What does a radiosonde measure and how is the data used in weather forecasting?

A radiosonde measures temperature, humidity, and pressure as a balloon carries it up through the atmosphere, while GPS tracking adds wind direction and speed at each altitude. The data is transmitted to a ground station in real time and used to build upper-air analysis charts, initialize numerical weather prediction models, and plot soundings on the Skew-T Log-P diagram. The National Weather Service launches radiosondes twice daily at 0000 UTC and 1200 UTC from about 90 U.S. stations.

How do you read the Skew-T Log-P diagram to determine atmospheric stability?

On a Skew-T, you lift a hypothetical surface air parcel along a dry adiabat until it cools to its dewpoint (the Lifted Condensation Level, or cloud base), then continue lifting it along a moist adiabat. If the parcel becomes warmer than the surrounding environment above the Level of Free Convection, the atmosphere is conditionally unstable and convection — including thunderstorms — can develop. The area on the diagram where the parcel is warmer than the environment represents Convective Available Potential Energy (CAPE), with larger areas indicating greater severe weather potential.

Why are the temperature lines tilted on the Skew-T diagram instead of being vertical?

The isotherms on a Skew-T are deliberately skewed about 45 degrees to the right so that the plotted temperature trace of a real atmospheric sounding spreads out and becomes easier to analyze. In the real atmosphere, temperature almost always decreases with altitude, which would cause the temperature trace to plot nearly vertically if the isotherms were straight up-and-down, making it hard to distinguish different soundings or assess stability angles. Skewing the isotherms separates the temperature and dewpoint traces visually and makes the critical reference lines — dry adiabats, moist adiabats, and mixing ratio lines — easier to work with.

Radiosonde Soundings and the Skew-T Log-P Diagram

Understanding the vertical structure of the atmosphere is one of the most critical tasks in aviation weather analysis. While surface charts and constant-pressure charts tell meteorologists what is happening at specific altitudes or pressure levels across a wide horizontal area, neither reveals exactly how temperature, moisture, and winds change with altitude at a single location. That vertical profile is captured by a radiosonde sounding and visualized on a specialized chart called the Skew-T Log-P diagram. Together, these tools form a cornerstone of upper-air analysis and are foundational to understanding atmospheric stability, cloud formation, icing, turbulence, and thunderstorm potential.

The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 25, identifies soundings — including radiosondes, dropsondes, pilot balloons (pibals), wind profilers, and Doppler radar VAD wind profiles — as one of the primary observation sources used to build weather analysis charts. Because weather systems are three-dimensional, both surface and upper-air data are required for a complete atmospheric picture. Radiosonde soundings supply the vertical dimension that no surface station can provide.

What a Radiosonde Is and How Soundings Are Collected

A radiosonde is an instrument package attached to a free-lift weather balloon. As the balloon ascends through the atmosphere — typically reaching altitudes of 60,000 to 100,000 feet (roughly 18–30 km) before bursting — the radiosonde measures temperature, humidity (dewpoint), and pressure at frequent intervals and transmits those readings by radio to a ground station. Modern systems also track the balloon's horizontal drift using GPS, which gives a detailed wind profile (direction and speed) at each altitude sampled. The complete vertical snapshot of these variables from the surface to the upper atmosphere is called an upper-air sounding.

The National Weather Service launches radiosondes twice daily from approximately 90 stations across the United States, typically at 0000 UTC and 1200 UTC. These synoptic launch times allow sounding data to be ingested into numerical weather prediction models and used to build the suite of upper-air (constant-pressure) charts that meteorologists and pilots rely upon. When special operations require real-time vertical data — such as forecasting severe weather or supporting aircraft accident investigation — additional launches or dropsondes (radiosondes released from aircraft and falling with a parachute) may be used.

The Skew-T Log-P Diagram: Structure and Logic

Raw sounding data — a list of pressure, temperature, and dewpoint at each altitude — becomes far more useful when plotted graphically. The standard tool for this is the Skew-T Log-P diagram, often simply called a Skew-T. Its somewhat intimidating appearance hides a logical design optimized specifically for atmospheric analysis.

The axes

The vertical axis is pressure plotted on a logarithmic scale, decreasing from roughly 1050 mb at the bottom to about 100 mb at the top. Using a log scale compresses the upper atmosphere while expanding the lower troposphere — the region where most aviation weather occurs — making details easier to read where they matter most.

The horizontal axis represents temperature, but the isotherms (lines of equal temperature) are tilted (skewed) approximately 45 degrees to the right as they rise. This skewing is intentional: in the real atmosphere, temperature nearly always decreases with altitude. If the temperature axis were vertical, the temperature trace would plot as an almost vertical line, making it difficult to distinguish between different soundings or to measure angles that indicate stability. Skewing the isotherms spreads the temperature and dewpoint traces apart, making the diagram much easier to read and interpret.

