Re: How does drought happen?

If the water cycle is the way we've all been taught, how is it that we 
can have
cloudless days and limited humidity if the water isn't on the ground?  
Where
does it go?  And, better yet, how does it come back?
Thanks,
Andrea
	



Hello Andrea,
If you could seal up your city in a glass box, then the amount of water 
in this 'local water cycle' would be fixed.  In that case, when most of 
the water condensed out of the air, you would find it on the ground (or 
in the ground, or soaked up by plants, etc).  When the ground heats up 
and water evaporates, then the humidity in the air would increase, as you 
expected.

However, in the real world, without glass walls, humidity in a particular 
location is really controlled by conditions over the entire earth.  The 
water cycle in your "real" city will behave to some degree like the glass 
walled city; that is, when water on the ground evaporates it will raise 
the humidity, but actual humidity conditions will change a great deal due 
to the movement of the air from global and local winds.  Even if it is 
calm on the ground level, high altitude winds are often blowing that will 
affect your humidity as well as temperature and other weather factors.

Where you live, in Tustin, your climate is strongly affected by the 
ocean.  (I know about Tustin- I live not too far from there!)In common 
weather conditions in this area, a low pressure is created as the land 
heats during the day. This draws in moist, cool air from the ocean. This 
moist air will give you high humidity conditions.  

In our famous weather called the "Santa Ana" wind condition, hot and very 
dry winds blow in from deserts in the east.  This will give us extremely 
low humidities right up to the ocean shore.  

So, in a simply summary, the local humidity in the air is greatly changed 
by being swept away by dry winds, or increased when moist winds blow in.

I hope that his helps,

Best Regards,

Jay Shapiro