Earth's Tilted Axis
The main reason Portland has seasonal temperature changes is that Earth's axis is tilted at an angle of about 23.5 degrees. As Earth orbits the sun, this tilt causes different parts of the planet to receive more or less direct sunlight at different times of year. When Portland's hemisphere tilts toward the sun during summer, the sun's rays hit more directly and the days are longer, making it warmer. When it tilts away during winter, the sun's rays hit at a lower angle and days are shorter, making it colder.
Daylight Hours
Portland's daylight hours change dramatically between seasons. In summer, the sun rises early and sets late, giving Portland up to 15 hours of daylight. In winter, daylight can be as short as 8-9 hours. More hours of sunlight means more time for the sun to warm the Earth, which is why summer is hotter than winter.
Sun Angle Effect
The angle at which the sun's rays hit Portland also matters for temperature. During summer, the sun is high in the sky, so its energy is concentrated in a smaller area and heats the ground more efficiently. During winter, the sun is lower on the horizon, so its energy spreads over a larger area and has less warming power. This is why the same amount of sunlight in winter does not warm Portland as much as summer sunlight.
Ocean Moderation
Portland's location near the Pacific Ocean has an important effect on its seasonal temperatures. The ocean heats up and cools down slowly compared to land. During summer, the cool ocean water moderates Portland's heat, keeping it from getting extremely hot. During winter, the ocean releases stored heat, keeping temperatures from dropping as low as they would in inland areas at the same latitude.
Latitude Influence
Portland is located at about 45 degrees north latitude, which puts it in a zone that experiences significant seasonal variation. Cities closer to the equator have smaller temperature differences between seasons because the sun stays relatively high in the sky year-round. Cities closer to the poles experience even more extreme seasonal differences. Portland's mid-latitude location gives it a moderate seasonal temperature range. 
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