15.1 The Role of Energy in Weather and Climate

Elements of Weather and Their Instruments

Weather is the condition of the atmosphere at a specific place and time, described through measurable elements. The five primary weather elements and the instruments used to measure them in a Texas 4–8 classroom weather station are:

• Temperature — measured with a thermometer; reported in degrees Celsius or Fahrenheit.
• Humidity (amount of water vapor in the air) — measured with a hygrometer; a psychrometer (sling or digital) compares wet-bulb and dry-bulb thermometers to compute relative humidity.
• Air pressure — measured with a barometer (mercury or aneroid); pressure is reported in millibars or inches of mercury.
• Wind speed — measured with an anemometer (cup or vane type); wind direction is measured with a wind vane.
• Precipitation — measured with a rain gauge; Texas teachers should note that a standard gauge is typically read daily at the same time.

Humidity, Dew Point, and Cloud Formation

Water vapor content is the weather variable students find hardest, and it drives most of what they experience on the Gulf Coast.

• Absolute humidity is the actual mass of water vapor per volume of air.
• Relative humidity is the percentage of the maximum vapor the air can hold at its current temperature. Warm air holds far more vapor than cold air, so relative humidity rises overnight as the air cools even when no water has been added.
• Dew point is the temperature to which air must be cooled, at constant pressure, for it to become saturated. When air temperature and dew point converge, condensation occurs — as dew, fog, or cloud.

A psychrometer measures humidity by comparing a dry-bulb and a wet-bulb thermometer: evaporation cools the wet bulb, and a large difference means dry air while a small difference means humid air. On a muggy August morning in Houston with a temperature of 27 °C and a dew point of 24 °C, the two bulbs read nearly the same and evaporative cooling — including sweating — becomes ineffective, which is the physiological reason humid heat is dangerous.

Clouds form when rising air expands and cools to its dew point and water vapor condenses onto microscopic condensation nuclei such as dust, salt, or pollen. The three lifting mechanisms are convection (surface heating), frontal lifting (one air mass overriding another), and orographic lifting (air forced up a mountain slope). Cloud names combine altitude prefixes (cirro- high, alto- middle) with form roots (cumulus heaped, stratus layered, nimbus precipitating), so a cumulonimbus is a heaped, precipitating storm cloud and a cirrostratus is a high, thin layered sheet.

Weather vs. Climate

A common TExES distractor treats a single hot day as evidence of climate change. The distinction is time scale:

Air masses, fronts, pressure systems, isobars, station models, and severe weather are developed in the next section, "Air Masses, Fronts, Weather Maps, and Forecasting."

Energy Transfers That Drive Weather and Climate

The Sun is the primary energy source for weather. Uneven solar heating between the equator (intense, direct rays) and the poles (low-angle, spread-out rays) sets up convection cells in the atmosphere: warm air rises at the equator, flows poleward aloft, cools, sinks around 30° latitude, and returns as surface trade winds. Three cells per hemisphere (Hadley, Ferrel, Polar) produce the major global wind belts — the trade winds, the westerlies, and the polar easterlies.

The water cycle transfers both matter and energy. Latent heat is the energy absorbed when water evaporates (cooling the surface) and released when water vapor condenses into clouds (warming the air and fueling storms). A single hurricane can release latent heat equivalent to many nuclear weapons per day, illustrating why the water cycle is an energy mover, not just a water mover.

Ocean currents also redistribute heat. The Gulf Stream carries warm water from the Gulf of Mexico toward northwestern Europe, making Britain far warmer than Canadian coasts at the same latitude. Texas students can connect the Loop Current in the Gulf of Mexico to hurricane intensification.

Earth's Position, Orientation, and Surface Features

Seasons come from Earth's 23.5° axial tilt, NOT from a change in Earth–Sun distance. Earth is actually closest to the Sun (perihelion) in early January, during Northern Hemisphere winter. The tilt causes sunlight to strike each hemisphere at a changing angle and day length through the year, producing the solstices and equinoxes.

Latitude determines how much solar energy a region receives on average, setting the broad climate zones: tropical (low latitude, hot), temperate (mid-latitude, seasonal), and polar (high latitude, cold).

Surface features modify that broad pattern:

• Mountains and rain shadows: the windward side of a mountain forces air to rise, cool, and drop rain; the leeward side descends, warms, and dries — creating deserts such as the Chihuahuan Desert in the rain shadow of the Sierra Madre.
• Oceans and coastal moderation: water's high specific heat means coastal cities such as Galveston have smaller daily and seasonal temperature swings than inland Dallas.
• Albedo: the fraction of sunlight reflected by a surface. Fresh snow has high albedo (~0.9) and absorbs little energy; dark asphalt has low albedo (~0.1) and absorbs most energy. Melting sea ice exposes dark ocean, lowers albedo, and amplifies warming — a feedback loop TExES items may probe.
• Jet stream: a high-altitude, fast-moving river of air that steers storm systems across the United States; a persistent dip in the jet stream can bring repeated cold fronts into Texas.