This document discusses microclimate and factors that influence it such as topography, elevation, ground cover, wind, water, and solar orientation. It explains how microclimate is the immediate local climate conditions defined by temperature, humidity, wind, etc. Topography such as hills, elevation changes, and ground cover including vegetation type influence microclimate. Bodies of water like lakes and oceans moderate local temperatures. Wind speed and direction are affected by topography. The nature of airflow as laminar or turbulent flow is determined by the Reynolds number, which depends on fluid properties and flow characteristics. Wind profiles near the ground follow logarithmic and power-law relationships affected by surface roughness.

1. Submitted by :
Abhilash Singh
Dept. of Agricultural Meteorology
CCSHAU, Hisar
Assignment
AGM – 505
UNIT – II
Atmosphere near the ground; laminar and turbulent flows; wind profile near the
ground.

2. Atmosphere near the ground

3. Microclimate
Immediate local
conditions for
temperature,
humidity, solar
radiation, wind speed,
wind direction,
pressure, rainfall,
vapour pressure etc .

4. Microclimate
Affected by:
– Earth
– Wind
– Fire
– Water

5. Earth: Topography
The leeward side of a
hill can protect from
unwanted winds

6. Elevation
Temperature changes about -9.8 ºC
with each +1000 meters change in altitude
Elevation Temperature
Salt Lake City 4200’ 95º F
Park City 6800’ ~86º F

7. Rain & Shadow affect
Form when moisture
precipitates out as air
moves up slopes

8. Ground Cover – Vegetation
Trees, shrubbery, and
grasses provide shade that
prevents moisture from
evaporating
Permeable surfaces reduce
temperatures through
evaporative cooling

9. Ground Cover
Impermeable surfaces
reduce evaporative cooling
opportunities

10. Surface Color
Lighter surfaces
reflect radiant
heat
Darker surfaces
absorb radiant heat

11. Urban heat island
Large concentrations
of thermal masses
with darker and/or
impermeable surfaces
create urban heat islands

12. Wind: Topography
Warm air rises
Cool air drops
Morning and afternoon
“diurnal winds” are
intensified in canyons and
on sloped surfaces and
diverted by hills and ridges

13. Convective Air Flow
Warm air rises
Cool air drops
Recognizing air channels can help reduce
discomfort or enhance comfort

14. Wind Direction & Speed
• Confirm mean (average) speed
direction
• Consult a wind rose for
prevailing direction

15. Wind Roses
January

16. Wind Roses
July

17. Sea Breeze Effect
Onshore winds occur
when water temperature
is lower than adjoining
air temperature over
land.
Offshore winds are the
reverse process.

18. Bernouli Effect
Air flow closer to the
ground is slowed by
frictional effects of
surfaces

19. Venturi Effect
Air follows the path of
least resistance
Wind speeds increase as
cross sectional area
decreases

20. Wind Breaks/Channeling
Terrain and vegetation can
block or channel wind
Plan Views

21. Solar Orientation
South facing slopes get significantly more solar
radiation than north facing slopes
N

22. Water: Thermal Mass
Large bodies of water
moderate local
temperatures
A thermal mass that
can act as at thermal
buffer/ heat sink

23. Lakes & Oceans
Warm
temperatures
are reduced in
summer
Cool
temperatures
are raised in
winter 10ºF
10ºF
72ºF
80ºF
35ºF
30ºF

24. Groundwater
Groundwater
table affects
temperature and
humidity

25. Rivers & Streams
Moving water
generates air
movement

26. Perspiration
Occurs as body core
temperature rises
In low RH conditions
evaporation occurs and
cools skin
In high RH conditions
evaporation is inhibited

27. Insolation
Humidity and cloud cover
affect the amount of solar
radiation entering or
leaving the local
environment

28. Evapotranspiration
Trees and
shrubbery give
off moisture that
increases local
humidity

29. The flow separates into "layers" that slide relative to one another without mixing.
If we introduce a coloured stream into the laminar flow, the colour will stay in the
stream.
The flow is called steady.
Laminar flow can be represented by a set of lines known as streamlines (flow lines). •
An individual particle will follow a streamline.
• The flow pattern does not change with time. All particles starting on a streamline will
continue to move on that streamline.
• A set of streamlines is called a flow tube.
• Streamlines can't cross nor intersect the "walls" of the flow tube.
• The instantaneous velocity of the particle is in the direction of the tangent to the
streamline.
LAMINAR (STREAMLINE) FLOW

