What makes one city by the bay foggy, while another city by the bay isn't?

It's fog that causes it :P In truth though it's a bit like rain isn't it.... at some point there has to be an edge of the rain where on one side it's raining and on the other it isn't. So, logically speaking fog may be similar, it's foggy/misty in one place but in another it isn't.

I hate to use Wiki, but here is the best I can find.

Minute droplets of water constitute this after-dark radiation fog, with the ambient temperature −2 °C (28 °F)
Up-close view of water particles forming fog

Fog forms when the difference between air temperature and dew point is less than 2.5 °C (4.5 °F).[5]

Fog begins to form when water vapor condenses into tiny liquid water droplets suspended in the air. Six examples of ways that water vapor is added to the air are by wind convergence into areas of upward motion;[6] precipitation or virga falling from above;[7] daytime heating evaporating water from the surface of oceans, water bodies, or wet land;[8] transpiration from plants;[9] cool or dry air moving over warmer water;[10] and lifting air over mountains.[11] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds.[12][13] Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass.[14]

Fog normally occurs at a relative humidity near 100%.[15] This occurs from either added moisture in the air, or falling ambient air temperature.[15] However, fog can form at lower humidities, and can sometimes fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus, the air will become supersaturated if additional moisture is added

Air currents and micro-climates   If a localized eddy forms in the atmosphere it can pull in drier air which can "evaporate" fog or prohibit it from forming.  

What makes yesterday cloudy and today sunny but the weather? Methink no simple answer. This post was edited by O-uknow at May 26, 2018 6:45 PM MDT

I think I've Mist the point of this question......it's overcast and clouded by an atmosphere that appears quite foggy ... 

I would suggest  waiting for a clearer view of things before commenting....:( 

Well, it's because one city has fog and the other doesn't. But what I wanna know is why Washington D.C. is "Foggy Bottom" and it's not on a bay at all.

have no idea