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A plume from an effective height of \( 100 \) m is being modelled at a point where \( \sigma_z = 100 \) m. The vertical exponential term \( \exp(-H^2/2\sigma_z^2) \) equals

  • A.0.368
  • B.0.135
  • C.1.000
  • D.0.607

Answer: D

Correct answer: 0.607

With \( H = \sigma_z = 100 \) m the exponent simplifies:

\[ \exp\left(-\frac{H^2}{2\sigma_z^2}\right) = \exp\left(-\frac{100^2}{2 \times 100^2}\right) = \exp(-0.5) = 0.607 \]

This is the point at which the vertical spread has grown to equal the release height, so the ground is one standard deviation from the plume centreline and the term has fallen only to about \( 0.61 \) - a substantial fraction of the plume is now reaching the surface. The value \( 0.368 \) is \( \exp(-1) \) and \( 0.135 \) is \( \exp(-2) \), which correspond to larger \( H/\sigma_z \) ratios.

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