How do weather conditions affect pouring a concrete foundation?
If it's too wet and water starts to accumulate in the foundation, you can end up with weakened concrete. 

One of the key metrics for a concrete mix is the free water to cement ratio. If the water content is too low, you struggle to get it to compact properly , resulting in air voids, and weak concrete. If the water content is too high, the water can leave pores in the concrete, which also reduces the strength of the concrete. 

If the temperature is low, the hydration reaction slows down. Further, if the temperature is too low, the water can freeze, which means that it is not available to hydrate the cement, again affecting the strength of the concrete.