Percentage in a query

Hi all, I got this query:

SELECT TEMPO, COUNT(NOME) AS NU FROM HISTORY GROUP BY TEMPO     ------ nome(nvarchar), tempo (nvarchar)

the results:

TEMPO  NU

L           13

O           8

F           25

I need another column, percentage

TEMPO NU PERCENTAGE 

where the pecentage is (Nu/SUM(nu) ) * 100

I tried with 

SELECT TEMPO, COUNT(NOME) AS NU, COUNT(NOME) OVER(PARTITION BY NOME) AS TOT FROM HISTORY GROUP BY TEMPO

but it doesn't work...any suggestion? Thanks