Delta T

This article is about the time difference, for the temperature difference see psychrometrics. Delta T, delta-T, deltaT, ΔT, or DT is the time difference obtained by subtracting Universal Time from Terrestrial Time.Universal Time (UT) is a time scale based on the Earth's rotation, which is somewhat irregular over short periods (days up to a century), thus any time based on it cannot have an accuracy better than 1 : 108. But the principal effect is over the long term: over many centuries tidal friction inexorably slows Earth's rate of rotation by about 2.3 ms/day/cy. However, there are other forces changing the rotation rate of the Earth. The most important one is believed to be a result of the melting of continental ice sheets at the end of the last ice age. This removed their tremendous weight, allowing the land under them to begin to rebound upward in the polar regions, which has been continuing and will continue until isostatic equilibrium is reached. This "glacial rebound" brings mass closer to the rotation axis of the Earth, which makes the Earth spin faster (law of conservation of angular momentum): the rate derived from models is about 0.6 ms/day/cy. So the net acceleration (actually a deceleration) of the rotation of the Earth, or the change in the length of the mean solar day (LOD), is +1.7 ms/cy. This is indeed the average rate as observed over the past 27 centuries.
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A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.