Diacritic (Modifier) Symbol for Variance Using a bar over a variable seems to be standard shorthand notation for the mean value:
$\bar x = \frac{1}{N}\sum_i x_i$
Now, what is  suitable shortand notation for the variance? I can only find $\sigma^2$ or $\mathrm{Var}$, but no diacritic (modifier) symbol that would make the notation more concise. Like this ring symbol, for example:
$\mathring x = \frac{1}{N}\sum_i (x_i - \bar x)^2$
What is a suitable symbol to denote the variance in such a compact manner?

Personnally, the thing I saw the most is $\mathbb{E}(X)$ for the mean value and $var(X)$ or $\mathbb{E}((X-\mathbb{E}(X))^2)$ for the variance. 
The use of symbols depends on the context (there are just context-wise habits, no conventions), you just have to precise what you are talking about. For instance, in topology, both the bar and the ring get a different meaning (respectively adherence and interior).
