Frequently Asked Questions (FAQs) on Monsoon
Q. What is southwest monsoon? What
causes the monsoon? Which are the prominent monsoon regions, other than India
A.
The seasonal reversal of winds and the associated rainfall. This word is derived
from the Arabic word “Mausim”. The annual oscillation in the apparent position
of the Sun between the Tropics of Cancer and Capricorn causes the annual
oscillation in the position of the thermal equator (region of maximum heating)
on the Earth’s surface. This is associated with the annual oscillation of
temperature, pressure, wind, cloudiness, rain etc. This is the cause of the
monsoons. On the Earth’s surface, there are asymmetries of land and Ocean. The differential heating of land and Ocean
cause variations in the intensity of the annual oscillation of the thermal
equator and hence regional variations in the intensity of monsoon. The
southwesterly wind flow occurring over most parts of India
and Indian Seas
gives rise to southwest monsoon over India
Q. What is Forecasting?
A. In
science, the forecasting means the process of estimation of the value of some
variable at some future time. One of the
primary functions of the national Weather services is forecast of weather parameters
such as rainfall, temperature, wind, humidity etc. over a region averaged over
a particular time period. For example forecast of daily rainfall (rainfall
averaged over a day).
Q.
What is Nowcasting?
A.
A weather forecast in which the details about the current weather and forecasts
up to a few hours ahead (but less than 24 hours) are given.
Q.
What is short range weather forecasting?
A.
Short range weather forecasts are weather forecasts valid up to 72 hours ahead.
This forecast range is mainly concerned with the weather systems observed in
the latest weather charts and also by considering the generation of new systems
within the time period.
Q.
What are medium range forecasts?
A. These are weather
forecasts generally valid for a period of 4 to 10 days (However, up to 7 days
in tropics). In this, the average weather conditions and the weather on each
day will be prescribed with progressively lesser details and accuracy than that
of the short range forecasts.
Q.
What is long range forecast?
A. As per
the World Meteorological Organization (WMO) definition, long range forecast is
defined as the forecast from 30 days’ up to one season’s description of
averaged weather parameters. The
seasonal forecast comes under long range forecast.
Q.
What is extended range forecast?
A. This is the forecast range lies
between long range (seasonal) and medium range. Thus, it starts generally from
days 10 (however, beyond days 7 in tropics) up to one month.
Q. What are the seasons defined by the India
Meteorological Department for the country as a whole?
A.
Meteorological seasons over India
Winter Season: January
– February
Pre Monsoon Season:
March – May
Southwest Monsoon Season: June - September
Post Monsoon Season: October - December
Q.
What is maximum temperature?
A.
It is the highest temperature attained during a day. It often occurs during the
afternoon hours.
Q.
What is minimum temperature?
A.
It is the lowest temperature recorded which usually occurs during the early
morning hours.
Q.
What is atmospheric pressure?
A.
The pressure of the atmosphere at any point is the weight of the air column
which stands vertically above unit area with the point as its centre. For
meteorological purposes, atmospheric pressure is usually measured by means of a
mercury barometer where the height of the mercury column represents the
atmospheric pressure. The pressure is
expressed in hPa (Hecta Pascal) which is defined as equal to 106 Newton/m2.
Q.
How do we express the quantity of rainfall?
A. Liquid rainfall
is expressed as the depth to which it would cover a horizontal projection of
the earth’s surface, if there is no loss by evaporation, run–off or
infiltration. It is expressed in terms
of mm or cm. It is assumed that the amount of precipitation collected in the
gauge is representative of a certain area around the point where the
measurement is made. The choice of the instrument and the site itself, the form
and exposure of the measuring gauge, the prevention of loss of precipitation by
evaporation and the effects of wind and splashing are some of the important
points to be considered in the correct measurement of precipitation.
Q.
What is a thunderstorm?
A. A Thunderstorm is defined as a meteorological phenomenon
in which one or more sudden electrical discharges manifested by a
flash of light (Lightning) and a sharp rumbling sound (thunder) occurs from a
cloud of vertical development.
