how to hide vertical scrollbars whe less than a certain number of rows.

I am surrounding a GridView with a <div> in order to provide vertical scrollbars. Is their a way to only display the scroll bars, if the GridView has more than 50 rows in it? If less than 30 rows, I want to hide the scrollbars.
 
<div style="overflow-y: scroll; height: 200px">
<asp:GridView ID="GridView1" runat="server" Font-Size="12px" BackColor="#FFFFFF" GridLines="Both" CellPadding="4" Width="560"> <HeaderStyle BackColor="#EDEDED" Height="26px" />
</asp:GridView>
</div>
...