I’m working on a world with a mild but near constant winter. This weather pattern moves down the latitude and causes deep snows and some ice at worst in the mountains to heavy monsoons in tropic regions. What kind of axis and rotation does the planet need to have for a winter season pattern to move north to south down the globe every 4-5 months?
There are several things you could tweak:
Seasons are caused by periodic shifts in how much star light reaches a particular area of a planet. Here are some of the most common things that effect this:
- Brightness of star
- The height of the planet's orbit
- The eccentricity of the planet's orbit (how elliptical it is)
- The planet's axial tilt
- The planet's axial precession (how the axial tilt wobbles over time)
Consider Earth as a starting point:
- The Sun is a little on the bright side for a main sequence star
- The Earth orbits in the hotter portion of the goldilock's zone (or circumstellar habitable zone if you prefer)
- Earth's orbital eccentricity has a relatively small effect on seasons
- Earth's tilt is about 23 degrees and is responsible for about a 7°C fluctuation in temperatures in the northern hemisphere and about a 3°C fluctuation for the southern hemisphere
- Earth's axial precession is very slow and operates on a geological timescale (a period of 26,000 years!)
So, I would suggest:
Since you just want things to be colder, but still have similar seasonal variation to Earth the two most obvious choices are to raise your orbit or to switch to a cooler star. Since you also want shorter season cycles, you actually want to orbit more closely to your star; closer orbits are faster orbits. Since Earth is already kind of on the hot side we are forced to swap out to a cooler star. That would then give us the ability to lower our planet's orbit such that it's on the far edge of that new, cooler star's goldilock's zone.
Alternatively, you could keep everything else the same, but raise the orbit a bit and give the planet an obscene precession of 5 months:
Be aware if you are going hard-sci then all these tweaks come with strings attached. For example, close-orbiting planets tend to be tidally locked. Just something to be aware of if you are going for absolute realism.
Some other suggestions:
- Variable stars
- Habitable satellites that are regularly eclipsed by their parent
- Binary star systems