Humidity
Water Vapor Content in the Atmosphere
Definition
Water Vapor Content - Invisible gaseous water in the air, expressed as relative or absolute humidity
Measurement
Relative Humidity (%) - Percentage of maximum moisture air can hold at current temperature
Range
0-100% - 0% is completely dry, 100% is saturated (at dew point)
Understanding Humidity
Humidity is a measure of the amount of water vapor in the air. Warmer air can hold more water vapor than cooler air - this is the key to understanding humidity. Relative humidity (the standard measure) is the actual water vapor content compared to the maximum possible at that temperature, expressed as a percentage. When relative humidity reaches 100%, the air is saturated and water vapor begins to condense into liquid droplets (forming clouds, fog, or dew).
Types of Humidity Measurements
Relative Humidity (%)
Definition: Ratio of actual water vapor to maximum possible at current temperature
Range: 0% (completely dry) to 100% (saturated)
Example: 50% RH at 70°F means air contains half the maximum moisture possible
Daily Variation: Lowest at mid-afternoon (warmest), highest at dawn (coolest)
Standard Measurement: Most commonly reported humidity measure
Absolute Humidity & Dew Point
Absolute Humidity: Actual mass of water vapor per unit volume of air (g/m³)
Dew Point: Temperature at which air becomes saturated (100% RH)
Importance: Dew point indicates moisture content independent of temperature
Example: 70°F with 50°F dew point has moderate moisture
Forecasting: Used to predict fog, frost, and precipitation
Temperature & Humidity Relationship
Critical Concept: Warmer air can hold more water vapor than cooler air. When air is cooled, its capacity for water vapor decreases, and relative humidity increases.
- • Air can hold more water vapor
- • Relative humidity decreases
- • Example: 70°F, 100% RH becomes 70°F warmed to 80°F drops to ~60% RH
- • Air cannot hold as much moisture
- • Relative humidity increases
- • Example: 70°F, 50% RH cooled to 50°F reaches near 100% RH
This relationship explains why mornings are often humid and foggy (cool temperatures) while afternoons are drier (warm temperatures).
Humidity & Human Comfort
Comfort Range
Ideal Humidity: 30-50% relative humidity
Comfortable Zone: 40-60% RH at 70-75°F
Too Dry: Below 30% - respiratory irritation, static electricity
Too Humid: Above 60% - sticky feeling, mold growth, dust mites thrive
Heat Index (Apparent Temperature)
Definition: How hot it feels when combining temperature and humidity
High Humidity Effect: Reduces evaporative cooling from sweat
Example: 90°F with 70% humidity feels like 106°F
Danger: High heat index increases heat-related illness risk
Humidity & Weather Effects
Precipitation Formation
Requirement: High humidity essential for clouds and precipitation
Dew Point: Higher dew points mean more moisture for storms
Instability: Humidity combined with heat drives convective storms
Dew Point 60°F+: Indicates favorable conditions for severe weather
Atmospheric Stability
Moist Air: Less stable, promotes convection and storms
Dry Air: Suppresses development of clouds and precipitation
Temperature Contrast: Humidity differences create boundaries
Dry Lines: Boundaries between dry and moist air trigger storms
Fog & Dew Formation
Dew: Forms when air cools to dew point near ground
Frost: Dew freezes when dew point below 0°C (32°F)
Fog: Dew point reached aloft creates ground-level clouds
Visibility: Fog reduces visibility when RH reaches 100%
Evaporation & Transpiration
Low Humidity: High evaporation from water surfaces and soil
Drought Conditions: Low humidity and dew point worsen dry conditions
Plant Stress: Plants transpire more in low humidity
Fire Risk: Very low humidity increases wildfire danger
Global Humidity Patterns
- • High humidity year-round
- • Dew points 60-70°F+ typical
- • Afternoon RH often 70-90%
- • Frequent thunderstorms
- • Generally very dry
- • Low absolute humidity despite high RH
- • Dew points often below 0°F
- • Little precipitation
- • Extremely low humidity
- • RH drops to 10-20% in afternoon
- • Large daily RH variation
- • Very low dew points
- • Moderate humidity variation
- • Higher in summer/fall
- • Lower in winter/spring
- • Seasonal moisture cycles