air masses can be significant overnight.  The valleys receive more cold air as heat rises.  (This problem doesn't exist in the southern extremities of Florida, of course, because it never really gets cold there).

Florida is extremely flat and low-lying, so a hill top here is not very high.  It is simply higher than the valleys.

Why, then, is it the opposite in Arizona and California?  For example, I see that the lowest parts of Arizona (Yuma especially) stay warm practically all year.  As you travel eastward in Arizona, and especially northward, the elevation rises and the winter temperatures drop dramatically.  Places further south, like Tucson, are colder than Phoenix (two hours to its north) because the elevation is higher in Tucson than in Phoenix.  This may not be accurate, but it is what a local person told me in Arizona.  Yet, Tucson is not the rocky mountains!  It is not so high that is would be up in the clouds and chilly ----- or is it half way there? 

Northern Arizona has even higher elevation and it is absolutely freezing cold in winter, especially when contrasted with the daily 70 degrees Fahrenheit of Yuma, Arizona, all winter long.

I have noticed the same pattern in Southern California.  What happened to "cold sinks" in the valleys.  Doesn't that phenomenon exist in Arizona and California?  It certainly does here. 

Here, you want to live on a hill top for the best winter climate, but in California or Arizona, you don't want to???  Help me wrap my head around this.  I also saw someone writing about the benefits of living half way down a hillside slope somewhere near Palm Desert, CA.  What is the distinction?   If everyone wants to live in low lying areas to maximize their tender, tropical gardening options in Arizona, I assume that no one is afraid of "cold drains" and "cold sinks" there.  (?). Please enlighten me.

Edited February 5, 2019 by Sandy Loam

[kinzyjr]

I've found it helpful to live on the slope, but not at the very top or bottom of a hill.  At the bottom, the cold air tends to collect and sit there since it has nowhere to go.  At the top of a hill, there tends to be fewer obstacles for wind.  Interesting question that may have different answers depending on the type and duration of the weather event in question.  I'm interested in some more knowledgeable opinions and observations on this myself.

[Silas_Sancona]

Yes, it is all about location.. on a slope that is..  Narrow Valleys  surrounded by steeply angled mountain slopes are always going to be among the coldest spots ( cold air moves down hill,  pools in valleys)  Hill tops can also be cold simply due to exposure, altitude, etc. Mildest areas are almost always somewhere in the middle on a slope. How mild that slope might be?  depends on how it is angled.. Looking at a map.. In CA ( Southern CA. / San Diego area esp.). this is where most Avocado groves are located. Cold air has a hard time *sitting* on a slope ( ..again, moving down hill to lower elevations below ) Here, this is where some of the more frost tender native plants can be found ( Bursera microphylla for example)

We here in Phoenix benefit from the Heat island effect, and the fact Phoenix sits in a very broad valley that generally slopes toward the west /southwest, following the Gila/Salt river toward Yuma. The L.A. Basin opens to the Ocean, thus the ocean helps modify any cold air moving toward it from the inland valleys east of there.  Here and there, cold air has a lot of warmer artificial surfaces to move over, also helping to modify it. No heat island and both areas might be colder under optimal radiational frost/freeze conditions / events. Advective freeze events can happen, but aren't as common as east of the Rockies.

Local hills here in AZ can experience similar slope-related conditions as in CA... I can start a hike ( like the one i did last week ) on a morning where it's only say 65F at the trail head / parking area.. Half way up the trail, through a canyon, there's no real breeze and the temp. easily feels 7-13 deg warmer. At the top of the trail / atop a nearby hill above the road, it is cooler again, with a fresh breeze blowing over the ridge, heading back down? hot again.  On a cold night, i'd bet you would experience a colder wind flowing down and out of that area. When i used to bike ride, i'd sometimes bike a paved trail near where i grew up in San Jose at night. It was almost eerily strange how i could feel the temperature drop as i passed some of the canyon openings, or when i'd descend down into a lower spot close to the creek nearby. The difference in the *feels like* temp. was quite dramatic at times, even over a length of a few hundred feet.