The reference lines

The Skew-T contains several families of lines, each representing a different physical process:

Isotherms — lines of constant temperature, tilted 45° to the right. Temperatures are in degrees Celsius.
Isobars — horizontal lines of constant pressure (mb), forming the log-P vertical scale.
Dry adiabats — curved lines showing the rate at which a parcel of unsaturated air cools (or warms) when it rises (or descends) without exchanging heat with its surroundings. The dry adiabatic lapse rate is approximately 3°C per 1,000 feet (roughly 10°C per km).
Moist (saturated) adiabats — more gently curved lines showing the slower cooling rate of a saturated air parcel rising through the atmosphere. The moist adiabatic lapse rate varies with temperature and pressure but averages roughly 1.5–2°C per 1,000 feet in the lower troposphere.
Mixing ratio lines (isohumes) — nearly straight lines connecting points of equal water vapor mixing ratio (grams of water vapor per kilogram of dry air). These are used to trace the dewpoint and determine the moisture content of air.

Reading a Sounding on the Skew-T

When a sounding is plotted, two traces appear on the diagram. The temperature trace shows how actual air temperature changes with altitude, and the dewpoint trace (plotted to the left of the temperature trace at each level) shows the dewpoint temperature at the same altitudes. The closer these two traces are to each other, the higher the relative humidity. Where the two traces touch or nearly converge, the air is saturated — a cloud or fog layer is likely present at that altitude.

To assess atmospheric stability, a meteorologist performs a parcel analysis: a hypothetical air parcel at the surface is lifted on the diagram along the dry adiabat until it reaches its Lifted Condensation Level (LCL) — the altitude at which the parcel cools to its dewpoint and condensation begins (i.e., cloud base). Above the LCL, the parcel is lifted along a moist adiabat. The Level of Free Convection (LFC) is the altitude at which the lifted, now-saturated parcel becomes warmer than the surrounding environment and can continue rising on its own buoyancy without further forced lift. If the parcel temperature exceeds the environmental temperature for a significant depth above the LFC, the atmosphere is conditionally unstable and thunderstorm development is possible. The area between the LFC and the Equilibrium Level (EL) — where the parcel finally cools to the environmental temperature — is called CAPE (Convective Available Potential Energy). Large CAPE values signal the potential for severe convection.

Why This Matters for Pilots and Aviation Weather

For pilots, the Skew-T provides actionable information across several hazards:

Icing: The vertical extent and temperature range of saturated layers (where temperature and dewpoint converge) reveal where structural icing is most likely. The freezing level (0°C isotherm) is easily read directly from the diagram.
Turbulence: Large changes in wind speed or direction with altitude (wind shear), visible from the wind barbs plotted on the right margin of the diagram, indicate turbulence potential. Instability layers also correlate with convective turbulence.
Thunderstorm potential: CAPE, LCL height, and the temperature difference between the surface and upper levels can all be assessed quickly. A sounding with a very cold upper troposphere combined with a warm, moist boundary layer is a classic severe-weather setup.
Cloud heights: The LCL approximates cloud base, and the EL approximates cloud top — critical for flight planning in and around convective activity.

Because the NWS launches soundings only twice per day, meteorologists also use output from numerical weather prediction models to generate forecast soundings (called model soundings or model profiles) for any location and any future time. These are widely available through aviation weather products and allow forecasters to anticipate tomorrow's stability profile before the balloon even leaves the ground.

Key Numbers and Rules

Radiosonde launches occur at 0000 UTC and 1200 UTC from approximately 90 NWS upper-air stations in the U.S.
The dry adiabatic lapse rate is approximately 3°C per 1,000 feet (10°C/km).
The standard environmental lapse rate is approximately 2°C per 1,000 feet (6.5°C/km); when actual lapse rate exceeds dry adiabatic, the atmosphere is absolutely unstable.
When the temperature trace on the Skew-T is parallel to dry adiabats, the layer is neutral for dry parcels. When the trace is more vertical (steeper) than the dry adiabats, the layer is absolutely unstable.
Saturated layers (icing, clouds) appear where the temperature and dewpoint traces converge to within approximately 2–3°C.
Surface analysis isobars are commonly drawn at intervals referenced to values divisible by 4 relative to 1000 mb (e.g., 1000, 1004, 1008 mb), though exact conventions can vary by chart and are not part of the Skew-T itself.

Common Test Traps

Confusing pressure altitude with actual altitude. The vertical axis of the Skew-T is pressure in millibars, not feet MSL. A level labeled 500 mb is approximately 18,000 feet MSL under standard conditions, not 500 feet.
Misreading temperature because of the skew. Students often read temperature values horizontally instead of following the tilted isotherms. Always trace along the isotherm diagonally to identify the correct temperature at a given pressure level.
Assuming one sounding represents the whole region. A radiosonde captures the vertical structure at one location at one moment. Horizontal variability can be significant, especially near fronts or convective boundaries.
Forgetting that radiosondes measure dewpoint, not relative humidity directly. Relative humidity must be inferred from the temperature-dewpoint spread plotted on the diagram.
Overlooking wind shear information. Wind barbs are plotted on the side of the Skew-T diagram and are just as important as the temperature and dewpoint traces. Ignoring them means missing critical turbulence and severe-weather indicators.