Hujan (Presipitasi) – Bambang Triatmodjo

Summary

This video lecture on applied hydrology focuses on precipitation, specifically rain, as a key aspect of the hydrological cycle. The speaker explains the science behind rain formation, highlighting the role of factors like temperature, wind, and atmospheric pressure in cloud condensation. The video covers various types of rain, such as convective, cyclonic, and orographic rain, explaining their causes and characteristics. It also delves into rainfall parameters like intensity, duration, and distribution, using graphs to show data analysis. The lecture provides a detailed understanding of rainfall patterns in Indonesia, with real-world applications and statistical insights into regional precipitation variations.

Takeaways

• 😀 Precipitation refers to the falling of water from the atmosphere to Earth's surface, including rain, snow, fog, dew, and hail, with rain being the most significant in tropical regions like Indonesia.
• 😀 Rain forms from water vapor in the atmosphere, and its quantity and form are influenced by climatological factors like wind, temperature, and air pressure.
• 😀 Two essential conditions for rain are the presence of moist air and the rising of that air to cooler atmospheric layers, leading to condensation.
• 😀 The process of rain formation involves solar heating of water bodies (like oceans and lakes), causing evaporation, and the rising of vapor which condenses into clouds before falling as rain.
• 😀 There are three main types of rain: convective, cyclonic, and orographic. Each type depends on the method of air ascent and resulting cooling.
• 😀 Convective rain occurs in tropical regions during the dry season when intense heating near the surface causes warm, moist air to rise rapidly, resulting in heavy, brief showers.
• 😀 Cyclonic rain happens when warm, moist air meets colder, denser air, causing the warm air to rise, cool, condense, and form rain, often occurring in colder regions with longer durations.
• 😀 Orographic rain happens when moist air from the sea is forced to rise over mountains, cooling as it ascends, leading to rainfall on the windward side, with the leeward side being drier.
• 😀 Rainfall intensity and distribution vary by region, and measurements are taken in millimeters (mm) of water depth, with terms like daily or annual rainfall to describe it.
• 😀 Rainfall can be measured by its intensity, duration, and total volume, with intensity often categorized as light, moderate, heavy, or very heavy depending on the rate of precipitation over time.

Q & A

What is precipitation, and what are its forms?

Precipitation is the process of water falling from the atmosphere to the Earth's surface. It can occur in the form of rain, snow, fog, dew, or hail. In tropical regions like Indonesia, rain is the most significant form of precipitation.

What are the two essential conditions for rainfall to occur?

The two essential conditions for rainfall are: (1) the presence of air masses containing water vapor, and (2) these air masses must rise into the atmosphere and cool down, leading to condensation and the formation of rain.

What is the role of climatology and meteorology in studying precipitation?

Meteorologists and climatologists study the processes that lead to precipitation, such as air temperature, pressure, and moisture content. Hydrologists, on the other hand, focus on the quantity and distribution of precipitation in space and time.

What is convectional rain, and where does it typically occur?

Convectional rain occurs when air near the Earth's surface heats up, becomes less dense, and rises, cooling and condensing into water droplets. This type of rain is typical in tropical regions during the dry season and is usually intense but brief.

How does cyclonic rain differ from convectional rain?

Cyclonic rain happens when warm, lighter air meets cold, dense air, causing the warm air to rise, cool, and condense into rain. It generally lasts longer and is not as intense as convectional rain. It occurs primarily in colder regions.

What causes orographic rain, and where is it commonly observed?

Orographic rain is caused by moist air from the ocean being forced upwards by mountainous terrain. As the air rises, it cools and condenses, forming clouds and rainfall. The western coast of Sumatra is a prime example of an area where orographic rain occurs due to the Bukit Barisan mountain range.

What is the definition of rainfall intensity, and how is it calculated?

Rainfall intensity refers to the amount of rain that falls in a given time period, usually expressed in millimeters per hour (mm/h). It is calculated by dividing the total rainfall by the duration of the rain event.

How is the distribution of rainfall categorized, and what does a hyetograph represent?

Rainfall distribution is categorized based on the variation in rainfall depth during a rain event. A hyetograph is a graphical representation of this distribution, where the x-axis represents time, and the y-axis represents rainfall intensity.

What is the difference between a cumulative rainfall graph and a hyetograph?

A cumulative rainfall graph shows the total amount of rainfall that has fallen up to a given time, while a hyetograph shows rainfall intensity for each time period, not the cumulative total.

How do regional rainfall patterns in Indonesia vary, and what factors influence these patterns?

Rainfall patterns in Indonesia vary significantly due to geographical and climatic factors. Areas like Sumatra and Papua experience high annual rainfall, while regions like Nusa Tenggara and Timor receive less. These patterns are influenced by the location's proximity to the ocean, the presence of mountain ranges, and seasonal wind shifts.