Module 3 - Hydrology.pdf
- 2. PRECIPITATION Precipitation is the general term for all forms of moisture emanating from the clouds and falling to the ground. The usual forms are rainfall, snowfall, hail, frost and dew.
- 3. PRECIPITATION For precipitation to form: • The atmosphere must have moisture; • There must be sufficient nuclei present to aid condensation; • Weather conditions must be good for condensation of water vapour to take place; and • The products of condensation must reach the earth.
- 4. PRECIPITATION Precipitation results when water droplets come together and coalesce to form larger drops that can drop down. The net precipitation at a place and its form depend upon a number of meteorological factors such as the weather elements.
- 5. TYPES OF PRECIPITATION CONVECTIONAL PRECIPITATION FRONTAL PRECIPITATION OROGRAPHIC PRECIPITATION CYCLONIC PRECIPITATION
- 6. CONVECTIONAL PRECIPITATION This type of precipitation 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’.
- 7. 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. 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’.
- 8. FRONTAL PRECIPITATION 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.
- 9. FRONTAL PRECIPITATION 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’
- 10. OROGRAPHIC PRECIPITATION Orographic precipitation is a type of precipitation that is caused by the uplift of moist air as it flows over a mountain or other elevated terrain. As the air rises, it cools and condenses, forming clouds and precipitation.
- 11. OROGRAPHIC PRECIPITATION The process of orographic precipitation begins when moist air is forced to rise as it encounters a mountain or other obstacle in its path. As the air rises, it cools at a rate of approximately 6.5°C per 1000 meters of elevation gain, which causes water vapor in the air to condense into cloud droplets. If the rising air is saturated, meaning that it contains as much water vapor as it can hold, the cloud droplets will continue to grow and eventually fall to the ground as precipitation.
- 12. 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.
- 13. CYCLONIC PRECIPITATION 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.
- 15. FORMS OF PRECIPITATION Drizzle: a light steady rain in fine drops (0.5 mm) and intensity less than 1 mm/h. Rain: the condensed water vapour of the atmosphere falling in drops from the clouds of sizes larger than 0.5 mm. The maximum size is about 6 mm. Glaze: freezing of drizzle or rain when they come in contact with cold objects.
- 16. FORMS OF PRECIPITATION Sleet: frozen rain drops while falling through air at subfreezing temperature. Snow: ice crystals resulting from sublimation. Snowflakes: ice crystals fused together. Hail: small lumps of ice larger than 5 mm in diameter formed by alternate freezing and melting, when they are carried up and down in highly turbulent air currents.
- 17. FORMS OF PRECIPITATION Dew: moisture condensed from the atmosphere in small drops upon cool surfaces. 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. Fog: a thin cloud of varying size at the surface of the earth by condensation of atmospheric vapour. Mist: a very thin fog.
- 18. MEASUREMENT AND DATA ANALYSIS
- 19. MEASUREMENT OF PRECIPITATION Precipitation is expressed in terms of the depth to which rainfall water would stand on an area if all the rain were collected on it. Rainfall may be measured by a network of rain gauges which may either be of non-recording or recording type.
- 20. NON-RECORDING RAIN GAUGE Symon’s Rain Gauge 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.
- 21. NON-RECORDING RAIN GAUGE Symon’s Rain Gauge 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. It is often desirable to protect the gauge from being damaged by cattle and for this purpose a barbed wire fence may be erected around it.
- 23. RECORDING RAIN GAUGE This is also called self-recording, automatic or integrating rain gauge. This type of rain gauge has an automatic mechanical arrangement consisting of a clockwork, a drum with a graph paper fixed around it and a pencil point, which draws the mass curve of rainfall. 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.
- 24. RECORDING RAIN GAUGE 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.
- 25. TIPPING BUCKET RAIN GAUGE This consists of a cylindrical receiver 30 cm diameter with a funnel inside. 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.
- 26. TIPPING BUCKET RAIN GAUGE
- 27. WEIGHING TYPE RAIN GAUGE In this type of rain-gauge, when a certain weight of rainfall is collected in a tank, which rests on a spring-lever balance, it makes a pen to move on a chart wrapped round a clock- driven drum. The rotation of the drum sets the time scale while the vertical motion of the pen records the cumulative precipitation.
- 28. WEIGHING TYPE RAIN GAUGE
- 29. 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. 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 the drum is also replaced. This type of gauge is used by IMD.
- 30. FLOAT TYPE RAIN GAUGE
- 31. ANALYSIS OF RAINFALL DATA Descriptive Statistics: Descriptive statistics can be used to summarize precipitation data, such as the mean, median, and standard deviation of precipitation amounts. Time-Series Analysis: Time-series analysis involves analyzing the changes in precipitation amounts over time, such as seasonal or annual trends.
- 32. ANALYSIS OF RAINFALL DATA Spatial Analysis: Spatial analysis involves analyzing the distribution of precipitation across a region or area, such as the variation in precipitation amounts between different cities or regions. Correlation Analysis: Correlation analysis involves examining the relationship between precipitation and other variables, such as temperature, humidity, or atmospheric pressure, to understand the factors that influence precipitation patterns.
- 34. INTENSITY Intensity: Precipitation intensity refers to the amount of precipitation that falls during a specific time period, typically measured in millimeters or inches per hour. Higher intensity precipitation events are often associated with more extreme weather conditions, such as thunderstorms, hurricanes, or heavy snowfall.
- 35. FREQUENCY Frequency: Precipitation frequency refers to how often precipitation events of a certain intensity occur over a specific time period, typically measured in terms of the number of events per year. Frequency analysis can be used to identify the probability of extreme precipitation events occurring in a given region, which can help inform planning and management decisions.
- 36. DURATION Duration: Precipitation duration refers to how long a precipitation event lasts, typically measured in hours. Longer-duration precipitation events can lead to more widespread and sustained impacts, such as flooding or erosion.
- 38. SPATIAL DISTRIBUTION The spatial distribution of precipitation refers to the pattern of where precipitation falls over a specific area. This can be influenced by a variety of factors, such as topography, distance from water sources, and prevailing wind patterns. In addition, some regions may experience localized convective precipitation, which is characterized by intense, short-duration storms that are highly variable in space and time.
- 39. TEMPORAL DISTRIBUTION The temporal distribution of precipitation refers to the pattern of when precipitation occurs over a specific time period, such as a day, a month, or a year. This can be influenced by seasonal and climatic factors, as well as weather patterns such as fronts and atmospheric disturbances.