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Hadley cells are thermally driven cells with rising air near the Equator
in the intertropical convergence zone (ITCZ), poleward flow in the upper
troposphere, subsiding air in the subtropics at around 30^{o}, and return flow
from the subtropics to the equatorial regions as part of the trade winds. See sections 1.2.2 and 2.1.5.2.

Halocarbons are organic compounds in which one or more carbon atoms are linked with one or more halogen atoms (fluorine, chlorine, bromine or iodine).

The Holocene is the name given to the latest interglacial period that started around 10 000 years ago and is still continuing. See section 5.5.

The hour angle *HA* indicates the time since the Sun was at its local meridian,
measured from the observer's meridian westward. HA is thus zero at the local
solar noon. It is generally measured in radians or in hours (2
$\pi $ rad = 24 hours). See equation 2.21 and figure 2.10.

Atmospheric humidity is the amount of water vapour in the air. Different definitions are available, based on the mass ratio of water vapour compared to that of air, or the partial pressure of the vapour. See sections 1.2.1 and 4.2.1. See also specific humidity , relative humidity, and saturation vapour pressure

The hydrosphere is the water on and underneath the Earth's surface (ocean, seas, rivers, lakes, underground water). See section 1.1.

On a large scale in the atmosphere and the ocean, the dominant terms in the vertical equation of motion are gravity and the force due to the vertical pressure gradient. The hydrostatic balance, which assumes these two forces balance each other, thus holds to a very good approximation:

$$\frac{\partial p}{\partial z}=-\rho g$$

In this equation, *p* is the pressure, $\rho $ the density and *g* the gravitational
acceleration. When this balance is achieved, the fluid is said to be in
hydrostatic equilibrium and, knowing the density, the pressure can be
computed by integrating the equation along the vertical. The equation
shows that sea level pressure depends on the mass of the whole air
column above the surface.
See sections 1.2.1, 1.3.1 and 2.1.5.1.