Question

Describe what would happen to a parcel of air that begins to rise. Your answer should discuss the normal lapse rate, dry and wet adiabatic lapse rates, and humidity. Include a sketch showing the parcel of air at different altitudes.

Short answer

As the parcel rises, it cools at the DALR until saturation, then at the WALR.

Step-by-step solution

Define Key Terms

Before tackling the exercise, we need to understand a few key terms: 1. **Normal Lapse Rate**: This is the average rate at which atmospheric temperature decreases with an increase in altitude, approximately 6.5°C per 1000 meters. 2. **Dry Adiabatic Lapse Rate (DALR)**: This is the rate of cooling (or warming) of unsaturated air as it rises (or descends), approximately 10°C per 1000 meters. 3. **Wet Adiabatic Lapse Rate (WALR)**: This is the rate of cooling of saturated air (once it has reached its dew point). Due to latent heat release, it cools more slowly than the DALR, about 5°C per 1000 meters.

Air Parcel Begins to Rise

Initially, as the parcel of air begins to rise, it is assumed to be unsaturated. As it ascends, it expands and cools at the Dry Adiabatic Lapse Rate (DALR) of 10°C per 1000 meters.

Reaching the Dew Point

The air parcel continues to rise and cools until it reaches its dew point, where condensation begins. At this altitude, the humidity reaches 100% and the parcel becomes saturated.

Transition to Wet Adiabatic Lapse Rate

Once the air parcel becomes saturated, it cools at the Wet Adiabatic Lapse Rate (WALR) of approximately 5°C per 1000 meters as it continues to rise. This rate is less than the DALR due to the latent heat released during condensation.

Sketch the Scenario

Draw a sketch that shows the parcel of air at three different stages: 1. **At Ground Level**: Indicate the air parcel as unsaturated. 2. **At Dew Point Altitude**: Show the transition where the parcel cools to its dew point and becomes saturated. 3. **Above Dew Point Altitude**: Illustrate the continued rise of the parcel and its slower cooling at the wet adiabatic lapse rate. Label these sections to show the changes in lapse rate and humidity.

Key concepts

Normal Lapse Rate

The normal lapse rate refers to the average rate at which air temperature decreases with an increase in altitude. As you move higher in the atmosphere, the temperature typically drops at about 6.5°C for every 1000 meters (or about 3.6°F per 1000 feet). This steady decline is due to the lower density of air at higher altitudes, which means there is less air to retain heat.
The normal lapse rate provides a baseline to compare other atmospheric lapse rates, such as the dry and wet adiabatic lapse rates. Understanding this rate is essential for meteorologists when predicting weather patterns and climate changes.

Dry Adiabatic Lapse Rate

When an air parcel rises without exchanging heat with its surroundings, it cools at the dry adiabatic lapse rate (DALR). This occurs because the air pressure decreases with altitude, allowing the air parcel to expand and cool as it rises. The DALR is approximately 10°C per 1000 meters.
An important point to consider is that this rate applies only to unsaturated air, meaning that the air has not yet reached its dew point. In the atmosphere, if an air parcel starts to rise, it initially cools at this rate until it becomes saturated. This process plays a crucial role in cloud formation and weather dynamics.

• Air remains unsaturated.
• Cools or warms at approximately 10°C per 1000 meters.

Wet Adiabatic Lapse Rate

Once an air parcel reaches its dew point, it becomes saturated and condensation begins. This releases latent heat, slowing the rate of cooling. The cooling rate at this point is known as the wet adiabatic lapse rate (WALR), which is approximately 5°C per 1000 meters. Due to this latent heat release, the WALR is significantly less than the DALR.
This latent heat is the major reason why clouds and storm systems can sustain themselves at higher altitudes. It also explains why the tops of clouds are much cooler than their bases. The WALR is crucial for understanding precipitation processes and storm development.

• The air is now saturated.
• Cools more slowly than unsaturated air.

Humidity

Humidity refers to the amount of water vapor present in the air. It plays a significant role in weather conditions and atmospheric processes. As the air parcel rises and cools, the relative humidity increases until it reaches 100%, known as the air's saturation point or dew point.
High humidity levels can lead to cloud formation and precipitation. Conversely, low humidity can result in clear, dry weather. Understanding humidity helps meteorologists predict weather changes such as fog, dew, or rainfall, as it dictates how likely it is for atmospheric water vapor to condense.

Dew Point

The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid form. This transformation marks the transition point for a rising air parcel from cooling at the dry adiabatic lapse rate to the wet adiabatic lapse rate.
When the air parcel cools to its dew point, it starts forming clouds, as the water vapor condenses on small particles in the air. This is a critical step in the atmospheric lifecycle, as reaching the dew point often precedes weather phenomena like fog and cloud formation. It is a vital indicator in weather forecasting for predicting visibility and precipitation.

Air Parcel

An air parcel is a small, defined volume of air. Think of it as an imaginary "bubble" that can move freely through the atmosphere. It can be used to visualize the processes of rising, cooling, and condensing discussed in lapse rates.
As an air parcel rises, it expands due to decreasing air pressure at higher altitudes. Initially unsaturated, the parcel cools at the dry adiabatic lapse rate. Upon reaching the dew point, the parcel reaches saturation, releases latent heat, and continues to rise, cooling at the wet adiabatic lapse rate.
Understanding how an air parcel behaves is crucial for demystifying complex atmospheric processes like cloud formation and differing weather conditions due to altitude.