What are the scientific reasons for Earth's modified seasonal states? It is common knowledge that weather conditions are based on the Earth's distance from the sun. However, it doesn't stop with that base knowledge. That is only a portion of 'why' the seasons change. Another reason is the 23.5 degree axial tilt. This tilt allows the earth to remain tipped in the direction of the Polaris star, or the North Star. Polaris is the brightest star at the end of the 'Little Dipper' handle [Ursa Minor]. Earth's tilt effects it's rotation around the Sun working in conjunction with other variables such as the moon and other planets. The magnetic force of the sun and the rotation of all planets effects the position of the Earth. There are four main instances notably known as the Vernal Equinox (September 23), Summer Solstice (June 21), Autumnal Equinox (September 23) and the Winter Solstice (December 22). This rotation creates cause and effect in regards to what we call weather and seasonal change.
The two extreme rotational points are called perihelion and aphelion. Perihelion is when the earth is the closest to the Sun making the hottest weather. Aphelion is the rotational point in which the earth is the furthest from the Sun making the coldest weather. Perihelion happens during the summer solstice and Aphelion happens during the Winter Solstice. The equinox represents the points of transition between two extremes [Winter and Summer] known as Spring and Autumn. All four seasons cause change in the atmospheric pressure resulting in what we understand as Spring, Summer, Winter, and Autumn.
Each season has a different effect on the atmospheric layer known as troposphere. Troposphere is the level of atmosphere that changes it's pressure to reflect regional needs or elevation changes. This layer contains the element precious to life known as water [in vapor]. The layers of atmosphere protecting the troposphere are known as stratosphere, mesosphere, thermosphere, and exosphere ... The stratosphere, perfect for flight, is cooler based on an object's velocity. The higher the object the cooler the surface temperature in this layer. The mesosphere is the next layer from the Earth's 'ground'. No life can survive in this layer because the temperature is below 130 degrees Kelvin. Next is the Thermosphere, the biggest of all layers, which ionizes radiation from ultraviolet rays. Exosphere is the outermost layer of Earth, or it's atmospheric shell. Portions of the mesosphere, thermosphere, and exosphere comprise the ionosphere. All of the atmospheric layers work juxtaposed based on the rotational point of the Earth's. The Earth readjusts all layers based on the gravitational pull from the polarity of the poles and it's relative place in rotation versus it's anchor point[the Sun].
Based on the region, or the location relative to the equator, the weather is created as a compartmental component of the season. Thus creating the time-space variable to explain that moment's seasonal outcome based on the location of rotation. The meteorologist[weather man] will explain outcomes of rain, sleet, snow, cloudy, sunny, hot, mild, or any other condition based on that day's weather patterns[known for that season].
[*] For a diagram of this check out
 Travis Metcalf – The Seasonal Merry-Go-round -
 Earth System Science Education Web Site - Earth's Season - Oklahoma State Unniversity | NASA http://www.esse.ou.edu/fund_concepts/Fundamental_Concepts1/Solar_System/Earths_Seasons.htm#Equinoxes
 Phi Plait – Phil Plait's Bad Astronomy: Misconceptions – Bad Astronomy [Seasons] -