Altitude also plays a big part in temperature gradients in AZ. One reason Tucson is generally considered a few degrees cooler than Phoenix, is due in part to being higher.. this and Tucson's Heat Island footprint is smaller than ours, ..and Tucson is surrounded by mountains relatively close to populated areas, thus aiding in more effective cold air drainage off those slopes / out of the canyons, down into the Tucson *basin*.  As you head south from Tucson,  toward the U.S. / Mexico Border, elevation rises, even if to some, a lot of areas look flat. From here in Chandler, here's how elevation plays out as you head south:

All areas with a * are smaller towns surrounded by mostly open wilderness.. very little to no *heat island* effects on temperatures / very effective radiational effect, even during the summer. 

Chandler: 1196'

Casa Grande: 1414'

Casa Adobes area of Tucson 2437'

Near Downtown Tucson: 2502'

*Vail, AZ. 3519'

*Sonoita, AZ 4843'

*Lochiel, on the AZ / Mexico Border, southern end of the San Rafael Valley: 4713'

**These 3 spots can get snow, more often than can occur in Tucson, but are also greener, esp. in the summer,  than here in the lower / warmer deserts. 

Elevation also increases as you head east into New Mexico.. The much higher Rim country mountains north / northwest of Phoenix,  favorably oriented valley gaps between sky island mountain ranges in the White Mountains to the east of town, also help keep any cold air that might spill over the rim from up over the higher deserts in the North East part of AZ  away from most of Phoenix under most situations. It has to be quite cold, and that cold air mass quite deep to drive enough cold air over the rim to reach us here. 

Here in Chandler, some spots just southeast of me in Chandler Heights can stay warmer during cold spells simply due to sitting on enough of a slope which is higher than Chandler itself, but not as high as some of the peaks in the San Tan Mountains where Chandler Heights sits beside. Casa Adobes and Catalina Foothills in Tucson can experience the same *thermal slope* benefit  during cold spells as well.  Come down the slopes those neighborhoods sit on, to where the Santa Cruz River runs through town and it can get a lot colder. Even the AZ. Sonora Desert Museum, out in a sparsely populated part of the general Tucson area can be a hair warmer than the lowest spots along the river (runs through town) at times. 

Yuma's generally milder winters are directly related to proximity to the Gulf of California.. It has to get really cold across the region for Yuma to see a significant freeze event. Keep in mind it gets extremely hot, they barely get any rain.. and experience lots of dust storms,  pretty much any time of year.. Wouldn't live there if you sold me the city for .25 cents.

[Chester B]

I can relate my own experiences here in Oregon.  If you live on the west side of the Cascades then you're on the warm side of the state where Portland is located.  In winter when we get colder weather intrude there are two things seen which coincides with what you both were saying.  Elevations about 1000' are far more likely to get snow.  Many of those areas require chains or snow tires by law when inclement weather hits the city.  From my house I look out at one of the small mountains or large hills that reaches about 1200' in elevation, and the homes near the top often will have snow on the rooftops whereas further down no snow at all.  For every 1000' in elevation you lose 3.3 F.

I live on another one of these extinct volcanoes about 300'-400' up.  On cooler nights where frost is a risk often I will have none but the homes on the street below me will have frosts on their rooftops and lawns.  This is where the cold air is flowing downhill and pooling in the low lying areas.

So I think there are two different factors at work here, but consensus seems to be partway up a hill is best.

 

Found this article that might be of interest.

https://www.onthesnow.com/news/a/15157/does-elevation-affect-temperature

[Palmy]

You are specifically talking about something termed, a "valley inversion." Essentially, cold air is denser and can sink and pool into a valley while the air aloft remains warmer. This usually occurs during high pressure aloft such as a ridge. Winds are usually calm due to a lack of mixing of air. This leads to an "inversion" in temperatures, so instead of what you'd call typical; air temperature decreasing with height, you actually have air temperature increasing with height, which inhibits convection and mixing in the atmosphere.

