Lapse Rate Definition

The temperature of the Earth's atmosphere does not remain constant with height. As you go higher in the atmosphere, the temperature generally decreases. This decrease occurs because the air pressure decreases with altitude, causing the air to expand and cool. The rate at which the temperature changes with increasing altitude in the atmosphere is a fundamental concept in meteorology.

Understanding the Lapse Rate

The specific term used to describe this decrease in temperature of the free atmospheric air with altitude is the lapse rate. It is typically expressed as a change in temperature per unit of height (e.g., degrees Celsius per kilometer or degrees Fahrenheit per thousand feet).

There are different types of lapse rates:

• Environmental Lapse Rate (ELR): This is the actual observed rate of decrease of temperature with altitude at a particular time and location. It varies depending on current weather conditions.
• Dry Adiabatic Lapse Rate (DALR): This is the rate at which an unsaturated parcel of air cools as it rises (approximately 9.8 °C per kilometer or 5.4 °F per thousand feet). It cools due to expansion without heat exchange with its surroundings.
• Moist Adiabatic Lapse Rate (MALR): This is the rate at which a saturated parcel of air cools as it rises. It is lower than the DALR (typically around 5-6 °C per kilometer) because latent heat is released as water vapor condenses during ascent.

Why Other Options Are Incorrect

Let's look at the other options provided:

• Saturation pressure: This refers to the pressure at which the gaseous phase of a substance is in equilibrium with its liquid or solid phase. It's relevant to concepts like dew point but does not describe the general temperature decrease with altitude.
• Coriolis force: This is an apparent force that deflects moving objects (like air currents) sideways due to the Earth's rotation. It affects wind patterns, not the rate of temperature change with height.
• Dew point: This is the temperature at which air becomes saturated with water vapor, and condensation begins. While related to atmospheric moisture and temperature, it does not define the rate at which temperature decreases with altitude.

Therefore, the correct term for the decrease in temperature of the free atmospheric air with altitude is the lapse rate.