[Nqp15hhu]

I have had my own weather station for a while now. I am aware of the drop in temperature here on a clear night. Usually temp drops in steps, with a drop of 3-4c in the first hour or so after sunset.

Beyond that the temperature stabilises for 5 or so hours, maybe even rising, then dropping slowly, with a sharp drop before sunrise.

Anyway, it’s now 4am. We have had a 1c degree drop in 5 minutes. Humidity fell by 10%.

I haven’t had this happen before. What would cause this random drop at night?

[Nqp15hhu]

Anyone?

[Gaoth Laidir]

It's just one point in time. Without knowing the setup of the station, changes in cloud cover, etc., it's impossible to say what caused it. My guess would be that shift in wind direction towards easterly brought in colder air from somewhere to your east, maybe away from the house and more an open field. Also, there was an area of low cloud to your south, moving westwards, moving probably out of direct line of sight from you at around that time. Click here and sequence the images forwards to see it moving westwards. In an otherwise clear night, this the disappearance of this cloud could theoretically have tipped the radiation balance in favour of extra cooling.

[Nqp15hhu]

Originally Posted by Gaoth Laidir

It's just one point in time. Without knowing the setup of the station, changes in cloud cover, etc., it's impossible to say what caused it. My guess would be that shift in wind direction towards easterly brought in colder air from somewhere to your east, maybe away from the house and more an open field. Also, there was an area of low cloud to your south, moving westwards, moving probably out of direct line of sight from you at around that time. Click here and sequence the images forwards to see it moving westwards. In an otherwise clear night, this the disappearance of this cloud could theoretically have tipped the radiation balance in favour of extra cooling.

With a 1c drop in 5 minutes!? Really?

[Oneiric 3]

I have noticed this trend myself on the very few clear nights we get in winter/early spring. Temps will drop dramatically up to and shortly after sun down, then stabilise and even rise during the small hours, only to fall sharply in the run up too and shortly after sunrise. Always wondered why that happened. It is almost like the air is saying to itself 'one last push before the sun comes up'

[Oneiric 3]

There was a push of cooler, dryer air into the north of Ireland around the time in question:

[Nqp15hhu]

Originally Posted by Oneiric 3

There was a push of cooler, dryer air into the north of Ireland around the time in question:

Hmmm interesting point.

[Nqp15hhu]

Hmmm there must be some sort of cold source that my station is pulling from... just dropped 1c again in only 6 minutes..

[M.T. Cranium]

We get used to watching the weather reports from airports. They naturally place airports on flat ground that is surrounded by open land, so the air mixes freely. One result of that is a rather uniform temperature trend at airport weather stations. At our own locations, we are more likely to live in less uniform surroundings, maybe in a setting with different types of terrain all around us. So the air movements, particularly at night, can introduce different samples of nearby micro-climates and make the temperature trends erratic and as you said random.

Not just theorizing this, I also had my own home weather stations which I would read at odd intervals, and correlate with less precise thermometers on windows that you can read more frequently without going to much bother. So I became aware of this same phenomenon. The temperature did tend to fall off most frequently around sunrise where I used to observe too (in a largely rural setting). I think the reason for that is a greater atmospheric stability setting in just before the heat from the Sun can begin to overturn the inversion layer that is usually present on a clear night.

Living in North America with perhaps greater air mass contrasts, I've seen some very rapid temperature changes, one in particular was actually visible on a mercury thermometer on a steadily warming April morning, while I was watching the mercury actually rose about ten degrees in less than fifteen minutes. I have also done numerous mobile traverses to study urban heat island boundary effects. You'd be surprised how different the temperature can be over the space of 100-200 metres in open country, especially if there is a bit of snow on the ground. Then with random puffs of wind these different regimes can slowly eddy around and mix, so if you're a stationary observer you might see all kinds of odd variations. In one experiment I found that calibrated thermometers were reading 2-3 deg different after placing them in identical screens on a clear night, in different parts of a large garden (more like an empty field but with a few clumps of trees, and some unused buildings). So if there was a breeze in that situation, air that was warmer or colder might be blown around and hit your one stationary instrument causing it to appear to be oscillating when in fact there were all sorts of micro-climates within a few feet of one another.

An airport is probably the last place these sorts of things might happen but I was part of a research study when the airport weather instruments were moved from one point to another on the large complex of Toronto's main airport, and despite the new setting looking visually almost the same as the abandoned one, the sample period showed that clear nights were running about 1.5 C deg colder (calibrated instruments were placed at both locations). There was no particular obvious reason why given that the whole complex was flat and well away from anything that might disturb the air, but it turned out to be something like the effects of being a bit closer to runways before the move, therefore the air tended to mix more, and mixing air at night usually gets warmer.

