|
What is La Nina/El Nino?
They are large-scale patterns
that result from changes in ocean temperatures in the equatorial
Pacific Ocean region.
In the tropical Pacific, trade
winds generally drive the surface waters westward. The surface
water becomes progressively warmer going westward because
of its longer exposure to solar heating. El Niño is
observed when the easterly trade winds weaken, allowing warmer
waters of the western Pacific to migrate eastward and eventually
reach the South American Coast of Peru. The cool nutrient-rich
sea water normally found along the coast of Peru is replaced
by warmer water depleted of nutrients, resulting in a dramatic
reduction in marine fish and plant life.
El Nino is caused when the
temperatures are warmer than normal. La Nina is when they're
cooler than normal.
Scientists are still trying
to find out what causes these phenomenon and their apparent
effects on the climate around the world.
However, during El Nino events,
the winters around the Pacific Northwest tend to be warmer
and drier than normal, as the jet stream tends to point further
south into central and southern California.
"Neutral years" --
winters where there's neither of those two events going on
-- tend to be more variable, as the jet stream tends to move
around more often. So we could have a stretch where it's warm
and dry, followed by a stretch of cold and snowy. In essence,
all bets are off, and we're open to anything.
Neither of the events appear
to have much of an effect on summer weather, but there's still
plenty of research going on in the field.
What's the difference between "Rain" & "Showers"?
This is kind of tricky. Even
though "showers" are indeed rain, there's a subtle
but important distinction between the two as far as weather
forecasts go.
When we call for "Rain"
(as in, "Rain at Times", "a rainy day",
"Occasional rain") is a more widespread event. Most,
if not all, of the area will see rain and it'll last for a
while.
What do the different precipitation
terms mean?
We've already covered the
difference between rain and showers, but what about the other
rain terms most often used around here?
Being known for our gray,
damp days, we have plenty of words to try and help give better
details on what it might do on a particular day.
- Drizzle is a term used for a very light,
fine rain. The drops tend to be very small and not very
dense.
- Mist is similar to drizzle, only the number
of drops in any given area is greater. This is usually the
case on heavy fog days.
- Sprinkles would be something akin to drizzle
-- just a few drops here and there.
As for showers, "isolated"
and "spot" showers mean just a couple of showers
spread over a wide area, with shower coverage less than 25
percent of the forecast area.
"Scattered" showers
mean more shower coverage -- maybe 25-50 percent coverage
of the area. "Fairly widespread" and "widespread"
showers mean a larger area of coverage, i.e. 50-75% and more
than 75% area respectively. Since showers are usually small
in size by definition, we might use our every popular "showers
and sunbreaks" forecast, which would mean that in between
showers, the sun might be out.
Hail forms when rain droplets
get pushed higher into the atmosphere by strong upward winds.
As the droplets go higher, it freezes into an ice stone known
as hail. It then becomes heavier, and falls downward, picking
up another coating of raindrops on its way down.
If the updrafts are strong
enough, it'll blow the hail back up again, where the coating
freezes, making the hail larger, then falling back downward.
This process repeats until the hail is heavier than the updraft
can support, when it'll finally fall to the ground. The stronger
the updraft, the larger the hail stone will become.
Winter Precipitation
Freezing rain is caused when
you have a warm mass of air in the middle altitudes between
the ground and the cloud deck, followed by a mass of freezing
air near the surface.
When the precipitation falls
from the cloud, it will generally be snow. As it encounters
the warm air, it will melt into the usual rain. But right
before it reaches the ground, it enters the below-freezing
air and quickly turns to ice. On impact, it usually freezes
to whatever it lands on. That can turn streets into skating
rinks in no time.
Sleet is snow that begins to melt as it reaches the ground,
but doesn't melt completely. This usually happens when it's
around 33-35 degrees at the surface.
Flurries are the drizzle equivalent
of snow -- very light snow that isn't strong enough to accumulate.
Why is there a rainbow around the Sun/Moon?
You've all seen rainbows on
those days where it's raining and the sun's out at the same
time. But what about those times when you see a rainbow-like
halo around the sun or moon?
It's the same physics, really.
The halos (or, sometimes known as "sundogs" around
the sun) are usually seen when there are high clouds overhead.
Those clouds are made of tiny ice crystals, which will refract
the sunlight much like a prism will. And voila! You have a
rainbow halo around the sun. It works the same way with moonlight.
It's usually a sign that rain
is on the way, as high clouds usually precede a storm front.
Sometimes the rainbow isn't in the form of a halo, but just
colors a streak of clouds -- the ice crystals in that cloud
were at just the correct angle from the sun to produce the
prism effect.
What is dew point, and
how does it relate to fog?
