Lesson 2.1: Precipitation PDF

LESSON 2.1: Precipitation Forms of Precipitation; Rainfall characteristics; Different Types of Rain gauges _____________________________________________________________________________________________________________________________________________ Precipitation is the process that...

LESSON 2.1: Precipitation Forms of Precipitation; Rainfall characteristics; Different Types of Rain gauges _____________________________________________________________________________________________________________________________________________ Precipitation is the process that occurs when any and all forms of water particles fall from the atmosphere and reach the ground. (w/ video presentation) FORMS OF PRECIPITATION a. Drizzle — a light steady rain in fine drops (0.5 mm) and intensity 0.5 mm, maximum size—6 mm) from the clouds. c. Glaze — freezing of drizzle or rain when they come in contact with cold objects. d. Sleet — frozen rain drops while falling through air at subfreezing temperature. e. Snow — ice crystals resulting from sublimation (i.e., water vapour condenses to ice) f. Snow flakes — ice crystals fused together. g. Hail — small lumps of ice (>5 mm in diameter) formed by alternate freezing and melting, when they are carried up and down in highly turbulent air currents. h. Dew — moisture condensed from the atmosphere in small drops upon cool surfaces. i. Frost — a feathery deposit of ice formed on the ground or on the surface of exposed objects by dew or water vapour that has frozen j. Fog — a thin cloud of varying size formed at the surface of the earth by condensation of atmospheric vapour (interfering with visibility) k. Mist — a very thin fog TYPES OF PRECIPITATION The precipitation may be due to: 1) Thermal convection (convectional precipitation)—This type of precipiation is in the form of local whirling thunder storms and is typical of the tropics. The air close to the warm earth gets heated and rises due to its low density, cools adiabatically to form a cauliflower shaped cloud, which finally bursts into a thunder storm. When accompanied by destructive winds, they are called ‘tornados’. 2) Conflict between two air masses (frontal precipitation)—When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occur at the surface of contact, Fig. 2.1. This surface of contact is called a ‘front’ or ‘frontal surface’. If a cold air mass drives out a warm air mass’ it is called a ‘cold front’ and if a warm air mass replaces the retreating cold air mass, it is called a ‘warm front’. On the other hand, if the two air masses are drawn simultaneously towards a low pressure area, the front developed is stationary and is called a ‘stationary front’. Cold front causes intense precipitation on comparatively small areas, while the precipitation due to warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of cold and warm air sliding against each other. This phenomenon is called ‘occlusion’ and the resulting frontal surface is called an ‘occluded front’. 3) Orographic lifting (orographic precipitation)—The mechanical lifting of moist air over mountain barriers, causes heavy precipitation on the windward side (Fig. 2.2). For example Cherrapunji in the Himalayan range and Agumbe in the western Ghats of south India get very heavy orographic precipitation of 1250 cm and 900 cm (average annual rainfall), respectively. 4) Cyclonic (cyclonic precipitation)—This type of precipitation is due to lifting of moist air converging into a low pressure belt, i.e., due to pressure differences created by the unequal heating of the earth’s surface. Here the winds blow spirally inward counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. There are two main types of cyclones—tropical cyclone (also called hurricane or typhoon) of comparatively small diameter of 300-1500 km causing high wind velocity and heavy precipitation, and the extra-tropical cyclone of large diameter up to 3000 km causing wide spread frontal type precipitation. Handout in Hydrology -1- Engr. Maria Senen D. Bongulto 2nd Sem, A.Y. 2024 - 2025 CHARACTERISTICS The characteristics of precipitation that are important to hydrologic forecasting and water resource data are: ✓ Depth ✓ Duration ✓ Intensity ✓ Frequency ❖ Depth of rainfall - Rainfall depth is a crucial parameter in hydrology, as it helps us understand the amount of precipitation in a given area over a specific period. Accurate rainfall depth measurements are essential for various applications, including flood forecasting, water resource management, and agricultural planning. - All form of precipitation is measured on the basis of the vertical depth of water that would accumulate on a level surface if the precipitation remained where it fell. ❖ The duration - The duration refers to the length of time rainfall occurs. Rainfall of longer duration can significantly affect infiltration, runoff, and soil erosion processes. ❖ Intensity (rate of rainfall) - Intensity is defined as the time rate of rainfall depth and is commonly given in the units of millimeters per hour (or inches per hour). - The intensity and duration can have a large influence on whether the rainfall infiltrates or becomes surface runoff. Higher rainfall intensity produces larger size raindrops which have more impact energy, and thus higher intensity storms can damage delicate vegetation and bare soil. High intensity storms can literally displace soil particles, causing soil crusting or starting the soil erosion process. High intensity storms may also overwhelm the soils ability to infiltrate the rainfall at the same rate, causing infiltration-excess runoff. ❖ Frequency - The frequency, or more specifically, the return period refers to how often rainfall occurs at a particular amount or intensity and duration. Handout in Hydrology -2- Engr. Maria Senen D. Bongulto 2nd Sem, A.Y. 2024 - 2025  - For example, rainfall return periods are referred to as 100 year-1 hour rainfall or 100 year-24 hour rainfall to define the probability that a given amount will fall within a given time period. HYETOGRAPH 0.8 - A hyetograph is a graphical 0.7 representation of the distribution of rainfall intensity over time. It is Incremental Rainfall (in per 5 min) 0.6 the plot of the rainfall intensity drawn on the ordinate axis 0.5 against time on the abscissa axis. 0.4 - The hyetograph is a bar diagram. - The area under the hyetograph 0.3 gives the total rainfall occurred in that period. 