On a summer day it is cloudy over New York City. A town with similar weather conditions 30 miles away is clear. What about cloud formation might explain the difference?
2 Answers
This is probably the result of a cold or warm front moving in, usually associated with mid-latitude anti-cyclones.
Explanation:
In North America, anti-cyclones develop as low pressure systems and generally move across the continent from west to east. They are usually related to warm and cold air masses colliding. Uplift happens when cold air catches up to a warm air mass and this lifts the warm air up. In turn, this tends to produce cloudy conditions and rain. As the anti-cyclone moves along, weather conditions tend to improve.
The kinds of clouds that form is somewhat predictable and depends on whether a cold front is approaching a warm, or the other way around. For example, see the kinds of clouds associated with an approaching cold front (2nd pic).
Something referred to as local effects.
Explanation:
In order for cloud to form you need 2 things. Humidity near 100% and condensation nuclei.
Water vapor does not condense to liquid spontaneously. In situations with perfectly clean air (no nuclei) it often requires super saturation of near 300% humidity for condensation to occur. Enter condensation nuclei.
Condensation nuclei are particles in the air that are a surface for water vapor to condense onto. Bits of dirt or salt or even ice crystals at higher altitudes are all surfaces that water will condense on. They work like the surfaces in the bathroom when you have a hot shower. After a shower you don't have water dripping out of the air you have it condensing on the surfaces like the mirror.
In this situation if the weather conditions are similar that means that there is no front or trough or trowal or anything like that. What is happening is that due to human activities in the city there is an abundance of condensation nuclei. Also water vapor is a by-product of combustion. So there is a lot more water vapor and condensation nuclei over the city than a rural setting. That is one example of local effects, and why there would be cloud over a large city when nearby small towns are clear.
