Equation-Environment creates unequally large white space

Question

Consider the following example:

\documentclass{article}
\begin{document}
Nulla et lectus vestibulum urna fringilla ultrices.
Phasellus eu tellus sit amet tortor gravida 
    \begin{equation}
    N(\alpha) = \sigma_1(\alpha) \cdots \sigma_d(\alpha)
    \end{equation}
Nulla et lectus vestibulum urna fringilla ultrices.
Phasellus eu tellus sit amet tortor gravida placerat.
    \begin{equation}
    N(\alpha) = \sigma_1(\alpha) \cdots \sigma_d(\alpha)
    \end{equation}
Nulla et lectus vestibulum urna fringilla ultrices.
\end{document}

I noticed that before and after each equation latex uses a different amount of vertical white space. How can I make latex use a fixed amount of white space, say 10pt, before and after each equation?

I found the following code, but I am unsure how to use it properly and I haven't had any success playing around with the numbers.

\setlength\abovedisplayskip{8.0pt plus 2.0pt minus 5.0pt}
\setlength\belowdisplayskip{8.0pt plus 2.0pt minus 5.0pt}
\setlength\abovedisplayshortskip{8.0pt plus 3.0pt minus 3.0pt}
\setlength\belowdisplayshortskip{8.0pt plus 3.0pt minus 3.0pt}

Answer 1

This is by design: when the line preceding the equation is short, it's considered better to have less vertical space before (and after) it. In that case the “short skips” are used. You could set them equal to the non short skips.

The settings are made by the commands \normalsize and similar. You can find them in the sizeXY.clo files for article. It mostly depends on the class you're using. A good way to set them equal to the non short variant is

\usepackage{etoolbox}
\AtBeginDocument{%
  \appto\normalisize{%
    \abovedisplayshortskip=\abovedisplayskip
    \belowdisplayshortskip=\belowdisplayskip}%
}

If you're also using display math in \small, \footnotesize or \large, you should add similar patches for these commands.

Don't do it.