The amount of force exerted over a surface area caused by the weight of air molecules above it. As elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height. A column of air in cross section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 (psi) pounds per square inch. The standard value for atmospheric pressure at sea level is equal to ...
1 atm = 760 mm Hg (millimeters of mercury) = 1013 millibars = 14.7 psi (pounds force per square inch) = 1013.25 hPa (hectopascals).
The following JavaScript form calculates the atmospheric pressure at any altitude.
Although everyone is different, for the vast majority of people altitude sickness is generally a non-issue for extended periods at elevations below 2500m. Above this elevation the situation is different, as in some people the blood oxygen level drops with increasing altitude.
Oxygen saturation is the percentage of blood that carries oxygen. At sea level this is 98% to 100%. At 3000m altitude normal levels are 90% to 95%. People with levels between 80% and 90% often have symptoms of A.M.S. As levels fall below 80% symptoms of H.A.P.E. and mental changes appear.
The forms of altitude illness seen most commonly are:
Acute Mountain Sickness is a mild form of altitude sickness which effects 20% to 30% of visitors to elevations above 2500m. The symptoms are headache, nausea, vomiting and trouble sleeping. A.M.S. looks and feels like the "flu". Most people experience the symptoms of A.M.S. in the first three days after arrival. The symptoms usually go away by the fourth day.
The incidence and severity of A.M.S. are related to altitude, speed of ascent, physical exertion and prior acclimatisation. Some people are particularly susceptible to A.M.S. and experience similar episodes with each exposure. Symptoms of headache, shortness of breath, anorexia or nausea, weakness, dyspnea (difficulty in breathing) and flu-like malaise may begin 6 to 48 hours after ascent.
Treatment of A.M.S.
Most people with mild A.M.S. get better with no treatment at all. People with moderate or severe symptoms should see a doctor. Things that help:
Prevention of A.M.S.
People who get symptoms from high altitude can take several steps to prevent symptoms.
High Altitude Pulmonary Edema (H.A.P.E.) is the more severe form of altitude illness. It can effect both children and adults, and seems to be the worst about the third night. The symptoms are cough with congestion, trouble breathing, and shortness of breath. H.A.P.E. looks and feels like pneumonia. Many people with H.A.P.E. show mental changes from lack of oxygen. H.A.P.E. is a serious illness. You should consult a physician for oxygen therapy and transport to a lower altitude. Diamox® (acetazolamide) may be helpful but is not a substitute for oxygen therapy and transport to a lower altitude.
H.A.P.E. is a serious illness that can be fatal. The incidence at altitudes between 2500m and 4500m is between 1% and 10%. Rapid ascent, sleeping at high altitudes, severe exertion and extreme cold temperatures are predisposing factors. Symptoms of exertional fatigue, dyspnea, cough, and altered mental status may occur on the second to fourth day after reaching high altitude. Repeated attacks are common. Sometimes somnolence (state of drowsiness) or coma without significant dyspnea or cough is the first sign of this disorder. Without treatment, death usually occurs six to twelve hours after onset of coma. Patients with H.A.P.E. should be moved rapidly to a lower altitude. Oxygen is helpful. For mild cases, home oxygen may allow the patient to stay at high altitude. For severe cases, immediate transport to 1500m or lower is required.
Prevention of H.A.P.E.
Prevention of H.A.P.E. is basically the same as listed for the prevention of A.M.S. In addition, it is wise to consider sleeping below 2500m and using home oxygen at night. Diamox, staging, and avoidance of alcohol, sleeping pills and narcotics is recommended. Avoiding heavy physical activity is helpful. Physical conditioning before ascent does not help. Many patients are young, healthy and energetic.
Cerebral edema usually in conjunction with H.A.P.E. is a complication of rapid exposure to very high altitudes seen mainly in mountaineers. Symptoms consist of severe headache that may precede mental dysfunction (hallucinations, bizarre behavior and coma) and neurological abnormalities (loss of coordination, paralysis and cerebellar signs). Immediate descent and oxygen are recommended to prevent permanent neurological damage or death.
Asymptomatic retinal hemorrhages can occur at altitudes above 4000m and are common above 5500m, especially with rapid ascent and strenuous exertion. Hemorrhages usually dissolve in seven to ten days at sea level without impairment of vision. Rarely, macular hemorrhages (spots) affecting vision may occur and require immediate descent to prevent persistent visual defects.
Children and young adults seem to be more susceptible to altitude illness. The symptoms are the same. Diamox is still the drug of choice although the risk of side effects (blurry vision, tingling of hands and feet) is higher in young people. Because of these side effects, oxygen should be preferred, especially in infants and toddlers. As with adults, oxygen is the cheapest, safest, eaisest way to deal with altitude illness.
Diamox® is a carbonic anhydrase inhibitor, taken in doses of 125 mg twice a day beginning one or two days before ascent and continuing at high altitude for 48 hours or longer, can prevent or diminish symptoms of A.M.S. in may patients. Diamox® causes a bicarbonate diuresis which prevents fluid retention, decreases alkalosis, stimulates ventilation and decreases hypoxemia during sleep. The effect is transient, but renal compensation for the primary respiratory alkalosis usually occurs before the effect wears off. Diamox® is a sulfonamide and can cause all the adverse effects associated with this class of drugs. Most common side effects are numbness and tingling of lips, fingers and toes. Visual effects may occur. Dehydration may occur if fluid intake is inadequate.
Procardia® (nifedipine)
Procardia®, a blood pressure medication, has received considerable attention in the prevention and treatment of H.A.P.E. Although it is not F.D.A. approved for this use, there has been several articles appearing in the New England Journal of Medicine about its use. It seems to work by relieving the pressure and fluid build up in the lungs. Most likely we will see increase use of this medication for H.A.P.E. in the near future.
Decadron® (dexamethasone)
Several articles appeared in 1987 and 1988 Decadron® (a type of steroid) for the prevention and treatment of altitude illness. Decadron®, 4 mg. every six hours greatly reduces the symptoms of A.M.S. most likely by reducing brain edema. Although quite effective, especially for the headache of A.M.S., DecadronL is not used frequently by local physicians.
Rapid ascent to high altitude can be uncomfortable for many people and dangerous for some. Headache and other symptoms of acute mountain sickness may be prevented by gradual ascent or by taking Diamox. Pulmonary edema occurs in a small percentage of people who quickly ascent to elevations above 2500m. Those patients need to see a physician for oxygen therapy and descent to lower altitude.
Note on the Barometric Pressure Calculator:
The above calculator lets you calculate the atmospheric pressure on other planets, should you want to visit Mars during your next holidays.
The ground level atmospheric pressure on a planet is not fixed by any of the standard planetary data such as gravity and temperature. It depends on just how much atmospheric gas is available and has not yet escaped into space. For example, Venus is a little smaller than Earth and has slightly lower gravity, yet its atmospheric pressure is about 90 times that of Earth. You are pretty free to set the ground level atmospheric pressure to whatever you like.
Once the ground level pressure is decided on, the atmospheric pressure P at other altitudes is governed by the following formula:
P = Po exp(-g h / H)where
Po = the ground level pressureOn Earth the scale height is 7400 metres. On other worlds it will be
g = the surface gravity in units of Earth gravities
h = the altitude
H = the atmospheric scale height
H = Ho Mo / Mwhere
Ho = the scale height on Earth (7400 metres)For breathable nitrogen-oxygen atmospheres (like the one we have on earth), M will be close to 29.
Mo = the mean molecular weight of Earth's atmosphere (29 atomic mass units)
M = the mean molecular weight of the planet's atmosphere