Q.
What is a Dust storm?
A. A Dust storm is
an ensemble of particles of dust or sand energetically lifted to great heights
by a strong and turbulent wind. Often the surface visibility is reduced to low
limits; the qualification for a synoptic report is visibility below 1000 m.
Q.
What are clouds and how they are classified?
A. Clouds are
aggregate of very small water droplets, ice crystals, or a mixture of both,
with its base above the earth’s surface. A classification is made in level –
high, medium, or low – at which the various cloud genera are usually
encountered. In temperate regions the approximate limits are high, 5-13 km
(16500 – 45000 ft); medium, 2-7 km (6500 – 23000 ft); low, 0-2 km (0 – 6500
ft). The high clouds are Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs). The
medium clouds are Altocumulus (Ac), Altostratus (As) (the latter often
extending higher) and Nimbostratus (Ns) (usually extending both higher and
lower);
The low clouds are Stratocumulus (Sc), Stratus (St), Cumulus (Cu), and
Cumulonimbus (Cb).
Q.
What are land and sea breezes?
A. Land and Sea
Breezes are local winds caused by the unequal diurnal heating and cooling of
adjacent land and water surfaces; under the influence of solar radiation by day
and radiation to the sky at night, a gradient of pressure near the coast is
produced. During the day, the land is
warmer than the sea and a breeze, the Sea Breeze, blows onshore; at night and
in the early morning the land is cooler than the sea and the land breeze blows
off shore.
Q.
What are the different methods used for long range forecasting?
A. In general, three approaches are
used. These are (i) statistical method (ii) numerical weather prediction or
dynamical method and (iii) dynamical cum statistical method.
Q.
Who is responsible for issuing operational long range forecast in India
A. India Meteorological Department is
solely responsible for issuing operational long range forecast for India
Q. Which are the
countries that use empirical models for long range forecasts?
A. In addition to India , there are several other countries like United States , United
Kingdom , Australia ,
South Africa
Q. What are the long
range forecasts prepared by IMD and when are they issued.
A. IMD issues operational long range
forecast for the rainfall during SW Monsoon Season (June- September). These forecasts are issued in two stages. The
first stage forecast is issued in mid-April and consists of quantitative
forecast for the season (June to September) rainfall over India India
IMD
also prepares forecasts for winter (Jan- March) precipitation (issued in the
end of December) over Northwest India and northeast monsoon (October-December)
rainfall over Southern Peninsula (issued in
October). However, these forecasts are issued only to the government.
Looking
at the potential of numerical models, IMD has also established an experimental
prediction system based on General Circulation Model (GCM) in addition to its
existing operational forecasting system based on statistical models. For this
purpose, IMD uses the seasonal forecasting model (SFM) developed at the
Experimental Climate Prediction Centre (ECPC), Scripps Institute of
Oceanography, USA
Q.
What is the accuracy of the long range forecast for monsoon rainfall issued by
IMD?
A. The monsoon prediction in our country
is being done with reasonable accuracy. The success rate of IMD forecasts since
1988 has been high. During the last21 years (1988-2008), IMD forecasts were
qualitatively correct in 19 years (i.e. 90% of years). The exception was during
years 2002 and 2004 both of which were drought years. However, in some years (1994, 1997, 1999,
2002, 2004 and 2007) the forecast error (difference between actual rainfall and
forecast rainfall) was more than 10%. The 2002 drought was due to exceptionally
low rainfall during the month of July (46% of long term period) caused by
unexpected sudden warming of sea surface over equatorial central Pacific that
started in the month of June. It may be mentioned that the exceptionally
deficient rainfall of July, 2002 was not predicted by any prediction group in India
A. Climate, in a narrow sense, can be defined as the
average weather conditions for a particular location and period of time. In a
wider sense, it is the state of the climate system. Climate can be described in
terms of statistical descriptions of the central tendencies and variability of
relevant elements such as temperature, precipitation, atmospheric pressure,
humidity and winds or through combinations of elements, such as weather types
and phenomena that are typical to a location, region or the world for any
period of time.
