Atmospheric Circulation and Weather Systems

Atmospheric Pressure

The weight of the column of air contained in a unit area from the mean sea level to the top of the atmosphere is called Atmospheric pressure

The atmospheric pressure is expressed in millibar. At sea level, average atmospheric is 1,013.2 millibar

Vertical variation of pressure

In the lower atmosphere, the pressure decreases rapidly with Increase in altitude. The decrease amounts are 1mb for 10m increase in height.

The vertical pressure gradient force is much larger than that of the horizontal pressure gradient

Horizontal distribution of pressure

Small pressure differences are highly significant in terms of the wind direction and velocity.

The Horizontal distribution of pressure is studied by drawing isobars at constant levels.

Isobars are lines connecting two equal pressure points, to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of combination

World Distribution of Sea-level Pressure

Near the equator sea level pressure is low and the area is known as equatorial low.

Near the poles the pressure is high so it is called Polar High, the Pressure belts are not permanent.

Forces Affecting velocity and wind

The air in motion is called wind, wind flows from High pressure to lower pressure. The wind at surface experiences friction, rotation of the earth also affects the wind movement

The force exerted by the rotation of the earth is called the Coriolis force

The horizontal wind near the earth surface responds to the combined effect of three forces

1. Pressure gradient force
2. Frictional force
3. Coriolis force

Pressure Gradient force

The difference in atmospheric pressure produces force. The rate of change of pressure with respect to distance is the pressure gradient

The pressure gradient is strong where isobars are close to each other and weak where the isobars are apart

Frictional force

It affects the speed of the wind. It is greatest at the surface and its influence is generally extends up to an elevation of 1-3km. over the sea surface friction is minimal.

Coriolis force

The rotation of earth to its axis affects the direction of the wind. The force is called coriolis force

1. It deflects wind right direction in Northern hemisphere and to the left in the southern hemisphere
2. Deflection will be more when the wind velocity is high
3. Coriolis force is directly proportional to the angle of latitude
4. It is maximum at poles and absent at equator
5. Coriolis force acts perpendicular to the pressure gradient force
6. Pressure gradient is perpendicular to an isobar
7. The higher the pressure gradient force, the more is the velocity of the wind and larger is the deflection in the direction of wind
8. Tropical cyclones do not form at equator

General circulation of the atmosphere

• The air at the Inter Tropical convergence zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created
• The winds converge at the low pressure zone, the convergent air rises along with the convective cell
• It reaches the top of troposphere up to an altitude of 14km
• Down below near the land surface air flows towards the equator as easterlies
• Easterlies from either side converge at Inter tropical convergence zone(ITCZ), such circulations from the surface upwards and vice-versa are called Such cells in the tropic is called Hadley cell
• In the middle latitudes circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from subtropical high. At the surface these are called westerlies and the cell is known as Farrel cell
• At polar latitudes the cold dense air subsides near the poles and  blows towards middle latitudes as polar easterlies. This cell is called the polar cell
• These three cells set the pattern for the general circulation of the atmosphere

El Niño:

El Nino is a routine climate pattern that occurs when sea surface temperatures in the tropical pacific ocean rise to above- normal levels for an extended period of time 

These warm and cool phases are part of a reccurring climate pattern that occurs in across this pacific region, known as the El-nino southern Oscillation (ENSO)

La Niña 

The opposite of El nino is, La Nina , is when sea temperature drops in the central pacific to a level less than normal 

Effects of La Nina

• The Horn of Africa and Asia will see below average rainfall due to La Nina
• It could also lead to increased rainfall in Southern Africa
• It could also affect southwest Indian Ocean Tropical Cyclone season, reducing the Intensity 
• Southern Asia, Some pacific Islands and the Northern region of south America are expected to receive above average rainfall
• For India La Nina means good and bad news because India will have more rainfall which can cause floods

Land and Sea Breezes

Land and Sea absorb and release heat differently

Mountain and valley winds

• During the day the slopes get heated up and it rises above, so to fill the gap air from valley will flow up the valley, this wind is known as valley breeze
• During the nights the slope gets cooled down and air starts moving into the valley and it is called mountain wind

  The cool air of the high plateaus and ice fields draining into the valley is      called a Ketabatic wind

• Moisture that these winds hold will give precipitation while they cross the mountains over

      When air descends down the lee side of the slope, the dry air gets warmed

       Up by the adiatic process

Air Masses

When air has distinctive type of characteristics in terms of temperature and humidity is called Air mass

It is defined as a large body of air having little horizontal variation in temperature and moisture

Air masses are classified according to the source regions those are

1. Warm tropical and subtropical oceans
2. Subtropical hot deserts
3. The relatively cold high latitude oceans
4. The very cold snow covered continents in high latitudes
5. Permanently ice covered regions in the Arctic and the Antarctica

Fronts

When two air masses meet, the boundary zone is called the front

The process of formation of the fronts is called frontogenesis. There are four types of fronts

1. Cold
2. Warm
3. Stationary
4. Occluded

• When front remains stationary it is called stationary front
• When cold air masses moves towards warm air mass, its contact zone is called cold front
• When the warm air zone moves towards cold air mass then it is called warm front
• when air mass is fully lifted above the land surface is called occluded front

The fronts occur in middle latitudes and are characterized by steep gradient in temperature and pressure. They bring abrupt change in the temperature and causes precipitation

Extra Tropical Cyclones

The systems developing in middle and high latitudes, beyond the tropics are called the middle latitude or extra tropical cyclones

The passage of fronts causes abrupt changes in the middle and high latitudes

Extra tropical cyclones formed along the polar front

• in northern hemisphere, warm air blows from the south and cold air from the north of the front
• when pressure drops along the front, warm air moves towards north and cold air towards south in a anticlockwise motion
• the cyclonic circulation leads to a well developed extra tropical cyclone with a warm and cold fronts
• Warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front which causes precipitation
• Cold air pushes warm air up from behind which results in the formation of cumulonimbus clouds which develops along cold front
• Cold front is faster than warm front

Tropical Cyclones

Tropical cyclones are violent storms which originates over a oceans in tropical areas and move over coastal areas and destruct on a large scale by violent winds and heavy rainfall and storm surges

The tropical Cyclone form and intensify over a warm tropical oceans

The following conditions are favourable for the formation and intensification  of tropical storms

• Large sea surface with temperature higher than 27°
• Presence of coriolis force
• Small variations in the vertical wind speed
• A pre-existing weak low pressure area or low level cyclonic circulation
• Upper divergence above the sea level system

Eye:

Eye is a region of calm with subsiding air, around the eye there is an eye wall, where there is a strong spiraling ascent of air to greater heights reaching the tropopause

Air reaches to maximum velocity in this region, it will reach as high as 250km/hr

Thunderstorm and Tornadoes

• Thunderstorms are caused by intense convection on moist hot days, it is well grown cumulonimbus cloud which produces thunder and lighting
• When clouds reach a height where sub zero current prevails hails starts to form when it reaches earth they’ll become hailstorms
• Tornadoes are vertical funnels of rapidly spinning the air , their speeds tops 250 miles an hour
• Tornado over the sea is called water sprouts
• The violent storms are manifestation of the atmosphere’s adjustments to varying energy distribution