A population dominated by people who are no longer economically active (65+). Caused by a falling birth rate and an increased life expectancy. See Ageing Population: Causes & Impacts
Stability and Instabilty
occurs when the atmosphere (environmental lapse rate) cools slowly than a rising air parcel (dry adiabatic lapse rate) which is cooling at a faster rate. For example, assuming that ELR is 5oC whilst the DALR is 10oC would mean that for every 1000m rise in height the air parcel's temperature is dropping faster therefore cold than the atmospheric air. The parcel of air
will not rise as far because of its weight (cold air is dense) therefore sinks back to lower layers producing calm weather, clear skies and occasional drizzles.
This occurs when an air parcel (DALR) cools slowly than the atmosphere (ELR) which is cooling faster. For example, assuming ELR is dropping at 10oC/1000m whilst the DALR is dropping at 5oC/1000m would mean that the air parcel is warmer than atmosphere.. The less dense warm air parcel continues to rise and eventually condenses producing unstable weather conditions. Cumulonimbus clouds may form resulting in heavy thunderstorms with frequent lightning. High wind speeds may result and tornadoes may even form.
This occurs when the ELR is lower (warm) than the DALR but higher than the SALR. The air parcel cools faster than the atmosphere (stable conditions) but then cools slowly than the atmosphere after condensation. The dense cold air must sink downwards but instead continues to rise given a constant uplift mechanism. The air parcel eventually condenses releasing latent heat which then used to warm the air parcel and cools at the SALR. This latent heat offsets the cooling of the air parcel and the ELR now overtakes the air parcel in terms of cooling. The parcel is unstable because it has risen to condensation in the first place instead of sinking..The weather associated with conditional stability is usually fine at areas below condensation level.