0.2 - This chart is very useful in 0.1 representing the characteristics of storm, and is particularly 0 important in developing the 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 design storm to predict extreme Time (min) floods. DIFFERENT TYPES OF RAIN GAUGES Rainfall may be measured by a network of rain gauges which may either be of non-recording or recording type. An example of non-recording rain gauge is the Symon’s rain gauge (Fig. 2.3). - It consists of a funnel with a circular rim of 12.7 cm diameter and a glass bottle as a receiver. The cylindrical metal casing is fixed vertically to the masonry foundation with the level rim 30.5 cm above the ground surface. The rain falling into the funnel is collected in the receiver and is measured in a special measuring glass graduated in mm of rainfall; when full it can measure 1.25 cm of rain. - The rainfall is measured every day at 08.30 hours standard time. During heavy rains, it must be measured three or four times in the day, lest the receiver fill and overflow, but the last measurement should be at 08.30 hours ST and the sum total of all the measurements during the previous 24 hours entered as the rainfall of the day in the register. Usually, rainfall measurements are made at 08.30 hr ST and sometimes at 17.30 hr ST also. Thus the non-recording or the Symon’s rain gauge gives only the total depth of rainfall for the previous 24 hours (i.e.,daily rainfall) and does not give the intensity and duration of rainfall during different time intervals of the day. ❖ Recording Rain Gauge - This is also called self-recording, automatic or integrating rain gauge. This type of rain gauge Figs. 2.4, 2.5 and 2.6, has an automatic mechanical arrangement consisting of a clockwork, a drum with a graph paper fixed around it and Handout in Hydrology -3- Engr. Maria Senen D. Bongulto 2nd Sem, A.Y. 2024 - 2025  a pencil point, which draws the mass curve of rainfall Fig. 2.7. From this mass curve, the depth of rainfall in a given time, the rate or intensity of rainfall at any instant during a storm, time of onset and cessation of rainfall, can be determined. - The gauge is installed on a concrete or masonry platform 45 cm square in the observatory enclosure by the side of the ordinary rain gauge at a distance of 2-3 m from it. The gauge is so installed that the rim of the funnel is horizontal and at a height of exactly 75 cm above ground surface. The self-recording rain gauge is generally used in conjunction with an ordinary rain gauge exposed close by, for use as standard, by means of which the readings of the recording rain gauge can be checked and if necessary adjusted. There are three types of recording rain gauges—tipping bucket gauge, weighing gauge and float gauge. Tipping bucket rain gauge. This consists of a cylindrical receiver 30 cm diameter with a funnel inside (Fig. 2.4). Just below the funnel a pair of tipping buckets is pivoted such that when one of the bucket receives a rainfall of 0.25 mm it tips and empties into a tank below, while the other bucket takes its position and the process is repeated. The tipping of the bucket actuates on electric circuit which causes a pen to move on a chart wrapped round a drum which revolves by a clock mechanism. This type cannot record snow. Weighing type rain gauge. In this type of rain-gauge, when a certain weight of rainfallis collected in a tank, which rests on a spring-lever balance, it makes a pen to move on a chart wrapped round a clockdriven drum (Fig. 2.5). The rotation of the drum sets the time scale while the vertical motion of the pen records the cumulative precipitation. Handout in Hydrology -4- Engr. Maria Senen D. Bongulto 2nd Sem, A.Y. 2024 - 2025 Float type rain gauge. In this type, as the rain is collected in a float chamber, the float moves up which makes a pen to move on a chart wrapped round a clock driven drum (Fig. 2.6). When the float chamber fills up, the water siphons out automatically through a siphon tube kept in an interconnected siphon chamber. The clockwork revolves the drum once in 24 hours. The clock mechanism needs rewinding once in a week when the chart wrapped round thedrum is also replaced. The weighing and float type rain gauges can store a moderate snow fall which the operator can weigh or melt and record the equivalent depth of rain. The snow can be melted in the gauge itself (as it gets collected there) by a heating system fitted to it or by placing in the gauge certain chemicals such as Calcium Chloride, ethylene glycol, etc. ❖ Automatic-radio-reporting rain-gauge This type of raingauge is used in mountainous areas, which are not easily accessible to collect the rainfall data manually. As in the tipping bucket gauge, when the buckets fill and tip, they give electric pulses equal in number to the mm of rainfall collected which are coded into messages and impressed on a transmitter during broadcast. At the receiving station, these coded signals are picked up by UHF receiver. ❖ Automatic Rain-gauge Automatic Rain Gauges observe rainfall amount every 10 minutes and automatically send the data to a collection server. Handout in Hydrology -5- Engr. Maria Senen D. Bongulto 2nd Sem, A.Y. 2024 - 2025  ❖ RADAR (Radio Detection and Ranging) A weather surveillance radar (WSR) is useful in locating precipitation and estimate its intensity and in determining the center of tropical cyclones. In addition to this capability, state- of- the-art Doppler radars are capable of estimating radial velocity which can be used to determine the wind strength of tropical cyclones as well as to analyse its structure. Assignment: Read about different forms of precipitation and how it is formed. What are the types of rain gauges that are typically used in the Philippines? Give at least 2 stations/locations where you can find those. (I will send the Google drive link for your submission). Handout in Hydrology -6- Engr. Maria Senen D. Bongulto 2nd Sem, A.Y. 2024 - 2025