<p>These are both derived fields from the Unified PostProcessor (UPP). Honestly the code there is a mess and I can't tell exactly where those levels are being defined, but just from the meta descriptions it seems like the &quot;0 °C isotherm level&quot; is the lowest height of 0˚C in the atmosphere (above ground level) for a given grid point, and the &quot;Highest tropospheric freezing level&quot; is the highest such level below the derived tropopause height. So that backs up what
Poutnik's answer suggested.</p>

CC BY-SA 4.0
2024-10-20T17:36:25.523Z
2024-10-20T17:36:25.523Z
Mike Kavulich
<p>Various processes affect the freezing point. Both CCN (cloud condensation nuclei) characteristics and drop size affect the freezing point.</p>

CC BY-SA 3.0
2016-04-08T00:25:16.967Z
2016-04-08T00:25:16.967Z
BarocliniCplusplus
<p>I'm playing around with NOAA's GFS forecast data and I see that it contains both a &quot;0 °C isotherm level&quot; and a &quot;Highest tropospheric freezing level&quot;.</p>
<p>Could someone tell me the difference between these two levels? I'd assume they are the same except in the case of an inversion.</p>

CC BY-SA 3.0
2016-04-06T15:53:26.257Z
One of the effects is the change of freezing point with pressure: http://physics.stackexchange.com/questions/60170/freezing-point-of-water-with-respect-to-pressure
arkaia
CC BY-SA 3.0
2016-04-07T07:29:35.727Z
Unfortunately that's not relevant in this situation, the temperature difference in freezing point is very small on atmospheric scales.
os1
CC BY-SA 3.0
2016-04-07T12:05:15.327Z
If you read the physics.stackexchange, that is exactly what they say.
arkaia
CC BY-SA 3.0
2016-04-07T12:55:19.847Z
From browsing a few OPeNDAP servers that provide both the "highest tropospheric freezing level" and the "0C isotherm level", that's exactly what I see (that the difference between those levels is rather small).
David Hammen
CC BY-SA 4.0
2022-02-03T13:42:19.757Z
I understand "0 °C isotherm level" as "Lowest tropospheric freezing level", so it is lowest versus highest. If temperature in the troposphere profile crosses  0 °C just once, their altitude is the same. But due various inversion layers, temperature can cross  0 °C multiple times.
Poutnik
<meteorology><models><weather-forecasting><gfs>
What is the difference between the freezing level and 0 C isotherm in NOAA's GFS?
os1
# What is the difference between the freezing level and 0 C isotherm in NOAA's GFS?

(Asked by: username_0 on 2016-04-06)

I'm playing around with NOAA's GFS forecast data and I see that it contains both a "0 °C isotherm level" and a "Highest tropospheric freezing level".

Could someone tell me the difference between these two levels? I'd assume they are the same except in the case of an inversion.

**Tags:** meteorology, models, weather-forecasting, gfs

> **Comments:**
> **username_1**: One of the effects is the change of freezing point with pressure: http://physics.stackexchange.com/questions/60170/freezing-point-of-water-with-respect-to-pressure  
> **username_0**: Unfortunately that's not relevant in this situation, the temperature difference in freezing point is very small on atmospheric scales.  
> **username_1**: If you read the physics.stackexchange, that is exactly what they say.  
> **username_2**: From browsing a few OPeNDAP servers that provide both the "highest tropospheric freezing level" and the "0C isotherm level", that's exactly what I see (that the difference between those levels is rather small).  
> **username_3**: I understand "0 °C isotherm level" as "Lowest tropospheric freezing level", so it is lowest versus highest. If temperature in the troposphere profile crosses  0 °C just once, their altitude is the same. But due various inversion layers, temperature can cross  0 °C multiple times.  

---

## 2 answers

### Answer by username_4 on 2024-10-20. Score: 1

These are both derived fields from the Unified PostProcessor (UPP). Honestly the code there is a mess and I can't tell exactly where those levels are being defined, but just from the meta descriptions it seems like the "0 °C isotherm level" is the lowest height of 0˚C in the atmosphere (above ground level) for a given grid point, and the "Highest tropospheric freezing level" is the highest such level below the derived tropopause height. So that backs up what
username_3's answer suggested.


---

### Answer by username_5 on 2016-04-08. Score: 0

Various processes affect the freezing point. Both CCN (cloud condensation nuclei) characteristics and drop size affect the freezing point.


---

