Explain temperature, wind and humidity effects
Use evaporation and diffusion to explain environmental changes while respecting plant responses.
Section B8.3 · Transpiration
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall diffusion gradients and controlled investigations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gradient drives outward vapour diffusion?
What you will learn
- Explain each environmental factor through the loss route
- Use matched conditions and stomatal-response limits
Temperature and wind act differently
Higher temperature can increase evaporation and the capacity of air to hold water vapour, often promoting loss under suitable matched conditions. Do not reduce the explanation to “heat makes roots drink”; connect the change with the leaf evaporation and vapour route.
Wind removes humid air from near the leaf and replaces it with drier surrounding air, maintaining a larger outward vapour gradient. In still air, a humid boundary layer can build up. Airflow effects depend on external humidity and whether pores remain open.
Humidity changes the gradient
Higher external humidity usually reduces the water-vapour concentration difference between the internal leaf air spaces and outside air, reducing net diffusion with other factors comparable. Lower humidity can increase that difference and promote loss.
These are conditional predictions. Severe water stress or heat can close stomata and reduce loss; interacting factors can change the result. Compare temperature, humidity, wind, light, hydration and leaf area carefully rather than claiming an unlimited response to one factor.
Sources for this lesson
Pause and explain
Why can wind increase loss in suitable conditions?
Worked example
Explain why a hydrated leaf with open stomata may lose less vapour in humid than dry air at the same temperature.
Show the worked solution
- Internal moist air has a relatively high vapour concentration.
- Humid external air reduces the inside-outside concentration difference.
Answer Higher humidity reduces the outward vapour gradient, so diffusion and loss can decrease under matched conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain a lower loss rate when external humidity rises at matched temperature.
Give me a hint
Use the internal-to-external concentration difference.
Compare my reasoning
- Higher external vapour concentration narrows the gradient.
- A smaller gradient can reduce outward diffusion.
Higher humidity decreases the outward vapour gradient and can reduce loss, with other conditions matched.
Look for these in your work
- Explain the gradient
- Connect it to diffusion
Give distinct explanations for increasing wind and increasing temperature.
Give me a hint
One changes surrounding air; one affects evaporation.
Compare my reasoning
- Wind removes the humid boundary layer and can maintain the gradient.
- Temperature can increase evaporation and vapour-transfer potential.
Wind: humid-air removal. Temperature: increased evaporation potential, under suitable controlled conditions.
Look for these in your work
- Explain wind
- Explain temperature without conflating them
A hotter dry treatment gives lower uptake after the leaves wilt and pores close. Does it disprove the usual conditional predictions?
Give me a hint
Check whether plant condition stayed comparable.
Compare my reasoning
- Wilting and closure change leaf condition and open pore area.
- The usual prediction assumes suitable hydration and comparable stomatal conditions.
No. Stress and closure can reduce loss; the treatment no longer isolates temperature with comparable functioning leaves.
Look for these in your work
- Identify the changed plant response
- Keep the prediction conditional
Common mistakes
- Saying humidity blocks every water molecule physically.
- Claiming temperature or wind always increases loss without limits.
What can you explain now?
Why can wind increase loss in suitable conditions?
Compare with the explanation
It removes humid air near the leaf
Replacement with drier air maintains the outward gradient.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain all three factors through evaporation or vapour diffusion, then give one response that limits the trend.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.