Understanding Precipitation: Types and Characteristics

• Precipitation refers to any form of water that falls from the atmosphere to the Earth's surface.
• Precipitation is a key component of the Earth's water cycle.
• Precipitation plays a crucial role in distributing water across the planet.
• Types of water forms include rain, snow, sleet, hail, and drizzle.

Precipitation Types:

• Rain: Liquid water droplets that fall when the temperature is above freezing.
• Snow: Ice crystals that form when the temperature is below freezing.
• Sleet: Small pellets of ice that form when raindrops freeze before hitting the ground.
• Hail: Larger balls or lumps of ice that form in strong thunderstorms.
• Drizzle: Light, fine rain.

Rainfall Characteristics:

• Drizzle: Water drops that are less than 0.02 inches (0.51 mm) in diameter with light intensity at less than 0.04 in/hr.
• Rain: Drops that are larger than 0.02 inches, with a max practical size of ~0.25 inches (6.35 mm).
• Glaze: Ice coating formed when drizzle or rain freezes on contact with surfaces.
• Sleet: Frozen raindrops cooled to the ice stage while falling through subfreezing air.
• Snow: Ice crystals formed from sublimation (direct water vapor to ice transformation).
• Hail: Ice balls over 0.2 inches (5.08 mm), formed by repeated freezing and melting in turbulent air.

Forms of Precipitation

• Rain is the most common form of precipitation in the Philippines due to its tropical climate.
• The Philippines experiences significant rainfall throughout the year, primarily influenced by monsoons i.e. Habagat and Amihan.
• Southwest Monsoon (Habagat): Brings heavy rains from June to October and contributing to typhoons and intense flooding.
• Northeast Monsoon (Amihan): Causes cooler, moderate rainfall from November to February.
• Typhoons: The Philippines is located in the Pacific typhoon belt and encounters an average of 20 typhoons annually, potentially leading to disasters.
• Light rain: Common during inter-monsoonal periods.
• Moderate rain: Occurs during monsoons and in the outer bands of typhoons.
• Heavy rain: Associated with typhoons, localized thunderstorms, and monsoon surges.
• Snowfall is virtually non-existent in the Philippines due to its tropical latitude.
• The lowest temperatures recorded in high-altitude areas, such as Baguio City or Mt. Pulag, can drop to single digits (°C).
• Frost can occur, but snow remains an anomaly of the country's climate.

Contextual Challenges

• Prolonged rainfall, especially heavy rains, contributes to flooding, landslides, and dam overflow in vulnerable areas.
• Urban centers like Metro Manila face challenges with drainage systems that cannot handle the volume during heavy rain events.
• Drizzle and light rains are often harmless but can cause reduced visibility for drivers and commuters.
• Managing heavy rainfall includes weather forecasting, early warning systems and infrastructure improvements, such as flood control projects.
• The Philippines' location and geography make its rainfall patterns distinct compared to other countries because of heavy dependence on both seasonal winds and tropical cyclone activity.
• The interplay of these factors makes rain the most impactful form of precipitation for the country.

Types of Precipitation

• Precipitation occurs when air masses lift. Precipitation is classified based on the factors responsible for the lifting:
• Cyclonic precipitation: Caused by lifting of air mass due to pressure differences.
  • Responsible for most of the winter rain in Haryana and Punjab.
  • It may be Frontal - When moving warm, moist air is obstructed by a zone of cold, air rises, condenses to cause heavy rainfall.
  • Non-Frontal - When air rushes into areas of low pressure, air rises to a higher altitude, and condenses to cause heavy rainfall.
• Convective precipitation - Warm air lifts unequally after ground surface is heated up in tropical countries. Warm air condenses at high altitude and causes heavy/short rainfall.
• Orographic precipitation - Air masses of moisture strike topographic barriers causing condensation and precipitation at higher altitude.

