Lagrange vs Eulerian coordinates I have to solve this exercise about the Eulerian and Langrangian approach and I would like if you can take a look on my answers:

In the evening, at $t=0$ say, the temperature increases southwards in Sweden at a rate of $0.1$ K/km. A wind from the south at $3.0$ m/s brings warm air with it. However, net radiation from the air (into the clear sky) cools the air such that material air particles, travelling with the wind at speed $3.0$ m/s, decrease their temperature with time at a given constant rate of $−0.36$ K/hour.
a) What is the time derivative of temperature in [K/hour] for an observer standing still on the ground?
b) In what direction, and at what speed in [km/hour] should an observer move in order to experience a constant temperature?

These are my answers

a) 

$$\frac{\partial T}{\partial t} = -0.36 K/hour$$

b) 

$$\frac{D T}{D t} = \frac{\partial T}{\partial t}  + u \frac{\partial T}{\partial x} = 0$$

$$-0.36 + u (-0.1) = 0$$

$$u(-0.1) = 0.36$$

$$u = \frac{0.36}{-0.1}$$

$$u = -3.6 km/h$$

I have some doubts about the second because I don't know if I have to include the velocity of the fluid and calculate the observer velocity relative to that.
Note I assume the positive direction in the same direction of the wind velocity.

Thank you for help.
Edit* I think you need to consider the observer's movement, as the temperature of the particle changes over a fixed distance, but that rate is subject to both the speed of the wind (3m/s) and the perceived rate of change would thus be dependent on the speed of the observer.

Also I presume you're in SG2214 this semester, if you'd like to work on problems together I'd be happy to have a work buddy :) my email is (shsng@kth.se)
Edit* I think you need to consider the observer's movement, as the temperature of the particle changes over a fixed distance, but that rate is subject to both the speed of the wind (3m/s) and the perceived rate of change would thus be dependent on the speed of the observer.

Also I presume you're in SG2214 this semester, if you'd like to work on problems together I'd be happy to have a work buddy :) my email is (shsng@kth.se)