WEATHER 01 / LEARN · EXPLORE · CHECK
Atmosphere & wind
Follow the heat. Understand the wind and the performance penalty.
Your learning goals
- Connect uneven heating, pressure, and wind.
- Explain why high pressure does not guarantee good visibility.
- Explain density altitude without confusing indicated and true airspeed.
Start with the sun
Land and water warm at different rates. Latitude, seasons, daylight, and terrain change how much energy reaches the surface. That uneven heating helps drive circulation: air moves vertically and horizontally as pressure and temperature patterns evolve.
Most flying weather is in the troposphere. Its upper boundary, the tropopause, varies in height. Stable air above often limits convection, but strong storms can overshoot it; it is not a solid lid. Dry air is mostly nitrogen and oxygen. Variable water vapor supplies the moisture for clouds and precipitation.
Pressure starts the push
The pressure-gradient force acts from higher toward lower pressure. Closely spaced isobars indicate a stronger pressure gradient. Earth’s rotation deflects moving air to the right in the Northern Hemisphere and left in the Southern Hemisphere.
Away from surface friction, large-scale winds often run approximately along isobars. Near the surface, friction slows the wind and permits flow across isobars toward lower pressure. The depth of this friction layer varies; there is no universal 2,000-foot switch.
In the Northern Hemisphere, surface flow generally spirals inward counterclockwise around a low and outward clockwise around a high. Rising air near lows often favors clouds; sinking air near highs can suppress them. A high can still trap haze or fog.
Hot, high, and humid: give performance more room
Lower pressure, higher temperature, and higher water-vapor content reduce air density when the other relevant conditions are held constant. High density altitude means the airplane performs as though it were at a higher standard-atmosphere altitude.
A normally aspirated engine generally develops less power, propeller performance suffers, and the airplane needs a higher true airspeed for the same indicated airspeed. At the same indicated airspeed, configuration, and angle of attack, lift is approximately unchanged at normal light-airplane speeds. The higher liftoff true airspeed and reduced performance increase the runway needed and reduce climb capability.
Use the aircraft’s POH performance charts with actual conditions, weight, runway, and obstacles. Humidity matters even when a simple chart or estimate does not explicitly include it.
FLIGHT SCENARIO
What would change your plan?
Your morning departure becomes a hot afternoon departure from the same runway. Weight and wind have not changed. What must you recalculate?
- NoticeWhat does this situation require?
- VerifyWhat evidence is still missing?
- DecideWhat keeps an option open?
Compare your reasoning
Recheck takeoff distance, climb performance, and obstacle clearance using the POH. A familiar indicated liftoff speed does not mean the same groundspeed or runway requirement. An earlier departure or lower weight may restore margin.
SUMMARY
Uneven heating gets the air moving.
Recheck takeoff distance, climb performance, and obstacle clearance using the POH. A familiar indicated liftoff speed does not mean the same groundspeed or runway requirement. An earlier departure or lower weight may restore margin.
Before moving on, explain the decision in your own words: what would you verify, and what would make you change the plan?
PRACTICE
Flashcards and knowledge check
Six flashcards, then five questions with explanations.
Enable JavaScript for flashcards and the knowledge check. The lesson and scenario remain available without it.
Sources & lesson notes
Sources & lesson notes
- FAA Aviation Weather HandbookAtmospheric processes, clouds, fronts, hazards, and aviation weather products.
- NOAA Aviation Weather Center: product guideReport formats, forecast coverage, and advisory definitions.
- FAA AIM: MeteorologyBriefing, datalink limitations, and weather avoidance guidance.
Curriculum framework: the supplied Weather Theory and weather products documents. All scenarios are educational examples, not current weather. Content reviewed September 2026.