Water Balance

• In hydrology, the water balance equation describes the flow of water in and out of a system.
• Systems can be hydrological, like soil columns or entire drainage basins.
• The equation analyzes water availability.
• It follows that the water entering a system in the specified amount of time be stored, consumed, or flow out as above or below the surface.
• Equation components can be expressed as the equation I = O + ΔS
  • I = Inflow in the system
  • O = Outflow in the system
  • ΔS = Change in storage in a given period of time
• Inflow includes precipitation, surface inflow and subsurface inflow.
• Outflow includes surface outflow, subsurface outflow, evaporation, transpiration, and evapotranspiration.
• Change in storage includes soil moisture, groundwater, snow cover, surface, depression, and channel storage.
• Over a set period of time for a fixed area, the water equation is expressed as: P = Q+E+ΔS
  • P = Precipitation
  • Q = Runoff
  • E = Evapotranspiration
  • ΔS = Change in storage.

Measurement of Rainfall

• Usually measured by a rain gauge.
• Rainfall and other forms of precipitation are measured by their depth in millimeters.
• Amount of rainfall collected over the small areas reflects the rainfall of a large area given identical meterological conditions.
• A non-recording type of rain gauge is also known as Symon's rain gauge, used at all government rain gauge stations throughout India.
  • Glass bottle capacity: 100mm
  • Bottle sits in casing
  • Top has a funnel with a brass rim.
  • Rainfall is measured every 24 hours- recorded at 8 AM.
• Types of rain gauges:
  • Non-recording
  • Recording/automatic.
    • Weighing bucket
    • Tipping bucket.
    • Float-type.

Tipping Bucket Rain Gauge

• Includes a sharp-edged receiver with a funnel on end.
• A pair of buckets pivot below with a tipper bucket receiving 0.254 mm, which makes it tip over, sending the contents to a reservoir and bringing the partner bucket into its old position.
• Bucket tipping completes a circuit, with pen marking on a clock, moving driven drum, recording.

Weighing Bucket

• The most common type of self-recording gauge.
• It consists of a receiver bucket supported by spring or weighing mechanism.
• Bucket movement due to increasing weight is transmitted to a pen tracing the record on a clock-driven chart.

Automatic

• A rain gauge for automatic rainfall recording on graph paper from mechanical arrangement.
• Has record of cumulative rainfall over specified amount of time.

Float Type

• Working is similar to the weighing bucket gauge.
• A funnel receives water, collecting it in rectangles.
• A float at the bottom container moves up with water level- has movement recorded by a pen moving on recording drum, clockwork actuated.
• When the water level gets too high, the siphon releases the water- entire box drains!

Rain Gauges used in the Philippines

• The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) primarily uses tipping bucket rain gauges for automated weather stations and standard manual rain gauges. This list includes:
  • Tipping Bucket Rain Gauge (Automatic)
    • Weather stations for real-time measurement, often in cities or remote regions. They can transmit data automatically to PAGASA's central system.
  • Standard 8-inch Rain Gauge (SRG)
    • Non-recording instrument where rain is measured manually, commonly used in rainfall observation stations in locations where there are no automated systems.
  • Weighing Rain Gauges
    • Used in research stations for accurate precipitation measurements. They can measure different precipitation forms.

Estimation of Missing Rainfall Data

• Arithmetic Average Method uses normal annual precipitation of adjacent stations, must be within 10% of the normal annual precipitation of the station under consideration.
• Px = 1/m [P₁ + P₂ +...+ Pm]
• Normal Ratio Method
• Px = Nx/m [P₁/N₁ + P₂/N₂+...+ Pm/Nm]
  • Px = missing precipitation of station X
  • P1, P2, P3,... Pm = precipitation values at m neighboring raingauges.
  • Nx = normal annual precipitation at station X.
  • N1, N2, N3,..., Nm,= normal annual precipitation at m surrounding raingauges

Influences or inconsistency

• Significant change in or around a rain gauge station. (Causes of inconsistency of records are:
  • Shifting of rain gauge to a new location
  • Change in instrument
  • Change in surrounding of the rain gauge

