I'm working on piecing together a game that involve sea trading, loosely based on historical conditions. As a part of making the game, I'm creating a map. From that map, I'm trying to determine what the temperature is, at least on average, for a number of reasons. I'm also going to include a seasonal model to it. What I'm trying to do is to find a first order estimate of the average temperature by day and by latitude, at sea level. I know that this varies tremendously based on a number of factors, but it seems to me that assuming equal distance to the sun, a perfectly spherical Earth with a known axial tilt, and some other reasonable assumptions, there should be a model that will work, which I can add to some noise to cover some of the other sources of error.
Bottom line, is there a relatively simple model of latitude and time of year vs average temperature at sea level? Bonus points if it includes a difference by altitude. Thanks!