The measurement of surface elevation (altitude) with a laser altimeter. Particularly when used to measure elevation change, laser altimetry has become a leading source of data for the measurement of m
ass balance by the geodetic method. If, for logistical or financial reasons, it is not possible to survey the whole glacier by airborne laser altimetry, it is necessary to extrapolate to obtain a glacier-wide geodetic mass balance.
The most recent time (15 000 to 80 000 years ago) during which continental glaciers covered subpolar regions and existed at elevations as much as 1000 m lower than today; corresponding to periods in w
hich oxygen isotopes from marine sediment cores indicate that global sea level was 50-150 m lower and global temperature 5-10C lower than today.
The period during the last ice age when the glaciers and ice sheets reached their maximum extent, approximately 21 ka ago. This period has been widely studied because the radiative forcings and bounda
ry conditions are relatively well known.
The most recent time (115 000 to 125 000 years ago) during which global temperatures were as high as or higher than in the postglacial, when continental glaciers were limited to the Arctic and Antarct
ic, and sea levels were near current positions.
With reference to the effect of energetic particles on spacecraft microcircuits, a serious type of single event upset in which the microcircuit is either permanently stuck or cannot be reset without b
eing turned off and on.
The specific enthalpy difference between two phases of a substance at the same temperature. The latent heat of vaporization is the water vapor specific enthalpy minus the liquid water specific enthalp
y. When the temperature of a system of dry air and water vapor is lowered to the dewpoint and water vapor condenses, the enthalpy released by the vapor heats the air-vapor-liquid system, reducing or eliminating the rate of temperature reduction. Similarly, when liquid water evaporates, the system must provide enthalpy to the vapor by cooling. The latent heat of fusion is the specific enthalpy of water minus that of ice and the latent heat of sublimation is the specific enthalpy of water vapor minus that of ice. The latent heats of vaporization, fusion, and sublimation of water at 0C are, respectively, L_v = 2.501 * 10^6 J/kg L_f = 3.337 * 10^5 J/kg L_s = 2.834 * 10^6 J/kg It is common to see an expression like "release of latent heat." In other thermodynamic terms in this glossary, such expressions are avoided in favor of others using enthalpy and temperature, which are measurable quantities.