- The sun sends us more than heat and light; it sends lots of other energy and small particles our way. The protective magnetic field around Earth shields us from most of the energy and particles, and we don’t even notice them.
- But the sun doesn’t send the same amount of energy all the time. There is a constant streaming solar wind and there are also solar storms.
- When a solar storm comes toward us, some of the energy and small particles can travel down the magnetic field lines at the north and south poles into Earth’s atmosphere.
- There, the particles interact with gases in our atmosphere resulting in beautiful displays of light in the sky. Oxygen gives off green and red light. Nitrogen glows blue and purple.
Air Pressure and Atmospheric Circulation Horizontal
Distribution of Air Pressure
- Horizontal distribution of air pressure is seen on the basis of isobars
- Air pressure – High air pressure or Low air pressure
- The distribution of pressure belts is not regular due to unequal distribution of land and water.
- Pressure Belts can be Thermally induced or Dynamically induced.
- Thermally induced – Equatorial Low, Polar High
- Dynamically induced – Subtropical high, Subpolar low
Subtropical high is caused due to rotation of the Earth and sinking and settling down of winds
Subpolar low is caused due to spreading out of surface air due to rotation of the Earth.
Tricellular Meridional Circulation
Shifting of Pressure Belts
Sea Breeze and Land Breeze
- Daytime = Wind blows from colder sea to land
- Night time = Wind blows from warmer land to sea
Mountain Breeze and Valley Breeze
- Day time = Warm air rises along the sides and cool air descends to take its place.
- Night time = Cool air descends valley sides and displaces warm air from valley floors.
Chinook and Foehn
Warm and dry local winds blowing on leeward sides of mountains. They are also called snow eater and the chinooks help in early sowing of spring wheat in USA.
Foehn blow down the leeward side of the Alps
Warm and dry winds blowing from the north east and east to the west in eastern parts of the Sahara. They help reduce the high temperature in the coast of Guinea and are also known as the doctor.
It is a warm, dry and dusty wind that blows in northerly direction from Sahara Desert across the Mediterranean Sea to reach Italy and Spain. It becomes very active at the time of origin of cyclonic storms over the Mediterranean Sea. It becomes extremely warm and dry while descending northern slopes of Atlas Mountains. It is known by different names – khamsin (Egypt), ghibli (Libya), chilli (Tunisia). The warm and dry dusty winds in the Arabian desert are called Simon. While passing over the Mediterranean Sea it picks up moisture and yields rainfall over southern part of Italy and the rain is called ‘blood rain’ due to the fall out of red sands with the rains. It is injurious to agricultural and fruit crops.
It is a cold local wind blowing in Spain and France from north-west to south- east direction. They become more common and effective during winter season because of development of high pressure over Europe and low pressure over Mediterranean Sea. They become extremely cold when they blow through the central plateau and descend into Rhone valley on the southern coast of France becoming stormy. They adversely affect air flights and cause sudden drop in air temperature to below freezing point.
|Bora||It is an extremely cold and dry north-easterly wind blowing along the shores of the Adriatic sea. It becomes more effective in north Italy where it descends through the southern slopes of the Alps and blow in southerly direction. It is relatively moist unlike mistral as it picks moisture over the Adriatic Sea.|
|Blizzard||Violent, stormy, cold and powdery wind laden with dry snow and is prevalent in north and south polar regions, Siberia, Canada and the USA.|
|Purga||A snow laden cold wind in Russian Tundra|
|Bise||cold wind in France|
|Levanter||easterly cold wind in southern Spain.|
|Pampero||northwesterly cold wind in the pampas of South America|
|Norwester||a warm, dry and gusty wind in New Zealand|
|Santa Ana||warm and dry wind in USA|
|Yamo||warm and dry wind in Japan|
|Zonda||warm wind in Argentina|
|Tramontane||warm wind in central Europe.|
The seasonal wind of the Indian Ocean and southern Asia, blowing from the southwest in summer and from the northeast in winter. (in India and nearby lands) the season during which the southwest monsoon blows, commonly marked by heavy rains; rainy season. any wind that changes directions with the seasons.
