Understanding the environmental lapse rate is crucial in meteorology and various environmental studies. This rate describes how temperature changes with altitude in the atmosphere. While it's not a fixed value and varies based on conditions, learning how to calculate it from observed data is an important skill. This guide will walk you through the process.
What is the Environmental Lapse Rate?
The environmental lapse rate (ELR), also known as the ambient lapse rate, is the rate at which the temperature of the atmosphere decreases with an increase in altitude. Unlike the adiabatic lapse rate (which describes the temperature change of a rising air parcel), the ELR represents the actual temperature profile of the atmosphere at a specific time and location. It's typically expressed in degrees Celsius per kilometer (°C/km) or degrees Fahrenheit per thousand feet (°F/1000 ft).
Key Differences from Adiabatic Lapse Rate: The ELR isn't constant; it fluctuates due to factors like solar radiation, cloud cover, and atmospheric stability. The adiabatic lapse rate, on the other hand, is a theoretical rate based on the expansion and cooling of a rising air parcel.
Calculating the Environmental Lapse Rate: A Step-by-Step Approach
Calculating the ELR involves obtaining temperature readings at different altitudes and then determining the rate of change. Here's how:
1. Gather Data:
You'll need temperature and altitude measurements from at least two different points in the atmosphere. This data can be collected using:
- Radiosonde data: Weather balloons equipped with sensors provide detailed temperature and altitude profiles.
- Weather stations at different elevations: Temperature readings from stations located at varying altitudes (e.g., mountaintop and valley floor) can also be used, but this provides a less detailed picture.
- Atmospheric sounding data: Datasets available from meteorological agencies.
2. Record Your Data: Organize your data in a table with columns for altitude (usually in meters or feet) and corresponding temperature (in Celsius or Fahrenheit).
3. Select Two Data Points: Choose two points from your dataset with significantly different altitudes. The larger the difference in altitude, the more reliable your calculation will be. However, ensure the points are close enough to represent a relatively homogenous atmospheric layer to avoid large-scale atmospheric variations skewing the result.
4. Calculate the Temperature Difference (ΔT): Subtract the temperature at the lower altitude from the temperature at the higher altitude.
5. Calculate the Altitude Difference (Δz): Subtract the lower altitude from the higher altitude.
6. Calculate the Environmental Lapse Rate (ELR): Divide the temperature difference (ΔT) by the altitude difference (Δz).
Formula:
ELR = ΔT / Δz
Where:
ELRis the environmental lapse rateΔTis the change in temperatureΔzis the change in altitude
Units: The units of ELR will depend on the units used for temperature and altitude. Common units are °C/km or °F/1000 ft. Ensure your units are consistent throughout the calculation.
Example:
Let's say you have the following data:
| Altitude (meters) | Temperature (°C) |
|---|---|
| 1000 | 15 |
| 2000 | 10 |
- ΔT = 10°C - 15°C = -5°C
- Δz = 2000m - 1000m = 1000m = 1km
- ELR = -5°C / 1km = -5°C/km
This means the temperature decreases by 5°C for every 1 kilometer increase in altitude. A negative value indicates a decrease in temperature with increasing altitude, which is typical.
Factors Affecting the Environmental Lapse Rate
Several factors influence the ELR, making it highly variable:
- Time of day: Solar radiation significantly impacts temperature profiles.
- Season: Temperature differences between seasons are reflected in the ELR.
- Cloud cover: Clouds can affect both incoming and outgoing radiation, influencing temperatures at different altitudes.
- Geographic location: Latitude and proximity to bodies of water play a role.
- Atmospheric stability: Stable atmospheres generally have lower ELRs.
Conclusion
Calculating the environmental lapse rate provides valuable insight into the atmospheric temperature profile at a specific time and location. Understanding this rate is crucial for various applications, from weather forecasting to aviation safety and climate modeling. By following the steps outlined above and considering the factors influencing the ELR, you can accurately determine this important atmospheric parameter. Remember to always double-check your data and units for accurate results.
