Exponential distribution.
Its probability density function is
for x > 0 and 0 elsewhere. is known as the rate parameter.
The exponential distribution is a continuous analogue of the geometric distribution. It describes many common situations, such as the size of raindrops measured over many rainstorms [61], or the time between page requests to Wikipedia [62].
Parameters:  scale : float
size : tuple of ints


References
[61]  Peyton Z. Peebles Jr., “Probability, Random Variables and Random Signal Principles”, 4th ed, 2001, p. 57. 
[62]  “Poisson Process”, Wikipedia, http://en.wikipedia.org/wiki/Poisson_process 