A mobile device comprising an environmental parameter sensor (1000,5000), the environmental parameter sensor comprising: a first acoustic transducer (106,800,806); a second acoustic transducer (108,802,808) arranged at a predetermined distance from the first acoustic transducer; and a controller (100,500, 550) coupled to the first acoustic transducer and the second acoustic transducer; wherein the controller is configured to: determine a first time-of-flight value of an acoustic signal between the first acoustic transducer and the second acoustic transducer in a first orientation of the environmental parameter sensor (1000,5000) and determine a first speed of sound (c for) from said distance and said first time-of-flight value; determine a second time-of-flight value of an acoustic signal between the first acoustic transducer (106,800,806) and the second acoustic transducer (108,802,808) in a second orientation of the environmental parameter sensor (1000, 5000), which is substantially opposite to the first orientation; determine a second speed of sound (C against) from said distance and said second time-of-flight; and determine at least one of ambient air temperature or wind speed from said first speed of sound (c for) and said second speed of sound (C against). a first acoustic transducer (106,800,806); a second acoustic transducer (108,802,808) arranged at a predetermined distance from the first acoustic transducer; and a controller (100,500, 550) coupled to the first acoustic transducer and the second acoustic transducer; wherein the controller is configured to: determine a first time-of-flight value of an acoustic signal between the first acoustic transducer and the second acoustic transducer in a first orientation of the environmental parameter sensor (1000,5000) and determine a first speed of sound (c for) from said distance and said first time-of-flight value; determine a second time-of-flight value of an acoustic signal between the first acoustic transducer (106,800,806) and the second acoustic transducer (108,802,808) in a second orientation of the environmental parameter sensor (1000, 5000), which is substantially opposite to the first orientation; determine a second speed of sound (C against) from said distance and said second time-of-flight; and determine at least one of ambient air temperature or wind speed from said first speed of sound (c for) and said second speed of sound (C against). determine a first time-of-flight value of an acoustic signal between the first acoustic transducer and the second acoustic transducer in a first orientation of the environmental parameter sensor (1000,5000) and determine a first speed of sound (c for) from said distance and said first time-of-flight value; determine a second time-of-flight value of an acoustic signal between the first acoustic transducer (106,800,806) and the second acoustic transducer (108,802,808) in a second orientation of the environmental parameter sensor (1000, 5000), which is substantially opposite to the first orientation; determine a second speed of sound (C against) from said distance and said second time-of-flight; and determine at least one of ambient air temperature or wind speed from said first speed of sound (c for) and said second speed of sound (C against).