1)    Does RAIN or SNOW or CLOUDS affect the reception of my GPS receiver?
Answer:  No.  Not so as the user can tell without instrumentation.

2)    I notice that when it rains and I turn on my windshield wipers my GPS has trouble locking.  Doesn't this mean that rain affects my GPS reception?
Answer:  No.  What is affecting your GPS signal reception is the fact that the wipers running back and forth across your windshield is intermittently blocking reception and making it difficult for your GPS to get a complete navigation data string.  Turning off your wipers until the signal locks will speed the initial lock.

3) Sometimes my GPS gets a thick coating of water on it and it does not receive as well.  Does this not prove that rain can cause signal degradation?
Answer:  No.  A coating of water,  even a fairly thin one is NOT the same as raindrops.  The GPS signal frequency of about 1575mhz was chosen expressly because it is a "window" in the weather as far as signal propagation is concerned.

4) This all sounds good,  but do you have a reference that supports these statements?
Answer:  Yes.
                See: Ref: http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm  (Courtesy of Sam Wormley)

Cloud, Rain, Snow, weather in general does NOT attenuate the GPS signals enough to effect accuracy.  As can be seen below,  the total atmospheric loss (from all causes including rain,  clouds,  snow,  fog, etc.) is but 2db.  This is small compared with other variables.
 

          L1 and L2 Navigation satellite Signal Power Budget

Parameter                                          L1 P-Code   L1 C/A-Code  L2 P-Code
-----------------------------------------------------------------------------------------------------
User minimum received power        -163.0 dBw   -160.0 dBw   -166.0 dBw
Users linear antenna gain                      3.0 dB           3.0 dB           3.0 dB
Free-space propagation loss              184.4 dB       184.4 dB       182.3 dB
Total atmospheric loss                           2.0 dB           2.0 dB           2.0 dB
Polarization mismatch loss                     3.4 dB           3.4 dB           4.4 dB
Required satellite EIRP                    +23.8 dBw    +26.8 dBw     +19.7 dBw
Satellite Antenna gain at 14.3°            13.5 dB         13.4 dB          11.5 dB
       worst case Block II off-axis angle
Required minimum satellite antenna +10.3 dBw   +13.4 dBw        +8.2 dBw
 Input power                                          10.72W        21.88W             6.61W

5) Ok then,  can I use my GPS underwater?
Answer:  No.  Just a few millimeters of  "solid" water will severely attenuate the GPS signal.

Joe Mehaffey