Atmospheric Weather Variables

• Agrometeorology studies various atmospheric elements or weather variables that influence weather, climate, crop growth, and agricultural production. These variables continuously change with time and place and together determine atmospheric conditions.
• Major Atmospheric Weather Variables

Atmospheric Pressure

• Atmospheric Pressure is the pressure exerted by the weight of atmospheric air on a unit area of the earth’s surface. Air has mass and is held around the earth by gravitational force; therefore, it presses downward and sideways in all directions. This pressure produced by the atmosphere is known as atmospheric pressure.
• Atmospheric pressure is one of the most important weather elements because it controls wind movement, cloud formation, rainfall, storms, and other atmospheric phenomena. Differences in atmospheric pressure between two regions cause the movement of air, resulting in winds.
• The pressure is maximum near the earth’s surface because the lower atmosphere supports the weight of the air above it. As altitude increases, air density decreases and atmospheric pressure gradually becomes lower.

Units of Atmospheric Pressure

Atmospheric pressure is commonly expressed in:

• Millibar (mb)
• Hectopascal (hPa)

Both units are numerically equal. Other units sometimes used are:

• Atmosphere (atm)
• Millimeter of mercury (mm Hg)

Instrument Used for Measurement

Atmospheric pressure is measured by an instrument called a: Barometer

Types of Barometer

1. Mercury Barometer: Uses mercury column to measure pressure.
2. Aneroid Barometer: Uses a sealed metallic chamber and is commonly used in weather stations.

Standard Atmospheric Pressure

The average atmospheric pressure at mean sea level is: 1013.25 hPa. This is equivalent to:

• 760 mm of mercury
• 76 cm Hg
• 1 atmosphere (atm)

Factors Affecting Atmospheric Pressure

1. Altitude: Pressure decreases with increase in height because the amount of air above decreases.
2. Temperature: Warm air expands, becomes lighter, and produces low pressure, while cold air is denser and produces high pressure.
3. Water Vapour: Moist air is lighter than dry air; therefore, humid regions generally have lower pressure.

Importance of Atmospheric Pressure

Atmospheric Pressure is one of the most important atmospheric elements because it controls wind movement, weather changes, rainfall distribution, and atmospheric circulation. Variations in atmospheric pressure greatly influence climate, agriculture, aviation, and human activities.

1. Causes Wind Movement

• Wind is produced due to differences in atmospheric pressure. Air naturally moves from regions of high pressure to regions of low pressure in an attempt to maintain pressure balance. Greater pressure differences result in stronger winds.
• Thus, atmospheric pressure is the primary driving force behind global and local wind systems.

2. Helps in Weather Forecasting

• Changes in atmospheric pressure are important indicators of upcoming weather conditions. Meteorologists use pressure measurements to predict storms, rainfall, cyclones, and clear weather.
• Falling pressure generally indicates cloudy weather and rainfall.
• Rising pressure usually indicates clear and stable weather.
• Therefore, pressure observations are essential in weather forecasting.

3. Influences Rainfall and Climate

• Atmospheric pressure strongly affects cloud formation, atmospheric circulation, and rainfall distribution.
• Low-pressure systems are associated with rising warm air, cloud formation, and precipitation.
• High-pressure systems are associated with descending air, clear skies, and dry weather.
• Seasonal pressure differences also control monsoon winds and climatic patterns.

4. Importance in Agriculture

• Atmospheric pressure has direct and indirect effects on agricultural production and crop environment. It influences:
• Crop evapotranspiration
• Wind circulation
• Moisture movement
• Cloud formation and rainfall
• Temperature and humidity conditions affecting crop growth
• Knowledge of pressure systems helps farmers in weather-based agricultural planning and crop protection.

5. Importance in Aviation and Mountaineering

• At high altitudes, atmospheric pressure becomes low, resulting in reduced oxygen availability. This may cause breathing difficulty, fatigue, and altitude sickness in humans.
• Atmospheric pressure is also important in aviation because aircraft performance, navigation, and altitude measurement depend on air pressure conditions.

Variation of Atmospheric Pressure with Height

• Atmospheric Pressure decreases continuously with increase in altitude. The atmosphere is densest near the earth’s surface because gravitational force pulls air molecules downward. As altitude increases, the amount and density of air above decrease, resulting in lower atmospheric pressure.
• Therefore, atmospheric pressure is highest at sea level and gradually becomes lower at higher elevations such as hills and mountains.

Causes of Pressure Variation with Height

1. Decrease in Air Density: Air density decreases with increasing altitude because air molecules become more widely spaced. Since fewer air molecules are present at higher elevations, the force exerted by air becomes lower, causing a reduction in atmospheric pressure.
2. Reduction in Weight of Overlying Air: At lower altitudes, air pressure is greater because the atmosphere supports the weight of a large column of air above it. As height increases, the overlying air column becomes smaller and lighter, leading to a gradual decrease in pressure.

Important Features of Pressure Variation

• Atmospheric pressure decreases rapidly in the lower atmosphere and more slowly at higher altitudes.
• Nearly 50% of the total atmospheric mass is concentrated below 5 km from the earth’s surface.
• Pressure does not decrease uniformly; it follows an exponential pattern.
• High-altitude regions have lower atmospheric pressure and reduced oxygen availability.
• Low pressure at higher elevations affects weather conditions, human respiration, and plant growth.

Effects of Pressure Variation with Height

1. Effect on Human Health

• At high altitudes, reduced atmospheric pressure results in lower oxygen availability. This may cause breathing difficulty, headache, fatigue, and altitude sickness.
• Example: People traveling to mountainous areas often experience shortness of breath due to low air pressure.

2. Effect on Agriculture

Pressure variation influences:

• Crop adaptation
• Evapotranspiration
• Temperature conditions
• Moisture availability
• Only certain crops are suitable for high-altitude cultivation.

3. Effect on Weather and Climate

Variation in pressure with height affects:

• Wind circulation
• Cloud formation
• Rainfall processes
• Atmospheric stability
• These factors are important in weather forecasting and climate studies.

Effects of Low Atmospheric Pressure

Atmospheric Pressure decreases with increase in altitude. Low atmospheric pressure at higher elevations affects human health, agriculture, and weather processes because the amount of oxygen and air density become lower.

1. Breathing Difficulty

At high altitudes, atmospheric pressure becomes low, resulting in reduced oxygen availability for respiration. Due to insufficient oxygen supply, people may experience:

• Shortness of breath
• Headache
• Fatigue
• Dizziness
• Altitude sickness
• These problems are common in mountainous and hilly regions where air pressure is low.
• Example: People traveling to mountain areas often experience breathing difficulty because oxygen concentration and atmospheric pressure decrease with height.

2. Effect on Agriculture

Low atmospheric pressure combined with low temperature at high altitudes influences crop growth and agricultural productivity.

Major Effects

• Reduced crop growth and development
• Lower evapotranspiration rate
• Slow physiological activities in plants
• Limited adaptation of crops to high-altitude conditions
• Only cold-tolerant and specially adapted crops can grow successfully in mountainous regions.
• Example: Temperate crops such as apple and potato grow well in high-altitude areas, whereas tropical crops cannot survive under low-pressure and low-temperature conditions.

3. Influence on Weather

Variation in atmospheric pressure with height strongly affects atmospheric circulation and weather processes.

Effects on Weather

• Controls wind circulation patterns
• Influences cloud formation
• Affects rainfall distribution
• Determines atmospheric stability and instability
• Low-pressure conditions are generally associated with rising air, cloud development, and precipitation.