There are two major methodologies used to observe the state of the atmosphere:
in situ and remote sensing. In situ instruments measure what is occuring in their
immediate proximity, e.g. a thermometer or a wind vane. Remote sensing uses detectors
to observe events distant from the instrument: e.g. weather radars, satellite imagers
and radiometers on different platforms. Some data are availablr wihjty one minute time
resolutions (e.g.weather radar) while some is only available at 12 hour intervals
e.g. ballon borne upper air measurements or RAOBS). Analyses of these data are presented typically
either as vertical profiles of significqant variables (generaly referred to as soundings), or as
interpolated horizontal fields of primary variables such as pressure and temperature or
as derived variables such as vorticity, thickness or vertical velocity.
National Weather Service (NWS) produces
surface weather maps every three hours. At each observing station
on the map, detailed weather information is plotted in a pre-defined configuration called a station model.
These data are sea level pressure, pressure tendancy, air temperature, dew point temperature, wind
speed and direction (at 10 m above ground), visibility, current weather, cloud
amount, cloud height and the amount of precipitation in the last 3 hours.
In Situ upper air weather data are obtained from "RAWIN" sonde systems borne aloft by
weather balloon every 12 hours at 00Z and 12Z. These data are presented as plotted
soundings on Skew-T log-p diagrams for individual statiosn and as maps of
temperature, wind, and height at standard pressure levels. The station
models on the upper air maps is similar to those at the surface, but containing only
temperature, dew point temperautre, height of the pressure surface and wind data.
.
In addition to the NWS three hourly surface observations,
there are hourly observations of surface conditions taken at selected airports.
Historically these were called sequence reports and given the identification
tag "SA". Several years ago these were reformated to conform to a worldwide standard
and are now called METAR reports or simply METARS. These are primarily used
to support aviation. If conditions are changing rapidly, special METARS will be
issued more frequently than once every hour. The meteorological information contained
in a METAR report is less detailed than that in an NWS observation, but
there are many more stations issuing METAR reports than there are NWS sites. Note: only
the NWS stations are poltted on the surface maps.
Wind: speed, direction
Wind speed is reported in knots, or nautical
miles per hour in the surface observations. Wind direction is reported in degrees clockwise
from north. North wind is zero degree, east wind is 90 degree,
etc.
Examples for wind plots in a station model:
In news reports, the wind speed is given
in miles per hours and the wind direction is given in north (N),
east (E) , south (S) or west (W). In between these four directions,
we can use northeast (NE), southeast (SE), northwest (NW) or southwest
(SW). The wind direction can be further refined as north-northwest
(NNW) etc.
In scientific computations, we use
meter per second (m/s) for the wind speed. Usually, the wind
is decomposed into the west to east component (u) and the south to north
component (v) in the Cartesian Coordinates for computational purposes. The equations to compute
u and v from wind speed (V) and direction (
)
are:
Sometimes it is more convenient to use
Natural Coordinates. Natural Coordinates are local coordinates formed by setting one axis
to be parallel to the direction of the wind and the second prependicular and to the left
of the wind direction at a point. These directions are
usually denoted as "s" the streamwise or along-wind direction and "n" the normal
to the wind direction, to our
left if the wind is at our backs. The wind speed in the Natural Coordinates
corresponds to the observed wind speed and it is usually denoted
by V. Natural Coordinates are used very often to describe local derivative quantities
such as vorticity and divergence in weather discussions.
Unit conversions
1 nautical mile = 1852 meter = 1 minute of the latitude.
1 mile = 5280 feet ~ 1609 meter.
1 degree of the latitude = 60 x 1852 = 111120 meter.
1 kts = 1852/3600 = 0.5144 m/s.
1 mph = 1609/3600 = 0.4469 m/s.
Temperature
The temperature on the surface map is plotted
in degree Fahrenheit (°F) but the surface temperature in
the METAR report is in degree Celsius (°C). The relations
between °F and °C are
A few reference numbers:
The absolute temperature, Kelvin (K) is
related to °C as
In news reports, we always use Fahrenheit.
In our weather discussions, we will use both Fahrenheit and Celsius.
Kelvin is used whenever we use the gas law, evaluate buoyancy or
perfom any radiative or thermodynamic computation. It is also the
typical unit used for potential temperature.
Pressure
Pressure is reported in mb (millibars), although in SI units,
and therefore in all calculations in the SI system, it should be in Pascals.