A. Five-day weather forecast today is generally as
reliable as a three-day forecast two decades ago. Outlooks of up to a week,
especially in temperate mid-latitude regions are becoming increasingly
reliable. Information can be disseminated around the world from one location to
another within three hours, while recently understood phenomena such as El Niño
Southern Oscillation (ENSO) (El Niño, La Niña and neutral phases) can be
forecast up to a year in advance. Seasonal climate predictions can be forecast
up to a month, three months or six months ahead although these climate
predictions are probabilistic in nature. Such forecasts, often from advanced
centres, are made available globally to all nations.
Q. Why are weather forecasts sometimes
inaccurate?
A. Air pressure, temperature, mountain ranges, ocean
currents and many other factors combine to produce an enormous quantity of
interacting variables all of which can alter the weather to a greater or lesser
extent. However, greater understanding of the science, plus the use of powerful
computer models, continue to improve our ability to make more accurate
predictions with longer lead times.
Q. What is the difference between climate
change and climate variability?
A. Climate variability is the term used to describe a
range of weather conditions that, averaged together, describe the “climate” of
a region. In some parts of the world, or in any region for certain time periods
or parts of the year, this variability can be weak, i.e. there is not much
difference in the conditions within that time period. However, in other places
or time periods, conditions can swing across a large range, from freezing to
very warm, or from very wet to very dry, thereby exhibiting strong variability.
A certain amount of this is understood and accepted by the region’s inhabitants.
Occasionally, an event or sequence of events occurs that has never been
witnessed or recorded before, such as the exceptional hurricane season in the Atlantic in 2005 (though even that could be part of
natural climate variability). If such a season does not recur within say, the
next 30 years, we would look back and call it an exceptional year, but not a
harbinger of change. For the scientific community to recognize a change in
climate, a shift has to occur, and persist for quite a long time. The Intergovernmental
Panel on Climate Change (IPCC) is conducting considerable efforts in trying to
determine, for various hydrometeorological hazards (e.g. tropical cyclones and
tornadoes) and for related events (e.g. flash floods), whether their occurrence
is affected by human-induced climate change. The IPCC Fourth Assessment Report
provides evidence that climate change affects the frequency and (or) intensity
of some of those events, but further work is under way to refine those findings
and prepare a more comprehensive assessment as part of a Special Report to be
published in 2011.
A. El Niño, Spanish for "boy child"
(because of the tendency of the phenomenon to arrive around Christmas), is an abnormal
warming of water in the Equatorial Pacific Ocean every three to five years and
can last up to 18 months. Severe cases of El Niño, as in 1997/98, are
responsible for drought, flooding, as well as areas of formation for tropical
cyclones and severe winter storms. The 1997/98 El Niño and its associated
impacts have been blamed for the deaths of hundreds of people and caused
billions of dollars of damage in an estimated 15 countries especially in the
Panama Canal region but also as far away as the east coast of Africa. La Niña
means “the little girl”, the opposite of El Niño, and refers to the abnormal
cooling of the ocean temperatures in the same Pacific region.
Q.
What are the different Nino regions for measuring El Nino ?
A. For the
measurement of strength of El Nino or La Nina the Sea Surface Temperature
anomalies are measured over eastern and central Pacific
Ocean at 4 different regions as depicted below graphically. These
four Nino regions are bounded by
NINO12 : 0-10S, 80W-90W
NINO3 : 5N-5S, 90W-150W
NINO4 : 5N-5S, 150W-160E
NINO34 : 5N-5S, 120W-170W
Q.
