Why does an empty line between text and "align" have an effect, but not between text and "equation"?

Question

So I played around a little bit recently and I noticed that empty lines between text and and a \begin{align} produce a big vertical whitespace which is not the case with empty lines between text and \begin{equation}, even though there might be a very subtle effect with equation, but I am not entirely sure. I do not have any issue with this, but I am curious as to why that happens. As far as I know, LaTeX does usually not ignore empty lines, to form paragraphs for example. So why does that seem to be the case when setting an empty line in front of an equation environment?

example code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
Line of text directly followed by "align"
\begin{align}
    a + b = c
\end{align}

Line of text followed by an empty line and then "align"

\begin{align}
    a + b = c
\end{align}

Line of text directly followed by "equation"
\begin{equation}
   a + b = c
\end{equation}

Line of text followed by an empty line and then "equation"

\begin{equation}
   a + b = c
\end{equation}

\end{document}

output:

Answer 1

Enrico is absolutely right: "do not use empty lines around display environments" (The LaTeX Companion, second edition, p. 481 bottom).

But to explain the difference:

The empty line before the equation/align gives an extra vertical space of \baselineskip (in fact the line is visibly empty, but not technically empty: it contains the \indent box). But the equation then uses a vertical space of \abovedisplayshortskip which by default is 0pt (plus some stretch).

The equation without empty line has a vertical space of \abovedisplayskip, which by default is 10pt (plus some stretch). This is almost the same as the \baselineskip.

The align environment uses a "display alignment" which is approximately

$$\halign{ . . .}$$

This is low level TeX's equivalent of the array environment. This, however, is always preceded by the vertical space \abovedisplayskip.

So the align with an empty line before it gets both the \baselineskip and the abovedisplayskip.

Check: Set \abovedisplayshortskip to \abovedisplayskip and the equation gets the same spacing as the align.

Answer 2

Sometimes, I find the standard spacings in LaTeX to be, not necessarily incorrect but, just not what I want, or consider aesthetically pleasing in the context: in aligning an image with the text line, or between a table or a text, or any number of other examples.

Perhaps these will change again with LaTeX3? Who knows?

In the meantime, I just use \vspace or \space*, sometimes with a negative value, to achieve the effect I want.