The following rules for environmental dangers are meant to be compatible with any version of D&D, from 1st Edition AD&D to 3rd Edition D&D. They're meant to make things like falling and exposure more perilous, and also more realistic (based on some research on my part). The weather and starvation rules are largely based on 3rd Edition DM's Guide concepts (pp. 85-89).
Damage from a fall is dealt out at 1d6 per 10' fallen, cumulatively (this is shown on the accompanying chart). This makes more sense than the conventional non-cumulative ruling, since 20' to 30' falls are in reality quite dangerous, and 50' falls are almost always fatal. Furthermore, it has been documented that this was the system Gygax himself used, and corrupted by an editing error in the original Players' Handbook (see articles in Dragon magazine issues #69-70, and Unearthed Arcana at the top of page 25).
Technically, this damage should increase up to the level of about 240' height (where we may assume that terminal velocity is reached, and our damage is at 300 dice).
The rule for drowning is that any character can automatically hold their breath for N rounds (a number based on their Constitution score). After this period, they must make a Constitution check each round (DC10 + 1 per subsequent round), or else fall unconscious and then die.
Depending on the length of one round, N may be equal to the Constitution score (for rounds of 10 seconds), twice the Constitution score (for rounds of 6 seconds), or one-third of the Constitution score (for rounds of 1 minute length; round up, and apply the penalty at -2 per round).
Hot and Cold Weather
Weather which is exceptionally hot or cold requires saving throws (vs. Paralysis, or Fortitude DC15) to avoid subdual damage. This damage is, like falling, cumulative based on the number of time intervals spent in the hostile environment (and the saving throw is also penalized by the number of intervals). A certain amount of damage from heat and cold will then also trigger the penalties from fatigue and exhaustion. The cumulative damage again serves to generally simulate real-life survival times (for example, scientific charts estimating survival time while immersed in water at the "Mild" temperature rank show an average of around 3 hours, with a range of from 2 to about 7 hours; no high-level character should be able to survive for 10 or 20 times longer than the average, which would amount to days and days in such a circumstance!)
Damage from heat and cold can only be recovered once the victim is removed from the hostile condition. Then, this subdual damage heals at the usual rate (1 hp per level each hour).
Starvation and Thirst
As stated elsewhere, active medium-sized characters require about a gallon of fluids and about a pound of food each day to avoid starvation effects. At the "Hot" temperature level, an active character needs to drink about two gallons per day.
A character can go without food for 3 days (in growing discomfort). After this time, the character must make a Constitution check each day (DC10 + 1 per added day) or take d6 subdual damage dice, the number of dice cumulative for each day passed after the 3rd (1d6 on day 4, 3d6 on day 5, 6d6 on day 6, etc.)
A character can go without water for 1 day plus a number of hours equal to his or her Constitution score. After this time, the character must make a Constitution check every hour (DC10 + 1 per extra hour) or again take 1d6 subdual damage, cumulatively for each check interval that has passed.
Characters who take any subdual damage are fatigued; those who have suffered half damage are exhausted (see the temperature chart for effects). The subdual damage from starvation and thirst can only be healed by natural recovery after receiving proper nourishment.
Anyone can move at double base speed for up to one hour without penalty. A second such hour in the same day causes 1 point of subdual damage. Each additional hour causes twice the damage taken in the previous hour.
Usually a person can walk up to 8 hours in a day. Marching longer than this requires a Constitution check each hour (DC 10 + 1 per added hour); failure means the character takes 1d6 subdual damage, cumulatively for each extra hour (so they risk 1d6 after 9 hours, 3d6 after 10 hours, 6d6 after 11 hours, and so forth). This subdual damage can't be recovered normally until the character halts and rests for at least 4 hours.
AppendixView a rich-text document of charts for falling and temperature effects here.
View a rich-text version of this page here.