What is Southern Oscillation Index (SOI)
and ENSO
A. The
Southern Oscillation is the atmospheric component of El Niño. It is an
oscillation in air pressure between the tropical eastern and the western Pacific Ocean
waters. The strength of the Southern Oscillation is measured by the Southern
Oscillation Index (SOI). The SOI is computed from fluctuations in the surface
air pressure difference between Tahiti (Over Pacific Ocean) and Darwin, (Indian Ocean near Australia). El Niño
episodes are associated with negative values of the SOI, meaning that the
pressure at Tahiti is relatively low compared to Darwin
Low
atmospheric pressure tends to occur over warm water and high pressure occurs
over cold water, in part because deep convection over the warm water acts to
transport air. El Niño episodes are defined as sustained warming of the central
and eastern tropical Pacific Ocean . This
results in a decrease in the strength of the Pacific trade winds,
and a reduction in rainfall over eastern and northern Australia
ENSO is
composed of both El Nino and Southern Oscillation. Thus, the oceanic component
called El Niño (or La Niña, depending on its phase) and the atmospheric
component, the Southern Oscillation.
Q. What is Walker
The Walker
circulation is named after Sir Gilbert Walker, former Director-General of
Meteorology in India during the
British rule who, in early 20th century, identified a number of relationships
between seasonal climate variations in Asia
and the Pacific region. He had shown from his many published papers during
1920s and early 1930s that the weather in Djakarta (Indonesia) and Santiago (Chile) was related in such
a way that when the pressure was higher than normal at one place it was lower
than normal at the other. As these cities are about 15 000 km apart it was difficult at that time to
visualize a mechanism for the connection between these distant locations. Later it is understood that what Walker Walker
The easterly trade winds are part of the low-level
component of the Walker Pacific Ocean .
The rising air is associated with a region of low air pressure, towering
cumulonimbus clouds and rain. High pressure and dry conditions accompany the
sinking air.
When the Walker
Q.
How does the ENSO affect
monsoon?
A.
In a typical ENSO, the strong easterly winds of the
equatorial Pacific weaken, which allows warm eastward-flowing subsurface waters
to rise, increasing surface temperatures 1-2°C
and sometimes in the central and
Eastern Pacific. Along the West coast of South America ,
El Niño's warm waters persist and deepen, and cold, upwelling, nutrient-rich
waters fail to reach surface waters; the resulting warm, nutrient-poor waters
devastate coastal fisheries. Heavy rain falls along the South American coast,
and heavy rainfall also moves from the western to central Pacific, causing
drier than normal conditions in Indonesia
and nearby areas including India
Q. What is Indian Ocean
Dipole (IOD)
A. The Indian
Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the Indian Ocean . It is normally characterized by anomalous
cooling of SST in the south eastern equatorial Indian Ocean and anomalous
warming of SST in the western equatorial Indian Ocean .
Associated with these changes the normal convection situated over the eastern
Indian Ocean warm pool shifts to the west and brings heavy rainfall over the
east Africa and severe droughts/forest fires
over the Indonesian region.
Q. What is Madden Julian Oscillation (MJO)? How
it influences monsoon activity?
A. The Madden
Julian Oscillation (MJO) is one of the most important atmosphere-ocean coupled
phenomena in the tropics, which has profound influence on Indian Summer
Monsoon. The MJO is the leading mode of tropical intraseasonal climate
variability and is characterized by organization on a global spatial scale with
a period typically ranging from 30-60 days, which was discovered by Madden and
Julian in 1971 in a published paper. It has the following characteristics :-
·
MJO is a massive weather event consisting of deep
convection coupled with atmospheric circulation, moving slowly eastward over
the Indian and Pacific
Oceans
·
MJO is an equatorial traveling pattern of anomalous
rainfall that is planetary in scale.
·
Each cycle lasts approximately 30–60 days. Also
known as the 30-60 day oscillation, 30-60 day wave, or intraseasonal
oscillation (ISO).
·
The MJO involves variations in wind, sea surface
temperature (SST), cloudiness, and rainfall.
·
Based on the place of convective activity the
period of MJO is divided into 1-8
phases with each phase roughly last for 7 to 8 days.
Since the MJO is the most important mode
of tropical intraseasonal variability with potentially important influences on
monsoon activity in the Asian regions on extended range time scale (beyond 7
days to on1 month), the capability of statistical or numerical models in
capturing MJO signal is very crucial in capturing the active/break cycle of
monsoon.
