A returning private pilot's question about the relationship between cold air, atmospheric pressure, and altimeter settings surfaces a conceptual gap that trips up pilots at every certificate level, and the confusion is understandable because the term "high pressure" legitimately means different things in different aviation contexts. The post correctly identifies that cold air is denser, meaning more air molecules occupy a given volume, which produces what meteorologists call a high-pressure surface system — reflected in altimeter settings above 29.92 inHg. However, the pilot's secondary claim — that cold weather generally produces lower altimeter settings — actually inverts the typical relationship. Classic cold-air high-pressure systems routinely push altimeter settings to 30.30, 30.50, or even higher. The confusion arises because there are at least three distinct "pressure" concepts in play simultaneously: surface pressure (what QNH/altimeter setting measures), the geometric height of constant-pressure levels aloft, and air density, which is a function of both temperature and pressure but is not synonymous with either.
The critical distinction that resolves the pilot's confusion is the difference between surface pressure and the vertical distribution of pressure through an air column. In cold air, the atmosphere contracts — molecules are packed more tightly — which compresses pressure levels downward toward the surface. This means that a given pressure level (say, the 500 mb level) sits at a lower geometric altitude in cold air than it does in warm air or in the standard atmosphere. The altimeter, however, is calibrated against the standard atmosphere's assumed temperature profile. When actual air temperatures are significantly below standard, the altimeter over-reads: it indicates a higher altitude than the aircraft is actually flying. This is the foundation of the well-known mnemonic "from high to low or hot to cold, look out below" — and it has direct, quantifiable safety consequences on instrument approaches and in mountainous terrain.
For instrument-rated and professional pilots, this is not an academic distinction. Cold temperature corrections to published approach minimums are required in many international jurisdictions and are increasingly being formalized in U.S. operations. Transport Canada mandates cold temperature corrections below certain thresholds, and the FAA has been progressively standardizing cold temperature altitude correction procedures through AIM guidance and advisory circulars. On a stabilized ILS or RNAV approach in temperatures substantially below standard — minus 20°C or colder — an aircraft holding an indicated 500-foot AGL on the altimeter may be meaningfully lower than that in geometric terms. The error scales with altitude above the altimeter setting source and with the magnitude of temperature deviation below standard. Operators flying into high-latitude destinations, mountainous airports, or simply conducting winter IFR operations need to account for this systematically, not as an afterthought.
At the operational level, the broader takeaway is that pilots must consciously track which pressure concept is being invoked in any given discussion. High-pressure weather systems bring cold, stable, descending air and high QNH values — good visibility, smooth air, and altimeter settings that make the altimeter read slightly low (the aircraft is geometrically higher than indicated at a given pressure reading). Cold air aloft compresses the pressure structure and makes the altimeter read high (aircraft geometrically lower than indicated for a given pressure altitude). Density altitude, a third related concept, decreases in cold air and improves aircraft performance relative to indicated altitude. Each of these concepts uses the word "pressure" or modifies altitude in a different direction, and conflating them produces exactly the kind of circular confusion the Reddit post describes. Structured recurrency training and IFR proficiency work should address these distinctions explicitly, as they are precisely the category of subtle systematic error that contributes to controlled flight into terrain accidents in cold-weather and mountainous environments.