Ultrasound refers to a sound wave with a frequency greater than the upper limit of human hearing, which is generally over 20 kHz. In general, humans can hear sounds with a frequency between 20 Hz and 20 kHz.
An isohyet or an isohyetal line is a line that joins places of same rainfall on a map within a given time period. A map of isohyets is also known as an isohyetal map.
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