Explain why duration of insolation in some city is always longer than the other city?
The duration of insolation time is the amount of time a location spends in sunlight during the day. That duration depends upon the latitude of the city and the day of the year so different cities at different latitudes experience different insolation times even on the same day.
One way to see this is to get a lamp with a bare ulb and a globe. The globe should tilt 23 degress. The lamp is the Sun and the globe is the Earth. Place the globe so that it axis points towards the lamp. The amount of insolation time is the time it spends in the lit part of the globe. This is summer in the Northern Hemisphere. Compare New York City and Mexico City. Both spend far more of their time on the sunlit side than in the shadow and so insolation time is long.
Now place the globe on the opposite side of the lamp. This is winter in the Northern Hemisphere. Now NYC and MC spend a lot less time in sunlight while a Southern hemisphere city such as Buenos Aires spends most of its day in sunlight.
This model doesn't encompass the whole picture because the Sun is so much larger than the Earth. You'd need a searchlight and a globe the size of a marble to see the extra lighting at the poles.