Q.
How are low pressure system classified in India
A. The low-pressure systems over Indian
region are classified based on the maximum sustained winds speed associated
with the system and the pressure deficit/ number of closed isobars associated
with the system. The pressure criteria is used when the system is over land and
wind criteria is used, when the system is over the sea. The system is called as
low if there is one closed isobar in the interval of 2 hPa. It is called
depression, if there are two closed isobars, a deep depression, if there are
three closed isobars and cyclone if there are four or more closed isobars. The
detailed classifications based on wind criteria are given in the Table below.
Considering wind criteria, the system with wind speed of 17-27 knot is called
as depression and the low pressure system with maximum
sustained 3 minute surface winds between 28-33 knot is called
a deep depression. The system with maximum sustained 3 minute surface winds of
34 knot or more is called as cyclone.
|
System
|
Pressure deficient
hPa
|
Associated wind speed
Knot (Kmph)
|
|
Low pressure area
|
1.0
|
<17 o:p="">
|
Depression
1.0- 3.0
17-27 (32–50)
Deep Depression (DD)
3.0 - 4.5
28-33 (51–59)
Cyclonic Storm (CS)
4.5- 8.5
34-47 (60-90)
Severe Cyclonic Storm (SCS)
8.5-15.5
48-63 (90-119)
Very Severe Cyclonic Storm (VSCS)
15.5-65.6
64-119 (119-220)
Super Cyclonic Storm
>65.6
>119 (>220)
Q. What is Satellite Meteorology?
A. Satellite Meteorology refers to the study of earth's
atmosphere and oceans using data obtained from remote sensing devices flown
onboard satellites orbiting the earth. Satellite make measurements indirectly
by sensing electromagnetic radiations coming from the surfaces below.
Q. Which satellites are being used to monitor the weather of
Indian region ?
A. Kalpana-1 located at Longitude 74° E and Insat-3A
located at 93.5° E both geostationary satellites are being used to monitor the weather
of Indian region.
For meteorological observation, INSAT-3A carries a
three channel Very High Resolution Radiometer (VHRR) with 2 km resolution in
the visible band and 8 km resolution in thermal infrared and water vapour
bands. In addition, INSAT-3A carries a Charge Coupled Device (CCD) camera which
operates in the visible, near infra Red and short wave infrared bands providing
a spatial resolution of 1 km. A Data Relay Transponder (DRT) operating in UHF
band is incorporated for real-time hydro meteorological data collection from
unattended platforms located on land and river basins. The data is then relayed
in extended C-band to a central location. Kalpana -1 Satellite has a 3- Channel
VHRR and DRT similar to INSAT -3A Satellite.
Q. Which products are being
derived from operational Indian Geostationary Meteorological Satellites?
A. The
following products are being derived from INSAT satellites
·
Outgoing Longwave
radiation (OLR)
·
Sea surface
temperature (SST)
·
Quantitative
precipitation estimate (QPE)
·
Cloud Motion
vectors (CMV)
·
Water Vapour Wind
(WVW)
·
Cloud Top
Temperature (CTT)
·
Visible Channel Image
·
Infrared Channel Image
·
Colour Composite Channel Image
·
Water vapour Channel Image
Outgoing
long wave radiation (OLR)
Majority of meteorological sensors operate in long
wave range of radiation so that:
1. They operate day and night
2. The problem of low albedo is not encountered
3. The earth radiation is maximum and thermal IR and
ocean emissive is nearly unity.
Keeping the above
into mind the outgoing flux of long wave radiation at the top of atmosphere is
an important parameter in the earth atmosphere radiation budget. This parameter
can be derived by physical/statistical algorithm from the narrow band.
Sea
Surface Temperature (SST):
As we know oceans are the major storage of heat in the
earth climate systems. Sea Surface Temperature (SST) is one of the key
controllers of climate variability and acts as a vast thermal reservoir. SST
regulates the transfer of long wave radiation to the atmosphere as well as the
latent and sensible heat fluxes into the lower atmosphere.