It can happen anywhere you have valleys and mountains. Sometimes the temperature gradient can be extreme. An example of this is a small mountain valley called Peter's Sink in Utah. Sometimes the temperature can change 50F like within a half a mile around Peter's Sink, but this is an extreme case. But this phenomenon happens often in the intermountain west where there are a lot of valleys and mountains/hills.

Edited February 6, 2019 by Palmy

[Cikas]

Here in Europe, the higher the atlitude, the lower the temperatures will be. Here there are no benefits with living on higher atlitude. 

[quaman58]

Here in San Diego, the canyons near the coast are traps for cooler air year 'round. That makes the summers pretty nice, but winter nights are another matter. I live toward the bottom of my street which is less than 1/4 mile long, but the temperature is often nearly 5 degrees cooler at my house than the top of the hill. Fortunately, it keeps rolling into an even deeper canyon downhill which mitigates the effects somewhat. Despite being a generally temperate climate, the 2 month period from mid December to mid February can really cause hurt to the garden.

[RJ]

32 minutes ago, quaman58 said:

Here in San Diego, the canyons near the coast are traps for cooler air year 'round. That makes the summers pretty nice, but winter nights are another matter. I live toward the bottom of my street which is less than 1/4 mile long, but the temperature is often nearly 5 degrees cooler at my house than the top of the hill. Fortunately, it keeps rolling into an even deeper canyon downhill which mitigates the effects somewhat. Despite being a generally temperate climate, the 2 month period from mid December to mid February can really cause hurt to the garden.

Wow 5* over a quarter mile is pretty dramatic. 

[Walt]

On 2/4/2019 at 9:36 PM, Sandy Loam said:

I have never quite figured this out.  If you live here in the northern Florida, it is beneficial for the tropical gardener to live on a hill top in winter because cold air masses can be significant overnight.  The valleys receive more cold air as heat rises.  (This problem doesn't exist in the southern extremities of Florida, of course, because it never really gets cold there).

Florida is extremely flat and low-lying, so a hill top here is not very high.  It is simply higher than the valleys.

Why, then, is it the opposite in Arizona and California?  For example, I see that the lowest parts of Arizona (Yuma especially) stay warm practically all year.  As you travel eastward in Arizona, and especially northward, the elevation rises and the winter temperatures drop dramatically.  Places further south, like Tucson, are colder than Phoenix (two hours to its north) because the elevation is higher in Tucson than in Phoenix.  This may not be accurate, but it is what a local person told me in Arizona.  Yet, Tucson is not the rocky mountains!  It is not so high that is would be up in the clouds and chilly ----- or is it half way there? 

Northern Arizona has even higher elevation and it is absolutely freezing cold in winter, especially when contrasted with the daily 70 degrees Fahrenheit of Yuma, Arizona, all winter long.

I have noticed the same pattern in Southern California.  What happened to "cold sinks" in the valleys.  Doesn't that phenomenon exist in Arizona and California?  It certainly does here. 

Here, you want to live on a hill top for the best winter climate, but in California or Arizona, you don't want to???  Help me wrap my head around this.  I also saw someone writing about the benefits of living half way down a hillside slope somewhere near Palm Desert, CA.  What is the distinction?   If everyone wants to live in low lying areas to maximize their tender, tropical gardening options in Arizona, I assume that no one is afraid of "cold drains" and "cold sinks" there.  (?). Please enlighten me.

In my part of Florida, elevation can mean 1/2 to 3/4 higher USDA zone. I've lived in Highlands Count ((specifically, Lake Placid) for 21+ years now. The Lake Wales Ridge runs right down the center of the county. While the elevation is only around 150' or so in the highest areas, it makes all the difference in the world on radiational cooling nights (windless with clear sky) compared to the lower outlying areas.

