Hydrology Week 4

Hydrometeorology is a branch of science that studies the interaction between atmospheric conditions and the hydrological cycle. Elements such as temperature, humidity, solar radiation, wind, and air pressure all influence rainfall and water availability. For instance, in Indonesia, monsoon winds carry water vapour from the Indian Ocean and trigger the rainy season. Without understanding hydrometeorology, it would be difficult to predict floods, droughts, or even daily water use in agriculture. Why is hydrometeorology important for Indonesia?

• It helps predict water-related phenomena such as floods and droughts.

• It explains tectonic activity in mountains.

• It reduces global temperatures.

• It prevents volcanic eruptions.

• It eliminates monsoon winds.

Air temperature plays a crucial role in determining how much water vapour the atmosphere can hold. According to the Clausius–Clapeyron law, for every 1 °C rise in air temperature, the capacity of air to hold water vapour increases by about 7%. This means that during hot days, evaporation is stronger, and more moisture is present in the air. Conversely, when the temperature drops at night, the atmosphere cannot hold as much vapour, which often leads to condensation, cloud formation, or even rainfall. What happens when air temperature increases, based on the passage?

• Air holds less water vapour.

• Air holds more water vapour.

• Air pressure decreases rapidly.

• Rainfall automatically occurs.

• Wind speed always weakens.

Humidity refers to the amount of water vapour present in the air. Absolute humidity is measured in grams per cubic metre, while relative humidity is expressed as a percentage of the air’s saturation capacity. When relative humidity reaches 100%, the air is saturated, and condensation begins. This is why fog, dew, or clouds often form early in the morning when the air cools. High relative humidity can make humans feel uncomfortable because sweat evaporates less efficiently, especially before rainfall. What does relative humidity measure?

• The exact mass of water vapour.

• The total rainfall in a year.

• The percentage of air’s water-holding capacity that is filled.

• The volume of cloud cover.

• The speed of wind movement.

Urbanisation leads to the creation of “urban heat islands,” where cities are significantly warmer than surrounding rural areas. Asphalt, concrete, and buildings absorb heat during the day and release it slowly at night. This additional heat enhances convection and can trigger localised rainfall events. Jakarta often experiences intense afternoon storms due to this phenomenon. Why do cities like Jakarta sometimes receive intense local rainfall?

• Buildings increase atmospheric pressure.

• The urban heat island effect intensifies convection.

• Cities block ocean currents.

• Urban areas eliminate humidity.

• Cities prevent dew formation.

Solar radiation varies by latitude. Equatorial regions receive more direct sunlight, resulting in higher temperatures and faster hydrological cycles. In contrast, polar regions receive slanted radiation, leading to lower temperatures and slower cycles. This explains why the tropics have abundant rainfall, while polar areas remain cold and dry. Why do tropical regions have higher rainfall?

• They experience only short days.

• They lack air pressure differences.

• They are influenced by glaciers.

• They have lower humidity.

• They receive stronger solar radiation near the equator.

Water balance in a region depends on the combination of precipitation, evaporation, and runoff. Understanding water balance is essential for sustainable water resource management. What condition leads to drought?

• Runoff decreases.

• Precipitation exceeds evaporation.

• Evaporation exceeds precipitation.

• Clouds disappear.

• Rivers overflow.

Altitude strongly affects both temperature and precipitation. Higher altitudes are generally cooler, which reduces the capacity of air to hold moisture. As moist air rises over mountains, it cools, condenses, and produces rainfall. This is why highland regions in Indonesia, such as Bandung or Dieng Plateau, often experience heavy rainfall and cooler temperatures compared to lowland coastal areas. Why do highland regions in Indonesia often receive more rainfall?

• Winds weaken at higher altitudes.

• Mountains absorb solar radiation directly.

• High altitudes reduce evaporation permanently.

• Rising air cools, condenses, and forms precipitation.

• Rainfall is unrelated to altitude.