There are two main types of
fog- Radiation type and Advection type, both of them are related
to dew point temperature.
The dew point is the temperature
at which the air becomes 100% saturated. At that point, the
air condenses into water droplets, which we see as fog.
The lower the dew point, the
drier the air, and vice versa.
If you have a temperature
of 35 degrees and a dew point of 32, that feels quite humid.
But if it's 35 degrees and a dew point of 28 degrees, that's
much drier and more comfortable.
Fog happens when the temperature
and dew point are equal or within a degree and the air becomes
saturated.
The fog comes in two varieties:
Radiation
Fog or Ground fog usually
begins on clear, calm nights. But, first, during the day,
you need the sun to warm the ground.
Then, as night falls and the
temperature drops, the air cools to where it meets the dew
point and becomes totally saturated to where fog forms.
This fog is more common in
valleys because since cold air sinks, it tends to pool in
low-lying areas and makes it easier to reach saturation point.
A breeze will hinder this
type of fog development though, as it will mix in the drier
air from higher altitudes. Cloud cover also goes against ground
fog development, as the clouds act like a blanket that keeps
heat from radiating away and keeps it warmer near the ground.
Advection
fog is created when a warm
air mass moves over a colder area.
The fog burns off as the sun
begins to warm the top of the fog layer, pulling the temperature
higher than the dew point and evaporating the fog
What's the difference between a "Watch"
and a "Warning"?
You'll see us mention a lot
of various "watches" and "warnings" during
the fall and winter months.
A "watch" means that
conditions are right for the event to happen. So, for example,
if we say there's a "High Wind Watch" for an area,
that means that high winds are possible, but not imminent.
It's an early heads up that something merits close attention.
A "warning" means
the condition is imminent or already happening. Thus, a "High
Wind Warning" means that high winds are happening now,
or are going to happen in the warning area.
Why might weather forecasts vary
so much between different sources?
The best way to think about
it is if you're standing in an art gallery, and there are
5 people looking at a painting of abstract art, and if you
were to ask each one what they though the painting was representative
of, you might get five different answers.
There are a host of different
forecast models that are put out by various institutions.
The National Weather Service puts out a few different computer
forecast models -- Some are more designed for short-term forecasts,
some are designed for more long-term forecasts.
To top that off, the University
of Washington runs their own model now, and we have one that's
exclusive to us (That "Futurecast" product we air)
-- they both run on a much higher resolution and provide short
term forecasts for the Pacific Northwest.
All of the models are available
to everyone in the world (except the Futurecast, which is
just for KOMO.) Each model takes a slightly different tact
in trying to compute the atmosphere and say what's going to
happen next.
Now, most of the time, the
models are in general agreement, with just a few different
tweaks here and there.
Think of it as asking four
people how to get from West Seattle to a restaurant in Auburn.
They might have minor differences in what streets they take
to get there, but they'll usually at least get you to the
restaurant, and then it's up to you to rely on experience
and expertise as to which is the shortest route, based on
traffic, etc.
With the models, you learn
over time which ones perform better in different situations.
There are some days, and some
weather patterns when it can become a meteorological. That's
because none of the models are on the same page -- it's utter
chaos.
It'd be like you want to go
to Auburn, but one person gives you directions to Tacoma,
the other to Arlington, the third to Bremerton, etc.
In that case, now you have
my first example. Everyone in the weather community looking
at a piece of abstract art and trying to figure out a forecast
amid the model chaos. Some may be taking model A, some may
be going for B, etc. Some may be going on just looking at
a satellite photo and throwing the models out of the equation
completely.
And to mix things up, sometimes
the models can vary wildly from model run to model run.
You may notice differences
especially during snow forecasts. Snow is the most difficult
weather to forecast around here, since the area waters tend
to fight the cooling needed to snow.
And when it's mostly cold
enough to snow, people demand much more accuracy in where
and when it will precipitate. During rain events, the difference
between .05" of rain and .15" of rain is no big
deal. But with snow, that could be the difference between
a dusting and 4", and everyone wants to know if it will
happen at their house, and how much, and if not, why not,
and why did you miss it, and how come Everett got 6"
and we got rain here in Federal Way?
Why is it so hard to forecast
the weather around here?
On the very rare occasion
a forecast doesn't go according to plan, we'll receive an
isolated e-mail or two questioning just how come we got the
forecast wrong, and that their uncle in remote village can
do a better job with his trick knee(thumb rule) forecasting
the weather here than we can.
One of the main problems in
the tropical weather is that it never does the same thing
at the same time for all the weather systems in our area surrounded
by seas on three sides where no regular weather observations
are available.