Warmer air rises up on the ridge while cooler air drains off the ridge. I live about 1-1/2 miles as the crow flies from town, which is about 80 feet higher than my property. I've consistantly recorded about 7-8 degrees warmer temperatures up in town (on the ridge) than at my place. I've done this by using my car thermometer, leaving my house and then driving up the hill to town.

Air temperature inversion is common on cold windless nights, but the inversion layer isn't very high, maybe 200 feet where the warmest air is, then the temperature starts to get colder with increasing elevation/altitude.

One example of higher air temperature at a given elevation is in the graphic below. This is the reading from the University of Florida's weather tower on December 8, 2010 at 9:30 p.m. As you can see, there is a 10 degree F difference between the 60 cm temperature sensor and the 10 meter temperature sensor. That's approximately a 32.5 feet difference in elevation. That's also about one USDA hardiness zone! 

But that temperature difference can and does vary, and is not as much pronounced. Sometimes the cold air is much deeper, so you have to go much higher in elevation to get into the warmest air. But by and large, on windless cold nights, it's always warmer, up to a point, at higher altitude in my area.

[Sandy Loam]

Walt, that's an interesting diagram from that very cold night in 2010:  ten degrees difference from top to bottom!

I suppose the main difference for those of us in Florida is that the topography is just so flat here than our hill top elevation is never high enough to cause an actual drop in temperature.  Several parts of the state are so close to sea level that there isn't really any real elevation to speak of.  As a result, the valley effect is the same here as elsewhere, but our hill top effect is different.

[NC_Palms]

I've noticed this phenomenon here. Our topography is flat and low lying but depressions do exist, mostly in the form of Carolina bays. Because cold air sinks, these Carolina bays trap cold air. Fortunately I don't live in a depression like this, but I know of many people who do, and unfortunately they've lost many palms 

[Walt]

21 hours ago, Sandy Loam said:

Walt, that's an interesting diagram from that very cold night in 2010:  ten degrees difference from top to bottom!

I suppose the main difference for those of us in Florida is that the topography is just so flat here than our hill top elevation is never high enough to cause an actual drop in temperature.  Several parts of the state are so close to sea level that there isn't really any real elevation to speak of.  As a result, the valley effect is the same here as elsewhere, but our hill top effect is different.

That's correct. The topography in Florida (Lake Wales Ridge) and up in northern Florida isn't high enough in elevation to be colder than at lower elevation on severe radiational cooling nights.  Many years ago I found a graphic on Google showing the air inversion layer. It's probably 200' of so above sea level, or lower. That's why helicopters are sometimes used to fly in the inversion layer over a grove, orchard, crops, etc., so to push the warm air downward and mix it up with the colder lower air.

When I first moved to Lake Placid there was a small citrus grove area nearby. The owner sold various potted citrus plants and fruit baskets, etc. His place was at a higher elevation than mine, and I noticed (after a hard freeze at my place) his bougainvillea shrubs weren't hurt, but mine frozen back to the ground. He confirmed that the elevation differences along the Lake Wale Ridge citrus groves varied, and that citrus trees grown in the lower elevation areas would sometimes get cold damage, but groves higher up on the ridge didn't incur damage.

Of course, over the years I've observed many radiational freezes. In fact, the coldest record setting low temperatures for me have always been on radiational cooling nights.

I have many tall slash pine trees with pothos vines growing all the way to the top. On most radiational cooling nights when the temperature near ground level drops below 30 degrees and into the high 20s, these vines will get cold damage up to about 15 feet on the trunk, then the damage would be less, transitioning up to about 18 feet, where there was no damage. A clear indication of the elevation where the air temperature was above 30 degrees. Same thing would happen on my large Ficus altissima 'variegata' tree. The bottom half would get leaf damage, but the upper half would have no damage.

At this link it states the warmer air is about 10 to 50 meters above the ground.

ttps://www.nzfrostfans.com/about-frosts

[Zeeth]

How much does this phenomenon help for tall palms whose canopy is above the freezing mark? Does the trunk damage caused by the freeze still translate to dead palm, or is it mostly foliage dependent?