Chapter 5

Direct Methods- measure properties of the air that are in contact with the instrument being used. Ex: Thermometer
Indirect Methods- do not come into physical contact, obtain data from afar. Can be both active and passive.

Temperature- Mercury Thermometer, Resistance Thermometer
Humidity- Dew Point Hygrometer, Psychrometer (measures relative humidity)
Pressure- Mercury Barometer, Aneroid Barometer
Wind- has both speed and direction. Anemometers measure wind speed and Wind Vanes measure wind direction. Cup Anemometer, Propellers, and Windsocks measure wind speed.
Precipitation- Rain Gauge

Radiosondes are instrument packages carried aloft by a helium-filled “weather balloon” that measure vertical profiles of air temperature, relative humidity, and pressure from the ground up to 30 kilometers.
Rawinsondes measure wind speed and direction, along with temperature and humidity, by tracking the position of the balloon
Indirect Methods types are:

Active sensors- emit energy into the atmosphere and then measure the energy that returns.
Passive sensors- measure radiation emitted by the atmosphere, the Earth’s surface, or the Sun.

Reflection is when rays are turned back in the direction in which they came.
Refraction is when transmitted rays change direction due to spatial variations in properties of the medium.
Scattering is when light rays change direction when they encounter small particles, such as aerosols. --- Causes sunrises and sunsets to look red.
Cloud Ceiling is the height of the lowest widespread cloud base.
Ceilometer is used to measure cloud ceiling and is an active remote sensing instrument.
Weather Satellites fly around the Earth in two basic orbits:

GEO-Geostationary Earth Orbit—orbits the Earth as fast as the Earth spins and hovers over a single point above the Earth at an altitude of 36,000 kilometers. Must be located over the Equator
LEO-Low Earth Orbit—orbit the Earth at an altitude of 850 kilometers. Travels from pole to pole.

Satellite instruments measure electromagnetic energy that the Earth and the atmosphere reflect, scatter, transmit, and emit and the most common weather satellite images are the visible, infrared, and water vapor.
Radiometers are passive remote sensing instruments classified by two types:

Visible- measures the amount of visible light from the Sun that is reflected or scattered back to space by the Earth’s surface or by clouds. Differences in albedo of clouds, land, water, etc. allow distinguishing of different features.
Infrared (IR)- measures the amount of radiation, or heat, emitted by the surface or clouds. Cold objects appear white, while hot objects appear black. The advantage of this is that it gives information during both day and night.

Radar (Radio Detection and Ranging)- actively senses radiowaves scattered by large water and ice particles and converts them into information about precipitation.
Doppler Radar can measure the location and intensity of precipitation, as well as the speed of the wind in precipitating regions.
The Doppler Effect refers to the change in the frequency of a wave pattern caused by the relative motion of the wave emitter and the observer of the wave. (i.e. the pitch becomes higher as the sound approaches you, and then the pitch goes down as the source of the sound moves away)
Wind Profilers are Doppler radars pointed skyward and can determine the wind speed even when precipitation is not present by measuring the change in the frequency of the returning radar beam.

Atmospheric Optics

Light can be reflected, bent (refracted), scattered, and diffracted, with its colors being dispersed, which can create optical effects in the sky.
A Mirage is an image formed by the refraction of light by layers of air with different temperatures.
A Halo is a whitish ring that encircles but does not touch the Sun or Moon and is caused by the refraction of light by ice crystals.
When sunlight passes through a triangular glass prism, it separates into colors of the rainbow. This is due to different colors, defined by their wavelength, refracting at different amounts with blue and violet refracting the most and red refracting the least. This separation of colors is called Dispersion.
Sundogs are an optical effect caused by refraction and dispersion where it appears that shiny, colored regions are on either side of the Sun.
A Rainbow is a single, bright, colored arc, which is a result of reflection, refraction, and dispersion of light in raindrops.
Diffraction occurs when light is bent around small objects.