Air under the influence of both the pressure gradient force and Coriolis force tends to move parallel to isobars in conditions where friction is low (1000 meters above the surface of the Earth) and isobars are straight. Winds of this type are usually called geostrophic winds. Geostrophic winds come about because pressure gradient force and Coriolis force come into balance after the air begins to move
- In westerlies, there are strong, narrow bands of high speed wind => Jet stream
- Speed of Jet stream up to 400 km h
Location of Jet Streams
- The meandering jet streams are called Rossby Waves.
- Rossby waves are natural phenomenon in the atmosphere and oceans due to rotation of earth.
- In planetary atmospheres, they are due to the variation in the Coriolis effect (When temperature contrast is low, speed of jet stream is low, and Coriolis force is weak leading to meandering) with latitude.
- Rossby waves are formed when polar air moves toward the Equator while tropical air is moving poleward.
- The existence of these waves explains the low-pressure cells (cyclones) and high-pressure cells
Jet Streams – Travelling Depressions
- Jet stream embedded in westerlies (Rossby waves) at high latitude, cause pressure variability
- That’s why they are called travelling depressions
- A polar vortex is a low pressure area—a wide expanse of swirling cold air—that is parked in polar regions. During winter, the polar vortex at the North Pole expands, sending cold air southward. This happens fairly regularly and is often associated with outbreaks of cold temperatures in North America and Europe.
- The breaking off of part of the vortex is what defines a polar vortex event. But it actually occurs when the vortex is weaker, not stronger. Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path.
- This current keeps the cold air up north and the warm air down south. But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.
That’s what happened in early 2014, 2017 and in 2019. The polar vortex weakened, and a huge highpressure system formed over Greenland. The high-pressure system blocked the escape of all that cold air in the jet stream, and allowed part of the polar vortex to break off and move southward. Places as far south as Tampa, Florida experienced a drop in temperatures due to this polar vortex.
Most of Canada and parts of the Midwestern United States had temperatures colder than Alaska
Humidity and Precipitation
- Water vapour in air varies from zero to five per cent by volume of the atmosphere (averaging around 2% in the atmosphere).
- The actual amount of the water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre. The absolute humidity differs from place to place on the surface of the earth. The ability of the air to hold water vapour depends entirely on its temperature (Warm air can hold more moisture than cold air).
- The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity. With the change of air temperature, the capacity to retain moisture increases or decreases and the relative humidity is also affected.
- Relative humidity is greater over the oceans and least over the continents (absolute humidity is greater over oceans because of greater availability of water for evaporation).
- The relative humidity determines the amount and rate of evaporation and hence it is an important climatic factor.
Humidity and Precipitation
- Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. At this temperature, the air cannot hold any additional amount of moisture. Thus, relative humidity of the saturated air is 100%.
- If the air has half the amount of moisture that it can carry, then it is unsaturated
Relative Humidity can be changed by:
- By adding moisture through evaporation (by increasing absolute humidity): if moisture is added by evaporation, the relative humidity will increase and vice versa.
- By changing temperature of air (by changing the saturation point): a decrease in temperature (hence, decrease in moisture-holding capacity/decrease in saturation point) will cause an increase in relative humidity and vice versa.
- The air containing moisture to its full capacity at a given temperature is said to be saturated.
- It means that the air at the given temperature is incapable of holding any additional amount of moisture at that stage.
- The temperature at which saturation occurs in a given sample of air is known as dew point.
Adiabatic Lapse Rate
- The adiabatic lapse rate is the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change, under the assumption that the process is adiabatic, i.e., no heat exchange occurs between the given air parcel and its surroundings.
- The Dry Adiabatic Lapse Rate (DALR) is the rate of fall in temperature with altitude for a parcel of dry or unsaturated air (air with less moisture, to keep it simple) rising under adiabatic conditions.
- When an air parcel that is saturated (stomach full) with water vapour rises, some of the vapour will condense and release latent heat [Additional Heat from inside]. This process causes the parcel to cool more slowly than it would if it were not saturated.