India is largely on the Indian
subcontinent situated on the Indian
Plate, the northerly portion of the Indo-Australian
Plate, in southern
Asia. India's northern and northeastern states are partially
situated in the Himalayan
Mountain Range. The rest of northern, central and eastern
India consists of the fertile Indo-Gangetic
plain. In the west, bordering southeast Pakistan,
lies the Thar
Desert. The southern Indian Peninsula is almost entirely
composed of the Deccan
plateau, which is flanked by two hilly coastal ranges,
the Western
Ghats and Eastern
Ghats.
India is home to several major
rivers, including the Ganga,
Brahmaputra,
Yamuna,
Godavari,
Kaveri,
and Krishna.
India has three archipelagos - Lakshadweep
off the southwest coast, the Andaman
and Nicobar Islands volcanic island chain to the southeast,
and the Sunderbans
in the Gangetic delta in West Bengal.
Climate in India varies from
tropical
in the south to more temperate
in the north, with elevated regions in the north receiving
sustained snowfall. India's climate is strongly influenced
by the Himalayas and the Thar Desert. The Himalayas, along
with the Hindu
Kush mountains in Pakistan, provide a barrier to the cold
winds from Central Asia. This keeps most of the Indian subcontinent
warmer than most locations in similar latitudes. The Thar
Desert is responsible for attracting the moisture laden southwest
monsoon winds in that provide most of India's rainfall
between June to September.
Other problem is that we don't
have lot of reporting stations out in the Indian Ocean where
our weather comes from especially during south west monsoon
season, so the computer models fill the missing data points
from its own repository of short range forecasts. If we have
a small error in that prescription of data, the error becomes
magnified over time.
What that means is that our
forecast ends up being pretty general because there is no
way we can cover every individual variation that can occur
around here.
Now having said that, we also
know that the only weather that matters to you is the weather
that is falling on your head! So, we ask that you bear with
us and understand that there are times where we're going to
be off for some people, but on the other hand, somewhere we
shall be performing just right!
What's the origin of ''It's
Raining Cats and Dogs''?
Our search of the Internet
found plenty of theories, but no definitive answer. Some seemed
to center on ancient Greek. One place said the word cat is
derived from the Greek word 'catadupe', meaning "waterfall".
That's great, but where's the dog?
Another site had a 17th century
British origin, where one writer used the phrase when heavy
rains would wash dead animals down the streets.
But on a brighter (and cleaner)
note: the most popular explanation seems to stem from Northern
European Myths that symbolized cats with rain and dogs with
wind. Thus, a heavy rain storm with strong winds were: "it's
cats and dogs out there."
What's the origin of "April
Fool's Day"?
There are dozens of theories
for how the annual tradition of "April Fool's Day"
came about, and a few even have to do with weather.
One of them has to do with
the weather "fooling" farmers. Apparently, some
inexperienced farmers would be tricked by Mother Nature during
a warm spell in early to mid-March and go ahead and plant
their crops, only to have them destroyed when another frost
came along. They were known as "April Fools" for
not waiting until April to plant their crops and be safe from
the frost. (Although here in Seattle, the last frost is typically
around March 24, so you can get a week's head start.)
Another theory is that many
people would be "fooled" by the constantly changing
weather around the first day of spring. (And thus, weather-casting
was born?)
Why aren't the sunrise and sunset equal on the equinox?
The first days of spring and
fall are a time when there is equal 12-hour daylight across
the planet. However, looking at a Seattle sunset chart shows
Seattle has 12 hours and 11 minutes of daylight on Sept. 21.
Why?
Sunrise and sunset are calculated
when the top of the sun is on the horizon. However, equal
daylight is measured from the center of the sun -- accounting
for a few minutes. The other discrepancy is from when the
sun is on the horizon, the Earth's atmosphere bends the sun's
light.
As a result, the sun still
looks to be above the horizon for a few minutes after it has
already set. The U.S. Naval Observatory takes this into consideration
when calculating sunrise and sunset times.
What causes lightning?
It's still not quite fully
understood and is incredibly complex, but one main theory
goes that as water droplets move about inside some clouds,
they can build up an electric charge -- much like how if you
wear socks on a carpet, you build up a charge. The negatively
charged particles tend to conglomerate at the bottom of a
cloud, while the positively charged ones are at the top.
As the storm intensifies and
more negative charges build at the base, it can actually push
the negatively charged particles on the Earth's surface further
into the ground (since similar charges repel each other) leaving
mainly positive charges along the surface. Then, a conductive
path will be generated between the negative cloud and positive
Earth, allowing a huge electrical current to flow between
the two charges. That's what we see as lightning.
What are hectopascals?
The SI unit for pressure is
a pascal. The worldwide meteorological community uses the
hectopascal, i.e. a hundred pascals, which is the metric equivalent
of a millibar. However, millibars (and inches) are still used
in some public forecasts in the UK and USA.
