Imager
Most instruments that orbit the Earth can produce pictures, but an imager
is an instrument designed specifically to do that. Typically it has more
spatial resolution than other instruments (that is, it can see smaller
features on the surface), and it also collects energy in only a small
part of the electromagnetic spectrum. Many imagers have channels in
red, green, and blue light, which can be used to reconstruct true color
imagery of Earth - as a person would see it. Many also have some other
channels, which can be used to produce false color images that bring out
specific features of interest. For more information, see
an introduction
to remote sensing.
Latitude and Longitude
Latitude identifies the north to south location of a point on the Earth.
Latitude can be defined precisely as the angle between the vertical at a
location, and the equatorial plane of the Earth.
There are several equivalent ways to express latitude. The most common has
latitude 0° at the Equator, 90° N at the north pole, and 90° S
at the south pole. In several places in North America, signs mark the location
of the 45th parallel or 45° N: halfway between the equator and the pole.
With the advent of computers, another common way to express this is from
+90 (at the north pole) to -90 (at the south pole). In programming, this
allows the latitude to be simply expressed as a real number; and does not
require a number and a character (N or S). The third way to express this is
the colatitude (less common), defined as colatitude = 90 - latitude. Thus,
colatitude goes from 0 at the north pole, to 90° at the equator, to
180° at the south pole.
Longitude identifies the east to west location of a point on the Earth, by
measuring the angular distance from the Greenwich meridian (or Prime meridian,
where longitude is 0), along the equator.
There are also three equivalent ways to express longitude. You may be most familiar
with longitude as 0-180 East, and 0-180 West. It can also be expressed
as 0-360 East - or just 0-360. In that case, 270 East is equivalent
to 90 West. The third system arose in the computer era, when carrying
both a number (0-180) and a character (East or West) was inconvenient.
The new convention of -180 to +180 was then developed. In this case,
-90 is equivalent to 90 West.
Both latitude and longitude can be expressed either as XX degrees, YY minutes,
ZZ seconds; or as a decimal. The conversion is Decimal = (XX + YY/60 + ZZ/3600).
The decimal form is used in the S'COOL database.
For locations in the US, you can look up your lat/long at the
Census Bureau.
Here is a link to a limited number of cities around the world.
Local Daylight Saving Time
The local standard time, plus an
hour. This change is made in some parts of the world during summer
months so as to cause the day's activites to begin earlier,
with the goal of providing more time for leisure and recreation.
Local Solar Time
The local time where you are using the sun as a reference. Noon would be
when the Sun is at its highest point. A sundial would help you measure this.
Local Standard Time
The time established for a region by
law, or general use - it's the time you see on a correctly set clock, if
you're not in daylight savings time (summer)
Optical Depth
The optical depth is a measure of the visual or optical thickness of a cloud;
that is, of the reduction of light or energy transmitted through the cloud due
to interactions with the cloud particles. Optical depth is much more significant
to Earth's energy budget than the physical thickness of cloud layers.
Unlike height, it is not measured in meters or feet; but is nondimensional.
An optical depth below 3 is considered to be thin. Anything above 10 is
definitely thick. The same term is also used to describe the optical depth
of aerosol (dust, smoke, etc) layers.
Orbits
Radiosonde
A balloon-borne instrument that measures meteorological parameters from the
Earth's surface up to 20 miles in the atmosphere. The radiosonde measures
temperature, pressure, and humidity, and transmits or "radios" these data
back to Earth. Upper air winds also are determined through tracking of the
balloon ascent.
Radiosonde observations generally are taken twice a day (0000 and 1200 UTC)
around the globe. NOAA's National Weather Service (NWS) operates a network
of about 90 radiosonde observing sites in the U.S. and its territories. When
the balloons burst, radiosondes return to Earth on a parachute. Approximately
25 percent are recovered and returned to NWS for reconditioning and reuse.
Remote Sensing
Obtaining information about a subject, as with a camera, without being in
contact with it. This term is now commonly used in conjunction with
electromagnetic techniques for acquiring information; that is, techniques
which image part of the electromagnetic spectrum (i.e., visible light,
infrared energy (heat), X-rays, ultraviolet light, etc...)
Passive Remote Sensing
This is the traditional way to do remote sensing. A sensor simply collects
whatever energy reaches it.
Active Remote Sensing
An instrument, for example a radar or lidar, sends out a beam of energy to probe
the thing it wants to sense; then measures the amount of energy that is reflected
(bounced) back to a detector. In space, active sensors have only been used
relatively recently as they involve additional technical challenges.
Satellite azimuth from North (sat azm fr. north)
The satellite azimuth angle from north. If: point A = the ground site
(where you are); point N = the North Pole; point S = the point on the Earth
directly under the satellite; then the satellite azimuth angle = angle NAS.
Here's another way of thinking of this: Say the angle = 50 degrees. If you
stand facing the North Pole and then turn clockwise by 50 degrees, then you
will be looking in the direction of the satellite. Click on image for a
larger version.
Solar Zenith
The angle between the local zenith and the line of sight to the sun.
If: point A = the ground site (where you are); point Z = any point directly
above point A (The zenith); point B = the sun; then the solar zenith angle
= the angle ZAB.
Viewing Zenith
The angle between the local zenith and the line of sight to the satellite.
If: point A = the ground site (where you are); point Z = any point directly
above point A (The zenith); point C = the satellite, then the viewing zenith
angle = the angle ZAC.
Universal Time or Greenwich Mean Time
Standard time at longitude = 0 (the longitude line which goes through
Greenwich, England) which is used as a standard around the world to
assist in synchronizing data collection. Look
here to figure out the time offset
from UT to your time zone.
