Radar remote-sensing instruments

Any of a range of active REMOTE-SENSING instruments that propagate radio or microwave radiation and measure the BACKSCATTERING (echoes) that returns sometime later. Such systems can provide two types of information. By measuring the time taken for each echo to return to the sensor, distance (range) from the sensor can be determined.

At the same time, the intensity and POLARISATION of backscattered radiation may also be detected. There are three generic types of spaceborne RADAR instruments: altimeters, scatterometers and imaging radars. Altimeters (or nadir [downward] looking radars) are used to derive elevation profiles under the orbit track. These profiles are constructed by accurately measuring the time delay for a radar echo (pulse) to propagate to the surface and return back to the sensor. This has applications in mapping surface TOPOGRAPHY and in OCEAN and GLACIER monitoring. 

Scatterometers measure the radar cross section of a target (surface reflectivity), which is a function of how the target interacts with microwave radiation, and are typically used to measure wind speed and direction over water or detect rainfall. Imaging radars, such as synthetic aperture radar (SAR), are used to acquire high spatial resolution (a few metres to a few tens of metres) images, measuring range, intensity and sometimes polarisation of backscattered radiation over large areas. SAR instruments emit radar pulses as the platform moves and, by correcting the pulses for the transmission and reception times, an aperture can be synthesised that has a much greater size than the physical size of the antenna, which results in a finer spatial resolution. Radar instruments are generally composed of several parts: A transmitting source; an antenna, which shapes the transmitted energy into a beam pointing in a certain direction and collects energy from this direction; and equipment for processing and storing the data.

Wave Creation

As the wind moves across the surface of the water, some of the wind’s energy gets transferred into the water. The more energy the wind transfers to the water, the bigger the waves will be.

• The faster the wind blows (wind speed), the more energy the wind has, and the bigger the waves it can generate.

• The longer the length of time the wind blows (wind duration), the greater the amount of time it can transfer energy to the water, and the bigger the waves it can generate.

• The greater the distance over the water that the wind travels (fetch), the more opportunity there is for air–water interaction, and the bigger the waves that can be created.

Thus, high winds blowing across a long length of water for a long time can transfer a lot of energy, which will move as very large waves. Once waves are generated, they move across the surface of the water until they encounter resistance.

Waves

Waves are one of the most important processes in the coastal zone, especially on open shorelines. A wave is simply movement of energy. And it is the delivery of energy to the shoreline via waves that makes them so important. The word “wave” can be applied to a wide range of phenomena: radar waves, microwaves, earthquake waves, light waves, radio waves, sound waves, shock waves.

All of these “waves” are energy moving from one place to another. The energy that moves as a wave across water is almost certain to have come from the wind. However, it is possible for other sources of energy to produce a wave. Boats make waves called wakes; earthquakes can generate tsunamis; throwing a rock into a pond will produce ripples. Nevertheless, the moving air in the atmosphere is responsible for almost all the transfer of energy into the water.

Dangerous Semicircle

This term, now somewhat archaic, is given to that portion of a tropical cyclone in which the winds and rain are most intense. Primarily used by mariners, the term is derived from the practice of dividing a hurricane, typhoon, or cyclone into dangerous and navigable semicircles, or right and left halves, based upon the system’s forward motion. In the Northern Hemisphere, where tropical cyclones spin in a counterclockwise direction, an observer facing into the winds of an approaching hurricane will find the dangerous semicircle on the right, or eastern side, and the navigable semicircle on the left, or western side. The reverse is true, of course, in the Southern Hemisphere, where the clockwise spinning of a cyclone will yield a dangerous semicircle on the left, or western side, and a navigable semicircle on the right, or eastern half.

While in actuality both halves of a tropical cyclone are dangerous, the half that finds itself strengthened by both the forward speed of the system’s steering current and the storm’s own forward velocity will possess significantly faster winds and higher seas. For this reason, those mariners who have divided an oncoming hurricane, typhoon, or cyclone into dangerous and navigable semicircles have been better able to guide their vessels away from the storm’s most furious aspects, thus greatly improving their odds of surviving it.

