First, under R, we have QNH — that's listed on pages 21, 23, and 129 — Page 655, Lesson 628

This is the index of the book, so I want to walk you through the entries that start with the letter R and then S, because these are the topics you'll be studying in detail later. Let's look at them one by one. First, under R, we have QNH — that's listed on pages 21, 23, and 129. QNH is the barometric pressure setting you dial into your altimeter so that it reads your altitude above mean sea level when you're on the ground. It's a fundamental setting for maintaining vertical separation and knowing your true height above the terrain. Next, Quasi-stationary Fronts on page 312. A quasi-stationary front is a weather front that is not moving, or moving very slowly — less than about 5 knots. It's a boundary between two air masses where neither is strong enough to push the other out of the way, often leading to prolonged periods of cloud and precipitation. Then we have Radar on page 238. In meteorology, radar — which stands for Radio Detection and Ranging — is used to detect precipitation, its intensity, and its movement. It's a critical tool for nowcasting and avoiding hazardous weather. Radiation on page 63. This is the transfer of energy in the form of electromagnetic waves. In our context, it's primarily about solar radiation coming from the sun and terrestrial radiation emitted by the Earth. Radiation Fog on page 267. This is a specific type of fog that forms on clear, calm nights when the ground cools rapidly by radiating its heat away into space. The air in contact with the cooled ground then cools to its dew point, and fog forms. We'll see a diagram of this. Radiosonde on page 58. A radiosonde is a small, expendable instrument package carried aloft by a weather balloon. It measures temperature, humidity, and pressure as it ascends through the atmosphere, and it radios that data back to a ground station. It's the backbone of upper-air observations. Rain Ice on page 287. This is a type of icing that occurs when rain falls through a layer of sub-freezing air near the surface and freezes on impact with an aircraft or other object. It's also known as freezing rain. Relative Humidity on page 78. This is the ratio of the amount of water vapour actually in the air to the maximum amount the air could hold at that same temperature, expressed as a percentage. It tells you how close the air is to saturation. Ridges on page 44. On a weather chart, a ridge is an elongated area of high pressure. It's the opposite of a trough, and it's typically associated with fair weather and stable conditions. Rime Ice on page 287. This is a type of ice that forms when supercooled water droplets — liquid water below freezing — instantly freeze on contact with an aircraft surface. It has a rough, milky, opaque appearance and can severely disrupt airflow over the wings. Runway State Group on page 478. This is a code used in aerodrome weather reports and forecasts to describe the condition of the runway surface — for example, whether it's dry, damp, wet, or contaminated with snow, slush, or ice. It's critical for landing performance calculations. Runway Visual Range, or RVR, on page 274. This is the distance a pilot can see along the runway from the touchdown zone, as measured by special instruments. It's a more precise measure of visibility for takeoff and landing than the general meteorological visibility. Now moving to the S entries. Satellites on page 465. Weather satellites provide images of cloud cover and other features from space. They are essential for monitoring large-scale weather systems, especially over oceans where we have few other observations. Saturated Adiabatic Lapse Rate on page 89. This is the rate at which the temperature of a rising parcel of saturated air — air at 100% relative humidity — decreases as it expands. It's slower than the dry adiabatic lapse rate because the condensation of water vapour releases latent heat, which warms the parcel. The typical value is around 1.5°C per 1,000 feet, but it varies with temperature. Saturation on page 77. This is the state where the air holds the maximum amount of water vapour possible at a given temperature and pressure. At saturation, the relative humidity is 100%. Saturation Mixing Ratio on page 78. This is the mass of water vapour required to saturate a unit mass of dry air at a given temperature and pressure. It's a specific value that increases as temperature increases. Sea Breezes on page 161. A sea breeze is a local wind that blows from the sea toward the land during the day. It's caused by the differential heating of land and sea — the land heats up faster, creating lower pressure over it, and the cooler, higher-pressure air over the sea moves in to replace it. Secondary Depressions on page 355. These are smaller, weaker low-pressure systems that form within the circulation of a larger, primary depression. They often develop along the cold front of the primary and can bring their own weather. SIGMET on page 518. SIGMET stands for Significant Meteorological Information. It's a weather advisory issued for all aircraft in flight, warning of hazardous weather conditions such as severe turbulence, severe icing, thunderstorms, or volcanic ash clouds. SIGWX chart on page 497. This is a Significant Weather chart, a forecast chart that depicts the expected significant weather for a specific area and time, including fronts, jet streams, turbulence, icing, and cloud cover. Sirocco on page 406. This is a local wind that originates over the Sahara Desert and blows northward across the Mediterranean Sea. It's hot, dry, and often carries dust, and it can bring very poor visibility. Solar Radiation on page 59. This is the electromagnetic radiation emitted by the sun. It's the primary energy source that drives our weather. Squalls on page 150. A squall is a sudden, sharp increase in wind speed that lasts for at least a couple of minutes and is often associated with a rapid change in wind direction. They are commonly found along cold fronts or in thunderstorms. Stability on pages 87 and 90. Atmospheric stability describes the tendency of a parcel of air to resist vertical motion. Stable air resists upward movement, leading to flat, layered clouds and smooth flying conditions. Unstable air encourages vertical motion, leading to cumuliform clouds, turbulence, and showers. Standard Pressure Setting on page 130, and also Standard Pressure Setting (SPS) on page 23. This is the pressure setting of 1013.25 hectopascals, or 29.92 inches of mercury. It's the reference pressure used above the transition altitude to ensure all aircraft are using the same datum for vertical separation, regardless of local pressure variations. Steaming Fog on page 270. Also known as sea smoke or Arctic sea smoke, this fog forms when very cold air moves over much warmer water. The water evaporates into the cold air, quickly saturates it, and the resulting condensation appears as steam or fog rising from the water surface. Stratiform cloud on page 196. This is a type of cloud with a flat, layered, and often featureless appearance. It covers large areas and is associated with stable air and continuous, steady precipitation. The name comes from the Latin word stratus, meaning "layer." And that covers the R and S entries in the index.