Is this an x86 machine? if so, most OS's will scale the CPU clock rate, lowering the temperature.
On OpenBSD, you can do this manually setting the
hw.setperf sysctl(1), or by configuring
apmd(8) in
/etc/rc.conf.local.
There was once a diff floating around the mailing lists to do automatic scaling in the kernel, but I'm not sure if it was committed.