Terrestrial weather

On Earth, regularly happening weather phenomena include such things as wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes and ice storms. Almost all standard weather phenomena on Earth occur in the troposphere. Weather does occur in the stratosphere and does affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.

The Earth's atmosphere is one large chaotic system so small changes to one part can have large effects elsewhere. This makes it very difficult to exactly predict short term weather changes more than a few days in advance, though weather forecasters are continually working to extend this limit through the science of the study of weather, Meteorology. Chaos theory explains that there are an infinite number of variables which affect the weather. The slightest variation in the motion of any molecule in the air influences the atmospheric situation. The slightest difference could affect the troposphere sufficiently to lead to utterly different conditions. Thus it will never be possible to forecast perfectly

Germ theory of disease

The germ theory of disease, also called the pathogenic theory of medicine, is a theory that proposes that microorganisms are the cause of many diseases. Although highly controversial when first proposed, it is now a cornerstone of modern medicine and clinical microbiology, leading to such important innovations as antibiotics and hygienic practices.

The ancient historical view was that disease was spontaneously generated instead of being created by microorganisms which grow by reproduction. One of the earliest references to this theory appears in On Agriculture by Marcus Terentius Varro wherein there is a warning about locating a homestead in the proximity of swamps which reads "...and because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases."

Medical ultrasonography

Medical ultrasonography or sonography is an ultrasound-based diagnostic imaging technique used to visualize muscles and internal organs, their size, structure and any pathological lesions, making them useful for scanning the organs. Obstetric sonography is commonly used during pregnancy.

The choice of frequency is a trade-off between the image spatial resolution and the penetration depth into the patient. Typical diagnostic sonography scanners operate in the frequency range of 2 to 13 megahertz.Whilst in physics the term "ultrasound" applies to all acoustic energy with a frequency above human hearing 20,000 Hertz,its common usage as a term of medical imaging applies to just a band of frequencies hundreds of times higher.