Lecture on Humidity in Southeast Texas

Introduction

• Moving to Southeast Texas can feel very humid compared to other parts of the U.S.
• Humidity along the Gulf Coast is notably higher.

Absolute Humidity vs. Relative Humidity

Absolute Humidity

• Definition: Mass of water per unit volume of air.
• Importance: Indicates potential rainfall if air becomes saturated.

Relative Humidity

• Definition: Percentage of water vapor in air relative to air's capacity at different temperatures/pressures.
• RH indicates how much of the air is saturated relative to what it could hold
• Formula: (Actual Vapor Content / Capacity) * 100
• Common in weather reports and apps.
• Importance: Indicates likelihood of air becoming saturated.
• Positive relationship between air temperature & moisture-holding capacity
• Higher Temperatures = higher holding capacity
• Lower Temperature = Lower holding capacity

Relationships Between Temperature and Humidity

Basic Principles

• Higher temperatures can hold more moisture; lower temperatures hold less.
• Elevation impacts vapor capacity due to temperature changes in the troposphere.

Examples

1. 80°F Day: Capacity = 20g, Actual Vapor = 10g → Relative Humidity = 50%.
2. 100°F Day: Capacity = 30g, Actual Vapor = 10g → Relative Humidity = 33.3%.
3. Cooler Day: Capacity = 15g, Actual Vapor = 10g → Relative Humidity = 66.66%.

• Principle: Higher temperature decreases relative humidity (if vapor content is constant). Lower temperature increases relative humidity.

Practical Implications and Weather Observations

Condensation Formation

• Air reaching 100% humidity results in condensation (above 32°F) or deposition (below 32°F).
• Examples: fog, clouds, morning dew.

Tools: Sling Psychrometer

• Measures humidity using dry and wet bulb temperatures.
• Principle: Larger temperature difference between bulbs indicates lower relative humidity.

Formation of Fog

• Higher relative humidity increases likelihood of fog.
• Conditions like wind and evapotranspiration can influence evaporation rates.

Dew Point

• Definition: Temperature at which air reaches 100% relative humidity.
• Importance: More stable than relative humidity throughout the day.
• Higher dew points signify more moisture in the air; can make air feel oppressive.

Weather Patterns and Dew Point

• Dew point is a reliable indicator of how humid it feels; less affected by daily temperature variations.
• High dew points in Southeast Texas due to proximity to Gulf of Mexico and high evapotranspiration levels.
• High dew points lead to higher heat index readings, contributing to discomfort.

Application to Weather Forecasting

Calculating Elevation for Dew Point

• Example: Surface temperature = 80°F, Dew Point = 73°F.
• Atmospheric cooling rate: 3.5°F per 1000 feet.
• Result: Dew point elevation = 2000 feet, which is where clouds would form.

Observing Cloud Levels

• Low clouds indicate higher relative humidity near the surface.
• High clouds in desert areas require significant elevation due to low dew points.