给一个div加动画为什么实现不了,详情请看代码

Question

css:
.flightNumber{

  display: none;

-moz-transition: opacity 2000ms ease-in-out;

-o-transition: opacity 2000ms ease-in-out;

-webkit-transition: opacity 2000ms ease-in-out;

/*transition: opacity 2000ms ease-in-out;

opacity: 0;

}

.runIn{

-moz-transition: opacity 2000ms ease-in-out;

-o-transition: opacity 2000ms ease-in-out;

-webkit-transition: opacity 2000ms ease-in-out;

/*transition: opacity 2000ms ease-in-out;*/

opacity: 1;

display: block;

}

html代码:<div class='flightNumber'></div>
js代码:$(".flightNumber").addClass("runIn")