### Physics Group  > Air Group

Air is one of two Physics Models used by the Physics group. The Air model is useful for making particles that move naturally through air. This group of parameters contains Air Resistance, Wind direction, and other air-based settings. Air contains two displacement systems called Turbulence and Spherical. There is also a control called Motion Path which sets down path-based movement for the particles.

Technical note: In explaining the Air parameters, we will often refer to the 'spin' of a particle. By this, we mean the circular, spring-like, random motion of particles. There is no 'Spin' setting, but there are many controls that affect this spinning motion.

At left, the full Air group. At right, the Air option in Physics Model.

Motion Path

Motion Path is an interesting setting. It make the particles follow a user-defined 3D path. The path is defined relative to the 'normal' path that particles would take if this setting was not used. Therefore it is important to move the Emitter to where the path actually starts. Read about how to use Motion Path.

Left to right, the Motion Path popup and a particle trail along a motion path.

Air Resistance

Air Resistance makes the particle velocity decrease over time. Useful for explosions and fireworks where particles should start with a high velocity and then gradually slow down. Velocity is set with the Particle group > Velocity parameter.

Resistance Off       Resistance On

Air Resistance Rotation

Air Resistance stops the particles' flight by decreasing their velocity, then Gravity takes over and particles start to fall. But as the particles stop flying, they should also not rotate. There's a new setting that takes care of this. If Air Resistance Rotation is checkmarked, this means that Air Resistance will also affect the rotation. The particles will rotate fast in the beginning, but as they get stopped by Air Resistance, their rotation slows down or stops. This behavior helps to make the movement look natural. Particle rotation is set with the Particle group> Rotation group.

Spin Amplitude
Spin Amplitude makes particles move in randomized, circular orbits. A value of 0 turns off the spinning motion. Low values make small circular orbits. High values make large circular orbits. This behavior is useful for adding randomness to particle motion, which makes the animation look more natural. Amplitude can also be used to create a swarming look.

Spin Frequency
Spin Frequency sets how fast the spinning particles move in their orbit. Low values mean the particles spin slowly in their orbit. Higher values make the particles spin faster. The clips below were rendered with Motion Blur to show the motion clearly.

 Amplitude off No spin Frequency low Amplitude 100 Frequency high Amplitude 100

Fade-in Spin sets how long the particle lives before it is fully affected by the spin. Measured in seconds. High values means the spin takes awhile to affect the particles, which makes the animation fades in gradually overtime.

Note: Fade-in Spin was previously called Time Before Spin in Particular 1.5 and below.

Wind X, Y, Z

The Wind controls makes all particles move uniformly in the Wind direction. To make the Wind more realistic, Wind can be keyframed so it varies a little over time. Adding some Spin or Turbulence also helps to increase the realism.

Visualize Fields

The Visualize Fields checkbox simplifies working with Turbulence Field or Spherical Field. When working with the Field groups, sometimes you need to know exactly how the displacement field looks. Turn this checkbox On or Off to make all fields viewable. Doing so lets you adjust the field so it works in the desired way.

Note: This checkbox was previously called Visualize and used to only affect the Spherical Field in Particular 1.5 and below.

Turbulence Field

The Physics Air group offers two kinds of displacement models to push along the particles. One is Turbulence Field. Turbulence is not based on fluid dynamics. Instead, it is a 4D displacement based on Perlin Noise fractal. Turbulence adds motion so the particles that are close to each other get a similar, but not equal, random motion.

This behavior helps tremendously when creating fire and smoke effects to make particle motion look natural. Unlike fluid dynamics, Turbulence computes fast and can be freely scrubbed back and forth in the Timeline. More on the Turbulence Field group page.

Spherical Field

The Physics Air group has another kind of displacement model for particle pushing. This is Spherical Field, a displacement field in the form of a sphere. The sphere can either push particles outward or suck them in. Spherical is not a force field. The effect of the field is instant, and does not leave a trace once the field is removed. More on the Spherical Field group page.