# [ferret_users] Re: HOw to convert potential temperature to in-situ temperature?

```Here's a response to an old unanswered question to this group:

```
To convert from potential temperature to in-situ temperature, it is very easy. It can be done simply by using the existing Ferret function that does the opposite.
```
```
As documented in the Ferret user manual, the function THETA_FO is designed to convert from in-situ temperature to potential temperature (only). The arguments are
```
THETA_FO( salt, temp, P, P_reference)

```
What is undocumented in the manual is that to do the opposite (convert in-situ to potential temperature), we only need to inverse the 3rd and 4th arguments. For example
```
let Tpot     = theta_fo(35,20,5000,0)
let Tinsitu  = theta_fo(35,19,0,5000)
list Tpot, Tinsitu
! Results should be Tpot=19 and Tinsitu=20

```
This might be useful for some, e.g., ocean modelers who typically store out potential temperature fields but sometimes need in-situ temperature in post-treatments. It would be nice if this currently undocumented feature of THETA_FO would be added to the user's manual.
```
Thanks,

Jim

-----------------------------------------------------------
Old unanswered Question from Ferret archive:
-----------------------------------------------------------
* To: ferret_users
* Subject: HOw to convert potential temperature to in-situ temperature?
* From: Yang Kun
* Date: Fri, 25 Feb 2000 15:13:43 -0600 (CST)

HI, all

I know theta_fo can calculate potential temperature.
Is there also a fuction to do the contrary thing in ferret?
Or, I have to write Fortran to do this?
Thanks.

yours,
Kun

```