Summary

Geography Class 22

Revision of the previous class (05:10 PM)

Temperature and Factors affecting the temperature (05:18 PM)

Terms related to the temperature
Temperature is the degree of Hotness or coldness.
The thermometer is used to measure the temperature
Diurnal range of temperature: Difference between daily max and daily minimum.
Mean Daily Temperature: Sum of 24 hours of temperature/24
Mean monthly temperature: It is the sum of a daily temperature of a month divided by the number of days in a Month
Annual Range of temperature: Highest-Lowest Mean monthly temperature.
The highest recorded temperature is 58 degrees in Al Azizia (Libya)
But as per WMO, it is 56.8 in Death Valley (USA California)
Lowest temperature: Vostok station, Antarctic (-88 degrees)
Distribution of Temperature
Variation in temperature, Is not uniform along the latitude at the different places due to a number of factors, and when we connect these lines of uniform temperature along the latitude is known as Isotherm.
Across the earth, Isotherms shift north and south with the apparent movement of the sun.
The Isotherms move northward between January to July and they shift south between July to January.
The shifting of Isotherms is more in the Northern hemisphere than in the southern hemisphere.
This indicates higher seasonal variations in the northern hemisphere.
This is due to the presence of a vast landmass in the Northern Hemisphere.
Moving from land to oceans, Isotherms bents along a particular latitude.
Isotherms bents polewards in January over the oceans.
Isotherm bent Equatorward in July over the oceans.
Near the Equator Annual range of temperature is very less, The Range of temperature increases from the Equator up to higher latitudes, This Range is more in the Northern hemisphere.

Temperature Inversion (05:57 PM)

The average rate of decrease in temperature up in the altitude is called as Normal lapse rate
Under certain special conditions, Reversal of the Normal lapse rate occurs so that temperature increases with altitude. This phenomenon is called temperature inversion.
The Zone of temperature change in the atmosphere is called a temperature inversion zone.
Types of temperature inversion
a)Radiation inversion
The conditions that favour radiation inversion are:
Long winter nights
Calm and windless conditions
Clear and cloudless sky.
In these conditions, the land surface radiates heat more rapidly during the night, This loss of Radiation by the land surface causes the land to cool down and the air just above the surface also cools.
This colder air is overlaid by warmer upper air which has absorbed heat from radiation loss.
Therefore temperature increases with increases in altitude creating a temperature inversion
b) Air drainage type of inversion(06:35 PM)
The mountains and hilltops experience rapid fall in temperature during the night.
The valley sides remain warmer due to radiation exchange between the valleys.
The cooler and denser air from the hilltops sinks to the valley floor due to Gravity.
After some time the warmer air rests over the cooler air resulting in a Temperature inversion.
c) Advection inversion
It is produced when a thick layer of warm air passes over the cold water's surface or snow-covered surface creating temperature inversion.
d) Frontal inversion
It is caused due to frontal convergence of air masses.
Along the Fronts, the warmer air is forced upward from the ground by the undercutting of cold air which leads to temperature inversion where warm air is lying above the cold air.

Significance of temperature inversion (06:52 PM)

Temperature inversion prevents convection by creating stability in the lower atmosphere and hence prevents precipitation.
The temperature inversion conditions with cold air near the surface are suitable for fog formation.
It also reduces wind activity and prevents the distribution of pollutants causing bad air quality, For Example: Around Delhi, during winters the pollutants are trapped for a longer duration creating bad air quality.
In urban areas during winter fog mixes up with smoke resulting in the formation of Smog which is a thick layer and reduces air quality.
The formation of Fog reduces visibility and disrupts the Transportation network.
Air Drainage type of inversion decides the pattern of settlements in Mountainous regions, Mountain slopes are preferred over valley floors or hilltops.
Agriculture in hilly areas is impacted by Air drainage inversion.
Example: Fruit orchards of Uttrakhand and Himachal, Coffee plantation of Brazilavoid valley floors

Pressure (07:26 PM)

Pressure is the force experienced per unit area.
Atmospheric pressure is the pressure exerted by the Earth's atmosphere.
The average atmospheric pressure near sea level is equal to 1013milibars.
Pressure is measured using barometers.
Factors that cause variation in pressure
a)Temperature: When Air is heated it expands and the outward pressure of molecules is spread over a larger area therefore the pressure of air decreases.
When Air is cooled it contracts to cause an increase in pressure
b)Altitude: The pressure experienced by Air at ground level is higher than that of Air at higher altitudes.
When Air Rises due to convection its volume increases and pressure decreases.
When air sinks due to subsidence its volume decreases and pressure increases.
c)Rotation: The rotation of Earth causes air at the poles to be thrown away towards the Equator.
If we consider the effect of only rotation air piling up around the Equator produces a belt of high pressure and along poles a belt of low pressure.

Formation of pressure Belts (07:53 PM)

The Belt of low pressure experienced along the Equator region is known as the Equatorial low
And the Zone of High pressure at the poles is known as Polar High.
Air Rises at the Equator and moves upward till tropopause and from there separates in two directions i.e two poles.
At 30 degrees (North and South) it sinks down due to the air being dense now as it is now occupying a smaller area so the pressure increases, so the region is Sub Tropical High.
From 30 degrees Air then divides into 2 directions again, One moving towards the equator and another moving towards the poles.
The air near the polar region is always subsiding in nature, Because there is no convection, There is a High-pressure belt so air never vertically rises,
Air near the polar region gets thrown due to the rotation of the earth and starts to move away from the poles, which earlier was occupying a smaller area now due to getting thrown away occupies a larger area and at a 60-degree belt has the low-pressure belt. (Temperate low)
And this air coming from the poles meet the air from the Sub tropics, This convergence causes convection and rises vertically upward and upon reaching the troposphere again divides into two one moving pole side and another towards the subtropics.
And again meeting the subtropics air at the upper atmospheric level descends at 30 degrees.
This all results in the formation of Atmospheric cells.
Between o to 30 degree=Headley cell
Between 30 to 60 degree= Ferrel cell.
Between 60 to 90 degree=Polar cell
Planetary winds flow along the surface.
0 to 30 degree =Trade winds
30-60 degree= westerlies
60-90 degree=polar Easterlies

The Topic for the next class: Winds-Planetary, Local and Seasonal winds.