just wondering whether the weather will affect a engine's compression/timing?

1. sea level with 68F temp and 50% humidity
2. sea level with 95F temp and 90% humidity

will engine compression and/or timing be affected by above 1 and 2?

The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine’s horsepower and torque. For tweaking the fuel/air mixture and compression ratio, the air density is the most important consideration.

Inputs:
The Air Temperature should ideally be the temperature of the air that is going into the intake of the engine.
The Absolute Pressure (also called actual pressure or station pressure) is the ambient air pressure.
Relative Humidity is a measure of how much moisture is in the air compared to the amount of moisture that the air could hold at saturation. Relative humidity is a function of temperature and therefore changes as the temperature changes, even if the amount of moisture in the air remains constant.
The air density is the actual weight of a given volume of air. This is a key parameter for engine tuning.

Resuming:
When the air density increases, you will need to richen the air-fuel mixture to compensate. When the air density decreases, you will need lean-out the air-fuel mixture to compensate.

Use the following as a guide to correcting your setting when the weather changes:
Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must lean the mixture to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.
Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should lean the mixture.
Altitude: In general, the higher the altitude the lower the air density. When driving at racetracks that are at high altitude, you should lean the mixture and increase the engine's compression ratio to compensate for the lower air density.

Compression ratio and hence timing will noticeably change over a 10% variation in altitude, not over changes in humidity and or temperature.

With changes in temp and humidity, your main concern is choosing the right plug, and proper carburation, provided you have your head shimming set accordingly to your nitro contents on your fuel.

How to select the proper plug:
•When the ambient temperature is high, we have to use a plug with thicker wire.
•With higher compression, we have to use a plug with thicker wire.
•Humidity determines if we use a Cold (F) thermal range, or Hot (C) thermal range. Up to 70% humidity use (F) bodied plugs, and for more than 70% use (C) bodied plugs.
•If we have high temperature and high humidity, we should use a plug with thick wire and a Hot(C) thermal range.

There you have it

AFM

thanks AFM, you've always been informative...

generally, high temp & high humidity = lower air density = leaner settings

I think it is too simple to set it in that way. I would say if you know how to read the engine (sound, temperature, smoketail and visable performance) you know what to adjust.