Double Mass Curve Analysis

• Used to check consistency issues
• Pcx - corrected precipitation at any time period t₁ at station X
• Px - Original recorded precp. at time period t₁ at station X
• Mc - corrected slope of the double mass curve
• Ma - original slope of the mass curve

Average Precipitation Calculation

• Point sampling may only represent an area's rainfall distribution. Rainfall must analyzed for area calculation. To convert rainfall values, the following methods are used:
  • arithmetic Mean Method
  • Thiessen polygon method
  • Isohyets method

Arithmetic average method:

• Simplest method to determining rainfall over a basin.
• Divide the total rain depths recorded at varying sites, divide by the # of those stations. - P = P1 + P2 + P3 +...+ Pn/N = 1/N (ΣΡi) - P = Rainfall at the ith rain gauge station - N = total # of rainfall stations

Theissen Polygon method

• Rainfall varies by intensity and duration from place to place.

• Rainfall recorded at each rain gauge should weighted according to area represented. Procedure:

  • Draw area of concern to scale.
  • Locations of rain gauges need to be identified.
  • Connect gauges to for a network.
  • Draw perpendicular triangles to bisect the sides.
  • Mark polygons.
  • Delinate the forms and measure the area.

• Compute the average rainfall by following equation: P=PA₁ + P₂A₂ +.....+ Pm A Am/A₁ + A₂ +.....+ Am - The ratio is the weight-age factor of station # i. - total area of a basin has: - area of a particular polygon - rainfall data of particular rain gauges

Isohyetal Method

• A line joining places where rainfall amounts are equal on a rainfall basin map.
• displays lines or contours of rainfall or accurate depiction of the rainfall.
• Isohyets are generated using interpolation and rainfall is marked at station.
• A = Area between each pair of Isohyets
• P = Average precipitation between pairs of isohyets.

Design of a Rain Network

• rain gauges must evenly distribute
• Number of gauges cannot yield unreliable data.
• Density of gauges depends on region.
• According to India Standards IS: 4987-1968.

1. station/520 km squared/ plain terrains 2)1 station/(260-360) km squared terrain of average elevation (1000 m average)
2. Hilly areas/ high volume rain (1 station/ 130 km squared

• Optimum Numbers of Rain gauges
• N = (Cv / E)2
  • Cv = rainfall coefficient station values
  • E = Allowable % of basic rainfall.

Rainfall Data Characteristics

• Rainfall can be described by:
• duration
• intensity - described frequency when the previously mentioned details are available.
• intensity of rainfall is rate is falling.
• the duration of rainfall described as the time of when the rainfall fell
• Frequency of rainfall is presented as times.

Mass Vs Hyeto Graphs

• A mass curve is a graph of accumulated precipitation over time.
• It shows both storm duration and storm magnitude.
• Intensity at different time intervals equals the slope of the curve.
• A hyetograph is a graph of rainfall intensity during a time interval and is derived from the mass over time.
• The area under the hyetograph reveals the total rainfall over a given time.
• Useful in rainfall estimation.

Point Rainfall

• Refers to station data and can include:
• Daily
• Weeekly
• Monthly
• Yearly.
• Represented by bar.

Practice Question

Question: Assuming rain is falling from straight above, figure the catch on a 20-degree gauge.

• With inclined precipitation catch is found by this equation:
• Inclined Gauge = Pvertical x cos. (0)
• Catch results in 93.97% of the vertical gauge.

The second problem involves a inoperable rain station:

• Find missing measurements.
• Must multiply measured rainfall at another location, by a ratio.
• Divide missing annual snowfall at nearby locations.

Question Type 3

• Find the consistency of data for past years- consider mean annual rainfall.
• To find average, first find a change.
• Adjustment can be needed to correct for inconsistency.
• Must refer to page 51.

1. Look for change in slope to determine a break. 2. Ratio is from a previous slope.