[riffmongous]

but it turned out to be something like the effects of being a bit closer to runways before the move, therefore the air tended to mix more, and mixing air at night usually gets warmer

What evidence did they actually have to support that conclusion? Did they make any turbulent measurements?

[Nqp15hhu]

You can see that my weather station had this affect again last night:
https://www.wunderground.com/dashboard/pws/ILIMAV3

The lower station did not:
https://www.wunderground.com/dashboard/pws/ILIMAVAD4

Not sure if this is due to being sort of on the bottom of a hill, so maybe I am getting influence from the airstream on the hill at times and the air stream in the valleys on other times?

[Nqp15hhu]

This higher elevation station also had similar drops:
https://www.wunderground.com/dashboard/pws/ILONDO315

Another valley station:
https://www.wunderground.com/dashboard/pws/IDERRY5

[Nqp15hhu]

Originally Posted by M.T. Cranium

We get used to watching the weather reports from airports. They naturally place airports on flat ground that is surrounded by open land, so the air mixes freely. One result of that is a rather uniform temperature trend at airport weather stations. At our own locations, we are more likely to live in less uniform surroundings, maybe in a setting with different types of terrain all around us. So the air movements, particularly at night, can introduce different samples of nearby micro-climates and make the temperature trends erratic and as you said random.

Not just theorizing this, I also had my own home weather stations which I would read at odd intervals, and correlate with less precise thermometers on windows that you can read more frequently without going to much bother. So I became aware of this same phenomenon. The temperature did tend to fall off most frequently around sunrise where I used to observe too (in a largely rural setting). I think the reason for that is a greater atmospheric stability setting in just before the heat from the Sun can begin to overturn the inversion layer that is usually present on a clear night.

Living in North America with perhaps greater air mass contrasts, I've seen some very rapid temperature changes, one in particular was actually visible on a mercury thermometer on a steadily warming April morning, while I was watching the mercury actually rose about ten degrees in less than fifteen minutes. I have also done numerous mobile traverses to study urban heat island boundary effects. You'd be surprised how different the temperature can be over the space of 100-200 metres in open country, especially if there is a bit of snow on the ground. Then with random puffs of wind these different regimes can slowly eddy around and mix, so if you're a stationary observer you might see all kinds of odd variations. In one experiment I found that calibrated thermometers were reading 2-3 deg different after placing them in identical screens on a clear night, in different parts of a large garden (more like an empty field but with a few clumps of trees, and some unused buildings). So if there was a breeze in that situation, air that was warmer or colder might be blown around and hit your one stationary instrument causing it to appear to be oscillating when in fact there were all sorts of micro-climates within a few feet of one another.

An airport is probably the last place these sorts of things might happen but I was part of a research study when the airport weather instruments were moved from one point to another on the large complex of Toronto's main airport, and despite the new setting looking visually almost the same as the abandoned one, the sample period showed that clear nights were running about 1.5 C deg colder (calibrated instruments were placed at both locations). There was no particular obvious reason why given that the whole complex was flat and well away from anything that might disturb the air, but it turned out to be something like the effects of being a bit closer to runways before the move, therefore the air tended to mix more, and mixing air at night usually gets warmer.

My setting - I live on the bottom of a hill, sort of on a step above the valley but not high. There is land at around 400 metres 3 miles away to my NE. 350m about 2, miles.

Commonly, I do observe a stepped dropping temperature where I live - on clear nights. I have rarely or never observed these dramatic drops that I have observed as of late.

For ex, down from 2.4c at 2:55am to 1.7c at 3:00am. This seems to only happen with a SE or SSE wind, which in my location is blowing from the land immediately to the south of the hill.

I do wonder if the hill is having this impact? I have observed other odd phenomena locally, for example, one night driving from derry home, 3c and heavy rain for over 15 miles, as soon as I hit my road the temp immediately drops from 3c to 1c and we have a intense snow.

I often ponder if that was caused by a downdraft off the top of the hill?

[Marine Layer]

Originally Posted by Nqp15hhu

With a 1c drop in 5 minutes!? Really?

Yes
I've a Davis since 2003 and often saw it
I dont often go out to look at why

[Danno]

I've never paid much attention to the humidity variations here in south Laois at night, my memory tells me at a glance that humidity values are relatively static and uniform throughout the night time hours.

I do though notice very common temperature drops, stalls and even subtle increases on cold nights. With the anemometer at standard 10 metre height I can observe subtle wind strengths that would not be detected at lower levels.

I've seen a 2mph gust cause temperatures to rise when blowing from any direction between 0 and 180 degrees (north, east, south), however that same gust from between 180 degrees and 360 degrees (south, west, north) can cause temperatures to fall. Hilly terrain is to my south and west.

The night we recorded -9.2c in January was one such night under almost calm conditions, any subtle breath of wind caused an effect on temperatures.