Humidity and Adiabatic Processes

Exercise 15: Humidity

Objective

• To study the relationship between the water vapor content of the air, temperature, and relative humidity.

Materials

• Sling Psychrometer (optional).

Reference

• Hess, Darrel, and Redina Finch. McKnight's Physical Geography, 13th ed., pp. 150-153.

Humidity

• Humidity refers to the amount of water vapor in the atmosphere, and it is described in several ways:

  • Mixing Ratio:

  • Describes the actual amount of water vapor in the air.

  • Expressed as the mass of water vapor in a given mass of dry air, in grams of water vapor per kilogram of air (g/kg).

  • Specific Humidity:

  • Similar to mixing ratio.

  • Expressed as grams of water vapor per kilogram of air, including water vapor.

  • Absolute Humidity:

  • Describes the mass of water vapor in a given volume of air, expressed in grams of water vapor per cubic meter of air (g/m³).

  • Preference for Mixing Ratio:

  • More useful to meteorologists as it remains constant regardless of changes in volume (as when air rises).

  • Rough comparison:

    • At sea level, 1 cubic meter of air at room temperature has a mass of about 1.4 kilograms.

Relative Humidity

• Provides a measure of how saturated the air is with water vapor.

  • Definition:

  • Ratio of the actual amount of water vapor in the air (mixing ratio) to the maximum amount of water vapor that can be held in the air at a given temperature (capacity).

  • Expressed as a percentage:

  • $ext{Relative Humidity (RH)} = \frac{\text{Actual water vapor content}}{\text{Water vapor capacity}} \times 100$

  • Example of relative humidity values:

  • 50% RH means the air contains 50% of the water vapor necessary for saturation.

  • 100% RH means the air is fully saturated, allowing for condensation and cloud formation.

  • Saturation Mixing Ratio:

  • Maximum amount of water vapor at a given temperature.

  • Independent of air volume; increases with temperature.

  • Implication:

  • Warm air can carry more water vapor than cold air—not as if it holds water, but due to increased capacity.

Calculating Relative Humidity

• Relative humidity is calculated using the formula:

  • $ext{RH} = \frac{\text{Mixing ratio}}{\text{Saturation mixing ratio}} \times 100$

  • Example Calculation:

  • Given: Mixing Ratio = 13.5 g/kg, Saturation Mixing Ratio = 22.5 g/kg

  • $\text{RH} = \frac{13.5 \, \text{g/kg}}{22.5 \, \text{g/kg}} \times 100 = 60\,\% \text{ RH}$

Key Concept: Dew Point Temperature

• The dew point temperature is when relative humidity reaches 100%.

  • Defined as the temperature at which the saturation mixing ratio equals the mixing ratio.

  • Example: A parcel of air with a mixing ratio of 11.1 g/kg has a dew point of 15.6°C (60°F).

  • Relationships:

  • If the temperature is known, you can find the water vapor capacity.

  • If the mixing ratio is known, you can determine the dew point temperature.

  • If the dew point is known, you can determine the mixing ratio.

Exercise 16: Adiabatic Processes

Objective

• To study adiabatic processes in the atmosphere and calculate temperature and humidity changes in moving parcels of air.

Adiabatic Processes

• As a parcel of air rises, it encounters lower pressure and expands, leading to adiabatic cooling (no heat loss or gain).

• Falling air compresses and warms adiabatically.

• Dry Adiabatic Rate (DAR):

  • Roughly 10°C per 1000 meters (5.5°F per 1000 feet).

• Saturated Adiabatic Rate (SAR):

  • Approximately 6°C per 1000 meters (3.3°F per 1000 feet).

  • SAR varies; it can decrease to as low as 4°C per 1000 meters (2.2°F per 1000 feet).

• Lifting Condensation Level (LCL):

  • The height at which condensation begins as air cools to its dew point.

• During condensation, latent heat is released, warming the air parcel and affecting cooling rates.

Example: Temperature Changes in Air Parcel

• As air rises, the temperature changes are shown graphically over elevations (Figure 16-1).

• Descending air typically warms at the DAR but may not if it descends through a cloud.

Stability

• Stable Air:

  • Does not rise unless forced; cooler than surrounding air.

• Unstable Air:

  • Rises on its own; warmer than surrounding air.

• Conditional Instability:

  • May be stable initially, but can become unstable post-condensation.

Exercise 17: Stability

Definitions

• Environmental Lapse Rate (ELR):

  • Average rate of temperature decrease with elevation, typically about 6.5°C per 1000 meters (3.6°F per 1000 feet).

• Differences in ELR can signal weather changes, including temperature inversions.

Stability Categorization

• Stable Air:

  • Parcel is cooler than surrounding air at all elevations, indicating stability.

• Unstable Air:

  • Parcel is warmer than surrounding air, indicating instability and ability to rise independently.

• Conditional Instability:

  • Stability can shift depending on moisture and temperature conditions.

Exercise 18: Midlatitude Cyclones

Overview

• Midlatitude cyclones are significant storm systems characterized by a low-pressure area that spans approximately 1,600 kilometers (1,000 miles).

  • Form where contrasting air masses (warm and cold) converge, leading to fronts: cold front, warm front, and occluded front.

• These cyclones migrate northeast within the prevailing westerlies.

Types of Fronts

1. Cold Front:

   • Cold air advancing under warm air.

2. Warm Front:

   • Warm air advancing over cooler air.

3. Occluded Front:

   • Occurs when a cold front catches up with a warm front.

4. Stationary Front:

   • Both air masses are equally strong; no active advancement.

Weather Implications

• Cold fronts are often associated with intense precipitation due to abrupt uplift of warm air, leading to adiabatic cooling and condensation.

• Warm fronts lead to widespread but less intense precipitation.

Weather Maps

• Fronts illustrate the boundaries between air masses with differing temperatures, humidity, and pressures. Charting these fronts helps in weather forecasting.