1 mb = 100 Pa (Pascal)
In our weather discussions, mb is always
used. In news reports, the atmospheric or barometric pressure
is often reportd as inches of mercury.
1 inches of mercury = 33.864 mb.
1013.25 mb = 29.921 inches of mercury.
In scientific computations, Pa has to be
used.
Pressure decreases with height logarithmically.
The pressure over a mountain top is always lower than that over
a valley. In order to compare the pressure at two stations, the
observed station pressures, i.e. those actually measured at a station,
are adjusted to an estimate of what the pressure would be if the station
were at sea level. The reported pressure is this pressure adjusted to
Mean Sea Level (MSL). The method for this adjustment will
be discussed later.
Humidity
In weather discussion or news reports, relative
humidity or dew point are used to represent the amount of humidity
in the air. On the surface map, the humidity is represented as
dew point in °F. In the METAR report, the humidity is represented
as dew point in °C.
Other observed surface parameters
There is a host of other variables reported
in the surface observation. They include pressure change, current
and past weather, cloud type and amount, precipitation type and
amount, and visibility.
Upper air observations are made primary
by radiosonde, a radio transmitter attached to a weather balloon.
It reports pressure, temperature and relative
humidity while the balloon ascends. The wind direction and speed
are computed from the change in location of the drifting balloon wiht time and altitude using either
a directional antenna or a satellite tracking system.
The upper air data are reported on pressure
levels (p as a coordinate) instead of height levels (z as a coordinate).
the T and Td information are analysed and tabulated for the standard
pressure levels used to prepare the pressure level maps: e.g. 1000,
850, 700, 500, 300, 250 and 200 mb. In Addition, significant levels
are tabulated which identify the altitudes
at which the lapse rates of temperature or dewpoint temperature change significantly.
With this system, one need only plot the standard and significant level data on a chart
such as a Skew-T lop-P diagram, and connect the points with straight lines to reproduce the
sounding.
On the constant pressure surface maps, the temperature is
plotted in °C and the humidity is plotted as dew point depression in °C.
Dew point depression is the dew point temperature subtracted from the sensibel temperature.
A small dew point depression denotyes high relative humidity and conversely.
Instread of pressure, the height of the pressure surface is plotted and height
contours are analysed showing the topography of the pressure surfaces.
The North America standard atmosphere is
an idea atmosphere close to the mean condition of the North America
atmosphere. The MSL pressure of the North America standard atmosphere
is 1013.25 mb. The surface temperature is 15 °C, and the
lapse rate of the temperature (the rate of decrease of temperature
with height) is 6.5 °C per km up to 11 km, the tropopause
level of the standard atmosphere.
Several types of meteorological satellites orbit the earth. They are distinguished by their orbits, Polar or geostationary, and by the types of instrumentation they carry. There are several geostationary satellites spread about over the equator about 25,000 mile above the surface. Since they move in their orbits with the same angular velocity as thh earth, their imaging systems always look down at the same points. The polar orbiters are lower and pass over the same points on earth twice per day. While they are called polar, the orbits actually are slanted relative to the lines of constant longitude and they do not pass directly over the poles. Being closer to the earth the polar orbiters have much higher spatial resolutions then the geostationary satellites. Descriptions of these satelites and their measurement systems can be found at www.wrh.noaa.gov and at http://orbit-net.nesdis.noaa.gov.
Weather radar systems have been updated over the past decade and the WSR-88D is now the
standard operating unit. Also known as NexRad, these modern systems are very high resolution
radars with rapid scan rates. They also can operate in several modes: reflectivity mode whereby they
measure the total mass of water in precipitation sized hydrometeors in a sample volume;
and doppler mode in which the speed of targets parallel to the line-of-sught can be measured in each
range bin. The reflectivity data can be used to estimate rainfall rates at the ground for the
entire area of radar coverage. This is particulary important in identifying areas of probable
flooding -- especially issuance of flash flood warnings as the rain is falling.
The doppler measurements are particulary important for identifying areas of vertical shear
in the horizontal wind, a hazard to large aircraft taking off or landing. They are alos very important
for identifying the
development of rotational motion in large cumulonimbus clouds -- the precursor of tornado development--
whereby timely warnings can be givien to areas specifically in danger from these storms. Describtions
of teh WSR-88D can be found on the web also at www.whr.noaa.gov and radar data at
www.RAP.ucar.edu/weather.