3 Temperature and Atmospheric Stability

Temperature measurement and scales

Temperature describes how warm or cold something is and is related to the average motion of its molecules. Meteorologists measure air temperature using thermometers or electronic sensors.

A surface sensor is placed in a ventilated shelter that shields it from direct sunlight and heat radiated by nearby surfaces. This helps the sensor measure the air rather than its exposure to sunlight. Upper-air temperatures are measured at different heights, often with instruments carried by weather balloons.

Common temperature scales are Celsius, Fahrenheit, and Kelvin. Their conversions are:

For example, $20\,^{\circ}\mathrm{C}$ equals $68\,^{\circ}\mathrm{F}$, or $293.15\,\mathrm{K}$. Kelvin is used in many scientific calculations, while Celsius and Fahrenheit are common in weather reports.

How air gains and loses heat

The Sun’s energy mostly passes through the atmosphere and warms Earth’s surface. The surface then warms nearby air. Several processes influence how temperature changes:

• Radiation transfers energy by electromagnetic waves. Sunlight warms land, water, and other surfaces, while Earth also emits infrared radiation.

• Conduction transfers heat through contact. It warms the thin layer of air touching the ground. Because air is a poor conductor, this affects only a shallow layer directly.

• Convection transfers heat as air moves. Uneven surface heating can cause warmer, less-dense air to rise while cooler air sinks or moves in to replace it.

• Advection is the horizontal transport of warm or cold air by wind.

• Evaporation uses energy and can cool a surface or nearby air. Condensation releases that energy back into the atmosphere.

Dark pavement typically warms faster than nearby grass in sunshine. Land also changes temperature more rapidly than water, contributing to daily temperature differences near coasts.

Temperature patterns with time and height

Temperature varies across the day, across seasons, and with location and altitude. At many land locations, the surface and near-surface air warm during the day and cool at night. The daily temperature range is often smaller over water, on cloudy days, and when wind mixes air through a deeper layer.

Clouds can limit daytime warming by reflecting sunlight. At night, they can reduce cooling by absorbing and re-emitting some of the outgoing infrared energy.

In the troposphere, the lowest atmospheric layer where most weather occurs, temperature usually decreases with height. The rate of temperature change with height is the lapse rate. A common average tropospheric lapse rate is about $6.5\,^{\circ}\mathrm{C}$ per kilometre, but the rate varies from place to place and time to time.

An inversion is a layer where temperature increases with height. An isothermal layer has little temperature change with height.

Atmospheric stability and vertical motion

Atmospheric stability describes whether air resists or supports vertical motion. Forecasters assess it by comparing the environmental lapse rate—the temperature change with height in the surrounding atmosphere—with how the temperature of a rising or sinking air parcel changes.

As an air parcel rises into lower pressure, it expands and cools. As it sinks, it compresses and warms. If the parcel is warmer and less dense than its surroundings, it tends to keep rising; if it is cooler and denser, it tends to sink.

An unsaturated rising parcel cools at the dry adiabatic lapse rate, about $9.8\,^{\circ}\mathrm{C}$ per kilometre. Once the parcel becomes saturated, condensation releases heat, so it cools more slowly. The moist adiabatic lapse rate varies with conditions; about $6\,^{\circ}\mathrm{C}$ per kilometre is a useful approximate value.

Comparing the environmental lapse rate with the parcel rates gives three broad cases:

• Stable: The environment cools slowly with height or has an inversion. A displaced parcel tends to become cooler and denser than its surroundings and return toward its starting level. Stable layers limit vertical mixing.

• Unstable: The environment cools rapidly with height. A rising parcel can remain warmer and less dense than the surrounding air, encouraging further ascent and mixing.

• Conditionally unstable: The environmental lapse rate lies between the moist and dry adiabatic rates. An unsaturated parcel may resist rising, while a saturated parcel may continue rising. Moisture and a way to lift the air therefore matter.

For example, suppose the environment cools by $7\,^{\circ}\mathrm{C}$ per kilometre. An unsaturated parcel rising $1$ kilometre cools by about $9.8\,^{\circ}\mathrm{C}$, more than its surroundings, so it becomes relatively cool and tends to resist further ascent. If the parcel is saturated, it cools more slowly—roughly $6\,^{\circ}\mathrm{C}$ per kilometre in this simplified example—so it may remain warmer than the environment and continue rising. This illustrates conditional instability; it does not by itself guarantee clouds or storms. Atmospheric stability is one factor forecasters consider when assessing vertical motion and thunderstorm potential.