- The moist adiabatic lapse rate varies considerably because the amount of water vapour in the air is highly variable.
Atmospheric Stability and Instability
- Different forms of precipitation (dew, fog, rainfall, frost, snowfall, hailstorm etc.) depend on stability and instability of the atmosphere. The air without vertical movement is called stable air while unstable air undergoes vertical movement (both upward and downward). An air mass ascends and becomes unstable when it becomes warmer than the surrounding air mass while descending air mass becomes stable.
- The stability and instability depend on the relationships between ‘normal lapse rate’ and ‘adiabatic change of temperature’. Adiabatic rate is always constant whereas normal lapse rate of air temperature changes.
- When the normal lapse rate is higher than dry adiabatic rate, the air being warmer rises and becomes unstable. On the other hand, when the normal lapse rate of temperature is lower than dry adiabatic rate, the air being cold descends and becomes stable.
Condition of Stability
At ground surface if the temperature of a parcel of air is 40°C, the dry adiabatic lapse rate and normal (environmental) lapse rate are 10°C per 1000m and 6.5° C per 1000 m respectively, then at the height of one kilometre (or 1000 m) from the ground surface the temperature of the ascending air would be 30°C (40° -10°= 30°C) while the temperature of surrounding air at that height would be 33.5° C (40°-6.5° = 33.5°C)
Sometimes, the normal lapse rate in a certain layer of the atmosphere is about 4.6° C per 1000 metres. In such conditions if the normal lapse rate is less than wet adiabatic lapse rate even at condensation point, further vertical motion of air is stopped and thus such air is said to be absolutely stable and such atmospheric condition is called absolute stability..
Condition of Instability
When normal lapse rate is greater than dry adiabatic lapse rate of ascending parcel of air the rising air continues to rise upward and expand and thus becomes unstable and is in unstable equilibrium. In other words, atmospheric instability is caused when the rate of cooling of rising air (dry adiabatic lapse rate) is lower than the normal lapse rate.
For example, if the temperature of a certain parcel of air at ground surface is 40°C, the dry adiabatic and normal lapse rates are 10°C and 11°C per 1000m respectively, then the temperature of ascending air at the height of 1000m (one kilometre) would be 30°C (40°-10° = 30°C) while the temperature of the atmosphere at that height would be 29°C (40°-11°C = 29°C).
Thus, the rising air being warmer (30°C) than the surrounding air (29°C) continues to rise and expand to cause atmospheric instability. If the wet adiabatic lapse rate is also less than normal lapse rate, the rising air further continues to rise upward. Such state of continued upward movement of air is called absolute instability.
Condensation and its forms
- The transformation of water vapour into water is called condensation.
- Condensation is caused by the loss of heat (latent heat of condensation, opposite of latent heat of vaporization).
Forms of Condensation
- Dew Frost
The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulus clouds are formed. This process releases latent heat of condensation which further heats the air and forces the air to go further up.
Convectional precipitation is heavy but of short duration, highly localised and is associated with minimum amount of cloudiness. It occurs mainly during summer and is common over equatorial doldrums in the Congo basin, the Amazon basin and the islands of south-east Asia.
When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands (because of fall in pressure); the temperature falls, and the moisture is condensed.
The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area (Some arid and semi-arid regions are a direct consequence of rain-shadow effect. Example: Patagonian desert in Argentina, Eastern slopes of Western Ghats). It is also known as the relief rain.
When two air masses with different temperatures meet, turbulent conditions are produced. Along the front convection occurs and causes precipitation (we studied this in Fronts). For instance, in north-west Europe, cold continental air and warm oceanic air converge to produce heavy rainfall in adjacent areas.
Cyclonic Rainfall is convectional rainfall on a large scale.
World Distribution of Rainfall
World Distribution of Rainfall
- Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons. In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily.
- The coastal areas of the world receive greater amounts of rainfall than the interior of the continents. The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water.
- Between the latitudes 35° and 40° N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 45° and 65° N and S of equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east.
- Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.