30. • Density of streamlines proportional to the velocity:
streamlines close together ⇒ high velocity, streamline far apart ⇒ low velocity.
• For an ideal fluid, no kinetic energy of the particles is lost.
Velocity profile for the laminar flow
of a non viscous liquid

31. •A vigorous mixing (stirring) of the fluid occurs.
•A complex flow pattern changes continuously with time.
•The velocity of the particles at each given point various chaotically with time.
•The circles called eddies (eddy currents) or vortices.
•Eddies are great deal of energy due their rotational kinetic energy.
•A coloured dye added to a stream will readily disperse very suddenly as the flow rate increases.
The flow becomes unstable at some critical speed.
•Turbulent flow occurs when there are abrupt boundary surfaces.
•The flow of blood through a normal artery is laminar.
•However, when irregularities occur the flow becomes turbulent.
•The noise generated by the turbulent flow can be heard with a stethoscope.
•When the flow becomes turbulent there is a dramatic decrease in the volume flow rate.
•When a fluid flows around an object is a very important parameter in determining the type of
flow.
TURBULENT FLOW

32. •What factors determine
whether a fluid will flow in laminar or turbulent motion?
REYNOLDS NUMBER
A British scientist Osborne Reynolds (1842 – 1912) established that the nature of the flow
depends
upon a dimensionless quantity, which is now called the Reynolds number Re.
Re = ρ v L / η
•η density of fluid
•v average flow velocity over the cross section of the pipe
•L dimension characterizing a cross section.
•Re is not a precise physical quantity.
•The quantities L and are only typical values of size and speed.
•It is often not possible even to say which length you are talking about.
•For a body moving through a fluid it might be either length or breadth or thickness - or any other
dimension you might think of. For a fluid flowing through a channel or a tube, it turns out to be
the diameter of the tube.

33. •How easily the fluid becomes turbulent is related to its viscosity η and density ρ.
•Re is a dimensionless number
•As a rule of thumb Re < ~2000 laminar flow and Re >~ 2000 turbulent flow. 2000 < Re < 3000
flow is unstable – may change from laminar to turbulent or vice versa).
•The Reynolds number also indicates whether the flow around an obstacle such a ship’s hull or an
aeroplane wing will be turbulent or laminar.
•In general, the Reynolds number at which turbulence sets in depends significantly on the shape
of the obstacle and is not given by the formula for Reynolds number above and the condition
above.

34. The logarithmic wind profile
Consider first the simplest case of the wind profile in a neutral surface layer.
In this situation the temperature variables will not play an active role, as indicated by the
fact that the Richardson number will be small (indicating that shear production of turbulence is
much larger than buoyant production or suppression).
Mixing length is dependent only on distance from the surface, z.
Substituting (where k is a dimensionless constant) into (2) we find that the wind shear is
inversely proportional to z.
.
We know that wind speed must fall to zero at some height we will call z0. Integrating
this expression:

z
z z
dz
k
uu
ud
0
*
0
kz
u
dz
ud *

yields the log wind profile:







0
*
ln
z
z
k
u
u

35. In this equation
k is von Karman’s constant and has a value, derived from observations, of around 0.4.
z0 is the roughness length, defined as the height
where the wind according to the log law falls to zero.
In fact z0 lies within the roughness sub-layer where deviates from the log law.
It represents the bulk effects of roughness elements in the surface layer and
very approximately has a value around 0.1 times the height of the roughness elements.
u* is
2
1






a

= frictional velocity

36. Wind profile over plant canopy:
The crop acts as barrier to the flow of wind. The nature of the crop stand, the stems,
leaves; reproductive organs etc. all influence the nature of the wind profile within and outside the
crop canopy. Essentially, the position of the active surface barrier is increased when wind blows
over a crop stand. The depth of this frictional layer depends on the roughness of the surface and
as the roughness increases the depth also increases.
(A) Wind profile over a tall crop
The wind profile over short crops may be expressed by a logarithmic equation.
0
2
1
ln
1
z
dz
ak
u










The length, zo depends upon the shape, size, stiffness and density of the crop components.
(B) Wind profile over short crops
For short vegetation and for relatively smooth surfaces,
the roughness parameter is relatively constant over a range of wind speeds.
0
0
2
1
ZHZ
ln
1








for
z
z
ak
u

