Topic: Coldest AFTER Sunrise

[Randall Kayfes]

Logic tells us it would stop cooling and start warming when the sun rises. However I had often heard that it would actually be cooler after Sunrise.  So decided to test the theory against my data. Here in Arizona we have the distinct pleasure of not having much cloud cover to interfere with the analysis of the data. To present the data in a manner where Sunrise is the constant, I had to match Time interval 1 at the archival interval that closest matched the time of Sunrise. I also had each interval post sunrise consecutively number from Sunrise.  So interval 1 represents the temp at the time of sunrise and interval 2 is ten minutes later etc. until 60 minutes later.  I used If, And, Match, Vlookup, formulas as I was not about to do it by hand for 365 days of 6 datasets.

Looking at the data it would appear the folklore darkest before dawn is false it is actually after sunrise...

[Sigdigit]

I typically don't realize my coldest temps until an hour or two after sunrise.  But I am in a valley.

[Weather Display]

the heat from the sun is at  very oblique angle across the surface of the earth (i.e spread over a large area) at sunrise
so does not compensate for heat losses to space until its high enough in the sky (that happens faster at the equator where its going straight up)

[Randall Kayfes]

Just to clarify I also did a correlation with the time of sunrise and first Solar Radiation reading and all time interval 1's had solar radiation readings.



There are many theories as to the phenomenons cause.  Brian I find the one you postulated as the most plausible. I just wanted to verify it with my own data - very fun!

Randall

[vaughanweather]

My grass minimum temperature is often a few degrees cooler than the 2m air temp. Once the sun rises and begins warming the earth, on calm days I will often see a bump as the temperature suddenly dips usually at or just after sunrise. While the 2m temp falls the grass temp begins to rapidly rise. This leads me to believe that the heat of the sun causes the warmer air at the ground to lift the cooler air now above it where it reaches the sensor just before convective mixing takes over and washes out the cooler air. On days with mechanical mixing from wind or precipitation no bump is observed.

[DaculaWeather]

Always a lag.

[WeatherHost]

Not into charts, but I don't see anything unusual about this.  Sunrise/light doesn't immediately equal heat.

Isn't that when frost is most destructive?  When the sun first hits it?  May be an Old Wive's Tale, but word has it that when frost is expected, if you hose plants down before or just at sunrise, frost won't do as much damage.

[Bushman]

Not into charts, but I don't see anything unusual about this.  Sunrise/light doesn't immediately equal heat.

Isn't that when frost is most destructive?  When the sun first hits it?  May be an Old Wive's Tale, but word has it that when frost is expected, if you hose plants down before or just at sunrise, frost won't do as much damage.

Not a tale at all:  think "latent heat of water"