Chapter 5
Energy, Matter, and Momentum Exchanges Near the Surface
2. What is the difference between mechanical turbulence and thermal turbulence?
Answer: Mechanical turbulence is convection caused by the friction, irregular flow, and vertical gradients
of momentum associated with roughness elements while thermal turbulence results from the unequal
heating of the surface. Thermal turbulence is less important in the roughness layer due to small
differences in surface heating from the shading of roughness elements such as trees, buildings and
mountains.
4. Describe the conductive flux that is important near the surface.
Answer: The substrate heat flux, the flux of energy by conduction, is important in transferring energy
toward and away from the surface. The substrate heat flux can sometimes transfer energy from the surface
downward when the surface is being warmed or from beneath the surface upward when the surface is
colder than the ground below. Conduction can quickly melt snow after a warm period when stored energy
in the ground is released. This conductive heat flux is usually positive during the daytime and negative at
night.
6. What does a sun path diagram tell us and how is it interpreted?
Answer: A sun path diagram can be used to indicate the position of the sun in the sky at any given day of
the year and any given time of the day for a particular latitude. The sun’s location is identified using the
zenith angle and azimuth angle coordinates. The zenith angle indicates how many degrees the sun is from
being directly overhead of a given location and the azimuth angle represents the number of degrees the
sun is located away from north.
8. Describe the different types of radiometers and their functions.
Answer: A radiometer is an instrument that measures the flux of radiation over a unit area of the surface.
Most radiometers operate on the basis that the amount of radiation received on a surface is proportional to
the temperature. A pyranometer measures only shortwave radiation and a pyrgeometer detects only
longwave radiation. A pyrradiometer measures all-wave radiation. Net radiometers usually require two
sensors to measure the net flux of energy both toward and away from the surface.
10. How can evapotranspiration be measured?
Answer: A lysimeter measures evapotranspiration by weighing the soil and vegetation on top of it. The
water content of the volume can be determined if the mass of dry soil and vegetation is known.
Lysimeters take extremely accurate measurements of evapotranspiration but are more expensive to install
and maintain than evaporation pans. Eddy correlation is a very sophisticated method of measuring
evapotranspiration that is usually only done experimentally. Evapotranspiration using eddy correlation is
measured using theoretical equations derived from wind speed, temperature and specific humidity
measurements. Potential evapotranspiration is commonly estimated with a Class A Evaporation Pan. This
crude method of measurement consists of an instrument with a metal pan that sits on a short wooden
platform that is exposed to the elements. An analog gauge placed inside the pan measures the vertical
displacement of water that has evaporated since the previous reading. The evaporation reading must take
into account the amount of precipitation that was added to the pan.
12. How is atmospheric stability evaluated?
Answer: Atmospheric stability can be evaluated by comparing the lapse rate of an adiabatically-moving
parcel of air to the lapse rate of its surrounding environment. If the air parcel is warmer than its
surrounding environment, it will want to rise and will represent unstable conditions, but if it is cooler than
its surrounding environment, it will have a tendency to sink and be associated with stable conditions.
These relationships can be discerned using a thermodynamic diagram.
14. What is potential temperature and how can it be used to simplify the evaluation of stability in the
atmosphere?
Answer: Potential temperature is the temperature that a parcel of air would have if moved dry
adiabatically from its height in the atmosphere to the 1000-mb level. Potential temperature provides a
convenient way to assess atmospheric stability. A stable atmosphere is characterized by increasing
potential temperature with height and unstable conditions are noted when potential temperature decreases
with height.