I found a way of fixing the problem using your help "baxslash" by making a big sphere, color it black,hide it, and position it where the camera is at all times. And when I don't need to see the sky I don't update the sky and I show the black sphere. Little example code:
sync rate 60:sync on
set camera range 1,30000
autocam off
Global Clouds_update=1
Global time#=7.0
dc set cloudy 0.6
dc set cloud height 3
brightness#=1.0
dc set brightness brightness#
dc set auto brightness 1
dc set wind 10,270
dc init 1,5
dc set cloud height 3.0
dc update time#
texture backdrop 1
set normalization on
make object sphere 999,2000
scale object 999,-1000,-1000,-1000
position object 999,0,0,0
color object 999,0
set object ambient 999,0
make object cube 1,20
position object 1,0,-20,60
do
turn object left 1,2
if upkey()=1 then Clouds_update=1
if downkey()=1 then Clouds_update=0
Update_Clouds()
text 50,50,"UP to show the sky , DOWN to hide it"
sync
loop
FUNCTION Update_Clouds()
time#=time#+100.0/(60.0*60.0*60.0)
if Clouds_update=1
backdrop off
dc update time#
hide object 999
else
backdrop on
color backdrop 0
show object 999
Endif
ENDFUNCTION