Quantitative
Precipitation Estimate (QPE)
It is one of
the key meteorological parameter. A detailed knowledge of its distribution in
space and time is essential for understanding weather & climate.
Information about rainfall is of great value in variety of discipline beside
being control to human survival scientifically The latent heat released during
the process of condensation water into cloud and rain drops is one of the
significant energy source responsible for atmospheric heat engine.
Q. What are Image Channels?
A. The satellites typically scan the earth using
different wave lengths (channels). Current INSAT geostationary meteorological
satellites have 3 channel imager with the following channels:
i)
VISIBLE
wavelengths (0.55 - 0.75 um) (reflected solar radiation).
ii) IR (thermal
infra-red) (10.5 - 12.5 um). (emission
channel. Each point on the earth emits radiation in proportion to its
hotness/coldness. So this channel gives a thermal image of the earth)
iii) WV (water-vapour) (5.7 - 7.1 um). (This is also an
emission band. The image shows differences in water vapour absorption in the
atmosphere).
Q. What is the difference between geostationary and polar orbiting
satellites?
A. A geostationary satellite is positioned above the
Equator and orbits the Earth at the same rotation speed as the Earth itself,
making it appear stationary from the point of view of an observer on the
Earth’s surface. It flies very high above the surface of the Earth (altitude
almost 36000 kilometers), and thus is able to capture the whole Earth disc at
once. A polar orbiting satellite circles the Earth at a near-polar inclination,
meaning that it always passes almost exactly above the poles. The satellite
passes the equator and each latitude at the same local solar time each day,
meaning the satellite passes overhead at essentially the same solar time
throughout all seasons of the year. The low Earth orbit (800 - 850 kilometers)
is much closer to Earth than a geostationary orbit, and thus can see a smaller
part of the Earth below than a geostationary satellite, but in finer detail.
Q. What is GPS?
A. The Global Positioning
System (GPS) is a satellite-based navigation system made up of a network of 24
satellites placed into orbit in six orbital planes at an altitude of 20200 Km
above the earth surface with an orbital period of 12 hrs by the U.S. Department
of Defense. GPS works in any weather conditions, anywhere in the world, 24
hours a day.
Q. How does GPS
receivers determine its 3D position from GPS signal?
A. The GPS receiver compares the time a signal was transmitted
by a GPS satellite with the time it was received. The time difference tells the
GPS receiver how far away the satellite is. Now, with distance measurements
from a few more satellites, the receiver can determine the user's position by
triangulation. Precise location of interest to geophysists required correction
of position errors due to atmospheric delays.
Q. What is a radar?
A. Radar is acronym for Radio Detection
and Ranging. It uses electro-magnetic
waves in microwave region to detect location (range & direction), height (altitude),
intensity (in case of weather systems) and movement of moving and non-moving
targets.
Q. What is the working
principle of radars?
A. Radars are used for detection of aircrafts, ships,
weather systems and a variety of other applications. Our discussion is restricted to weather
radars only. Radar transmitter transmits
electro-magnetic waves through a directional antenna in any given direction in
a focused manner. A part of the
transmitted energy is absorbed by the atmosphere. Some of the energy travels further through
the atmosphere and a fraction of it is scattered backward by the targets and is
received by the radar receiver. The
amount of received power depends upon radar parameters like transmitted power,
radar wavelength, horizontal and vertical beam widths, atmospheric
characteristics etc., In case of weather echoes like clouds it depends on
physical state (raindrops, snow, hail etc.) and drop size distribution hydro
meteors. The amount of return power
provides information about the intensity of weather systems and azimuth and
elevation of the antenna gives the location and height of the cloud
systems. The time taken in to and fro
journey of the electromagnet waves gives the range (or distance from radar) of
the targets. Modern day radars, viz.,
Doppler Weather Radars, employ Doppler principle to provide information about
the speed and direction of the moving targets.
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