What is UTC?
UTC stands for Universal Time
Coordinated and it is equivalent to GMT.
What is a Geostationary Satellite?
Geostationary satellites are
positioned at an exact height above the earth (about 36000
Km). At this height they rotate around the earth at the same
speed as the earth rotates around its axis, so in effect remaining
stationary above a point on the earth (normally directly overhead
the equator).
As they remain stationary they
are ideal for use as communications satellites and also for
remote imaging as they can repeatedly scan the same points
on the earth beneath them.
Polar Orbiting satellites by
comparison have a much lower orbit, moving around the earth
fairly rapidly, and scanning different areas of the earth
at relatively infrequent periods.
How many Meteorological Geostationary Satellites are there?
Currently there
are 5 or 6 satellites positioned at regular intervals around
the equator so that the whole earth is covered.
The main satellites
are:
Meteosat
The European community satellite
operated by EUMETSAT (formally part of the European Space
Agency) in Germany is positioned above Europe/Africa (approx
0 degrees Longitude). This is the satellite that we receive
data from in Dundee. Meteosat also rebroadcasts data from
other geostationary satellites (although at less regular intervals
than its own data). EUMETSAT are alone in deciding to encrypt
the majority of their transmissions. Not very friendly behaviour
we think. Although we can receive and decrypt such data our
decryption license agreement restricts us to providing only
their unencrypted data on the web. These are transmitted every
6 hours.
Meteosat transmits
HRI data at 1694.5 MHz, at a data rate of 166.66 kbits/s.
Further information on Meteosat can be found at EUMETSAT's
web site
GOES-EAST
Operated by the US NOAA agency, positioned over USA/S.America
(75 deg. West. Further information on GOES-E can be found
at NOAA's web site.
GOES-WEST
Positioned over the Pacific (135 deg. West). Further information
on GOES-W can be found at NOAA's web site
GMS
operated by the Japanese Space Agency NASDA positioned over
Japan/Australia (140 deg. East). See also JAXA and MSC
IODC
Meteosat-5, now re-positioned at (63 deg. East) for Indian
Ocean Data Coverage
Indian National
Satellite System (INSAT) is a series of multipurpose Geo-Stationary
satellites launched by ISRO to satisfy the telecommunications,
broadcasting, meteorology, search and rescue needs of India.
Commissioned in 1983, INSAT is the largest domestic communication
system in the Asia-Pacific Region. Active satellites of this
series include INSAT-2E, INSAT-3A, INSAT-3B, INSAT-3C, INSAT-3E,
KALPANA-1 (METSAT), GSAT-2, EDUSAT (GSAT-3) and INSAT-4A.
INSAT satellites provide transponders (about 150) in various
bands (C, S, Extended C and Ku) to serve the television and
communication needs of India. Some of the satellites also
have the Very High Resolution Radiometer (VHRR), CCD cameras
for metrological imaging. The satellites also incorporate
transponder(s) for receiving distress alert signals for search
and rescue missions in the South Asian and Indian Ocean Region,
as ISRO is a member of the Cospas-Sarsat programme.
INSAT
milestones
- INSAT 1 Series
o 1A April 10, 1982
o 1B August 30, 1983
o 1C July 21, 1988
o 1D June 12, 1990
- INSAT 2 Series
o 2A July 10, 1992
o 2B July 23, 1993
o 2C December 7, 1995
o 2D June 4, 1997
- 2DT ARABSAT-1C is
acquired and renamed as INSAT-2DT
o 2E April 3, 1999
- INSAT 3 Series
o 3A April 10, 2003
o 3B March 22, 2000
o 3C January 24, 2002
o 3E September 28, 2003
- INSAT 4 Series
o 4A December 22, 2005 (Image & details of the satellite)
o 4B (To be launched by Ariane Space)
o 4C
o 4D (Spare)
o 4E
o 4F
o 4G
o GOMS: positioned at (76 deg. East)
What are Image Channels?
The satellites typically scan
the earth using different wave lengths (channels). Most geostationary
meteorological satellites scan using:
- VISIBLE wavelengths (0.5 - 0.9 um) (similar to that use
by the human eye).
- IR (thermal infra-red) (10.5 - 12.5 um). These wavelengths
show differences in temperature.
- WV (water-vapour) (5.7 - 7.1 um). These wavelengths show
differences in water vapour absorption in the atmosphere.
Can I extract useful scientific (quantitive) information from
these images?
Not in their present form.
The images have been heavily compressed using an algorithm
that degrades the pixel values.
If you require data for quantitive analysis (ie temperature,
albedo, wind vectors, etc), we will shortly be able to supply
the original raw data in a number of different forms.
|
