Caused by the localized heating of the earth's surface, which warms the air above it. The warm, moist air expands, becomes lighter, and rises violently, leading to rapid cooling, condensation, and heavy, localized showers or thunderstorms.

Occurs when a moving mass of moist air is forced to rise over a topographic barrier, such as a mountain range. The rising air expands and cools, leading to precipitation on the windward side, while the leeward side (rain shadow) remains relatively dry.

Results from the lifting of air converging into a low-pressure area (cyclone). When two air masses of different temperatures and densities meet (a front), the warmer, lighter air is lifted over the colder, denser air, leading to widespread, moderate precipitation.

Precipitation occurs in various forms. Rain is liquid water droplets with a diameter greater than 0.5 mm, whereas Drizzle consists of fine droplets smaller than 0.5 mm. Snow is formed by ice crystals resulting from sublimation (vapor directly to solid). Hail consists of hard pellets of ice, usually formed in violent cumulonimbus clouds, and Sleet is frozen raindrops or refrozen melted snow water.

The slope of the mass curve at any point represents the rainfall intensity at that specific time. A steep slope indicates high intensity, while a horizontal line indicates no rainfall. Mass curves are derived from recording rain gauges and are useful for extracting maximum intensities for various durations.

It is typically derived from the mass curve and represents the input driving the hydrologic system. The area under the hyetograph equals the total precipitation over the given period. It is the primary input for generating runoff hydrographs.

The accumulated annual rainfall of the suspect station ($Y$) is plotted against the accumulated average annual rainfall of a group of surrounding reliable index stations ($X$). If the record is consistent, the plot will be a single straight line. A break in the slope indicates a change in the regime of the suspect station. The data can be corrected by multiplying the values after the break by the ratio of the slopes (original slope / new slope).

These gauges, such as Symons' Gauge, collect rain for manual measurement at fixed intervals (typically daily at 8:30 AM). They are simple and robust, but only provide total depth, not intensity or variation.

These provide a continuous record of rainfall over time (hyetograph or mass curve). Examples include the Tipping Bucket, which tips after a specific volume collects (excellent for intensity), the Weighing Type which weighs accumulated rain or snow (good for cold climates), and the Float Type where a float rises as the water level increases in a chamber, tracing a mass curve on a rotating drum.

Modern hydrology increasingly relies on remote sensing. Weather Radar estimates rainfall intensity over large areas by transmitting microwaves and measuring the energy reflected back by raindrops (reflectivity). Satellites use infrared and microwave sensors to estimate rainfall globally, especially useful over oceans and remote, ungauged catchments.

Radar does not measure rainfall directly; it measures reflectivity ($Z$), which is a function of the size and number of raindrops. This is converted to rainfall intensity ($R$) using the empirical Z-R relationship, often written as $Z = a \cdot R^b$. The constants $a$ and $b$ vary depending on the type of storm and geographical location (e.g., standard Marshall-Palmer relation is $Z = 200 \cdot R^{1.6}$).

Used when the normal annual precipitation at index stations differs by more than 10% from the normal annual precipitation at the station with missing data. The missing precipitation ($P_x$) is estimated by weighting the precipitation at index stations ($P_1, P_2, \dots, P_m$) by the ratio of their normal annual precipitations ($N_x / N_i$).

For a given duration, the average depth of rainfall decreases exponentially with an increase in area. For a given area, the maximum depth of rainfall increases with an increase in duration. DAD curves are crucial for determining the design storm for large catchments where point rainfall is not representative.

Engineers use IDF curves to determine the 'Design Storm'—the rainfall intensity a structure (like a culvert or sewer) must withstand for a specific return period. For example, a highway culvert might be designed for a 50-year storm.

While IDF (Intensity-Duration-Frequency) curves plot the rate of rainfall (mm/hr) on the y-axis, DDF (Depth-Duration-Frequency) curves plot the total accumulated depth of rainfall (mm) on the y-axis. They convey the exact same statistical information but are used differently. DDF curves show that total depth increases with duration, whereas IDF curves show that average intensity decreases with duration.

A widely used statistical approach for estimating PMP based on historical rainfall data. It applies a frequency analysis equation adapted for extreme values, using a frequency factor ($K_m$) that represents the maximum observed deviation from the mean.