Covalent Bonds

Recall that an atom is chemically stable when its outermost energy level is full. A state of stability is achieved by some elements by forming chemical bonds. A chemical bond is the force that holds together the elements in a compound. One way in which atoms fill their outermost energy levels is by sharing electrons. For example, individual atoms of hydrogen each have just one electron. Each atom becomes more stable when it shares its electron with another hydrogen atom so that each atom has two electrons in its outermost energy level.  How do these two atoms stay together? The nucleus of each atom has one proton with a positive charge, and the two positively charged protons attract the two negatively charged electrons. This attraction of two atoms for a shared pair of electrons that holds the atoms together is called a covalent bond.

Molecules A molecule is composed of two or more atoms held together by covalent bonds. Molecules have no overall electric charge because the total number of electrons equals the total number of protons. Water is an example of a compound whose atoms are held together by covalent bonds. The chemical formula for a water molecule is H2O because, in this molecule, two atoms of hydrogen, each of which need to gain an electron to become stable, are combined with one atom of oxygen, which needs to gain two electrons to become stable. A compound comprised of molecules is called a molecular compound. Polar molecules Although water molecules are held together by covalent bonds, the atoms do not share the electrons equally. The shared electrons in a water molecule are attracted more strongly by the oxygen atom than by the hydrogen atoms. As a result, the electrons spend more time near the oxygen atom than they do near the hydrogen atoms. This unequal sharing of electrons results in polar molecules. A polar molecule has a slightly positive end and a slightly negative end.

Ions

Sometimes atoms gain or lose electrons from their outermost energy levels. Recall that atoms are electrically neutral because the number of electrons, which have negative charges, balances the number of protons, which have positive charges. An atom that gains or loses an electron has a net electric charge and is called an ion. In general, an atom in which the outermost energy level is less than half-full — that is, it has fewer than four valence electrons — tends to lose its valence electrons.

When an atom loses valence electrons, it becomes positively charged. In chemistry, a positive ion is indicated by a superscript plus sign. For example, a sodium ion is represented by Na+. If more than one electron is lost, that number is placed before the plus sign. For example, a magnesium ion, which forms when a magnesium atom has lost two electrons, is represented by Mg2+.

An atom in which the outermost energy level is more than half-full — that is, it has more than four valence electrons — tends to fill its outermost energy level. Such an atom forms a negatively charged ion. Negative ions are indicated by a superscript minus sign. For example, a nitrogen atom that has gained three electrons is represented by N3‒. Some substances contain ions that are made up of groups of atoms—for example, silicate ions. These complex ions are important constituents of most rocks and minerals.

Isotopes

Recall that all atoms of an element have the same number of protons. However, the number of neutrons of an element’s atoms can vary. For example, all chlorine atoms have 17 protons in their nuclei, but they can have either 18 or 20 neutrons. This means that there are chlorine atoms with mass numbers of 35 (17 protons + 18 neutrons) and 37 (17 protons + 20 neutrons). Atoms of the same element that have different mass numbers are called isotopes. The element chlorine has two isotopes: Cl-35 and Cl-37. Because the number of electrons in an atom equals the number of protons, isotopes of an element have the same chemical properties.

Scientists have measured the mass of atoms of elements. The atomic mass of an element is the average of the mass numbers of the isotopes of an element. Most elements are mixtures of isotopes. The atomic mass of chlorine is 35.453. This number is the average of the mass numbers of the naturally occurring isotopes of chlorine-35 and chlorine-37.

Radioactive isotopes The nuclei of some isotopes are unstable and tend to break down. When this happens, the isotope also emits energy in the form of radiation. Radioactive decay is the spontaneous process through which unstable nuclei emit radiation. In the process of radioactive decay, a nucleus can lose protons and neutrons, change a proton to a neutron, or change a neutron to a proton. Because the number of protons in a nucleus identifies an element, decay changes the identity of an element. For example, the isotope polonium-218 decays at a steady rate over time into bismuth-214. The polonium originally present in a rock is gradually replaced by bismuth.