Dew Point, Humidity, Relative Humidity and Hygrometry | Physics for JAMB PDF Download

Introduction

In the field of atmospheric physics, the concepts of dew point, humidity, relative humidity, and hygrometry play a significant role in understanding and measuring the moisture content in the atmosphere. In this set of notes, we will explore these concepts, their formulas (where applicable), and their practical applications, including problem-solving examples.

Dew Point

• Dew point refers to the temperature at which the air becomes saturated and condensation occurs, leading to the formation of dew or frost.
• It is an important parameter for weather forecasting, as it indicates the potential for fog, precipitation, or frost formation.
• The formula for dew point temperature is not required for JAMB Physics.

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Humidity

• Humidity is a measure of the amount of moisture present in the air.
• It is influenced by factors such as evaporation, condensation, and air temperature.
• Humidity is expressed in various units, including grams of water vapor per cubic meter (g/m³) or as a percentage (%).
• The formula for humidity is not required for JAMB Physics.

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Relative Humidity

• Relative humidity (RH) compares the amount of moisture in the air to the maximum amount the air could hold at a given temperature.
• It is expressed as a percentage and indicates how close the air is to saturation.
• The formula for relative humidity is: RH = (Actual Vapor Pressure / Saturation Vapor Pressure) × 100
  Note: Saturation Vapor Pressure is temperature-dependent and not required for JAMB Physics.)

Differentiating between Dew Point, Humidity, and Relative Humidity:

• Dew point is the temperature at which air becomes saturated and condensation occurs, leading to dew or frost formation.
• Humidity refers to the amount of moisture present in the air, expressed in various units.
• Relative humidity compares the amount of moisture in the air to the maximum amount it could hold at a given temperature and is expressed as a percentage.

Hygrometry

• Hygrometry involves estimating the humidity of the atmosphere using hygrometers, such as wet and dry bulb hygrometers.
• Wet and dry bulb hygrometers consist of two thermometers, one dry and the other with a wet cloth covering its bulb.
• The wet bulb thermometer measures the evaporative cooling effect, providing an indication of the atmospheric moisture content.
• The difference between the dry bulb and wet bulb readings is used to determine the relative humidity.

Estimating Humidity Using Wet and Dry Bulb Hygrometers:

• The wet bulb temperature (Tw) and dry bulb temperature (Td) are measured using the hygrometer.
• Using the readings, the difference between the two temperatures (Td - Tw) is determined.
• The difference is then compared to a psychrometric chart or table to find the corresponding relative humidity.

Solving Numerical Problems

Problem: The dry bulb temperature is 30°C, and the wet bulb temperature is 25°C. Determine the relative humidity.
Sol:
Step 1: Calculate the difference between the dry bulb and wet bulb temperatures:
Difference (Td - Tw) = 30°C - 25°C = 5°C
Step 2: Using a psychrometric chart or table, find the relative humidity corresponding to the temperature difference.
Let's assume the chart/table shows a relative humidity of 55% for a 5°C difference.
Step 3: Therefore, the relative humidity is 55%.

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Conclusion

Understanding the concepts of dew point, humidity, relative humidity, and hygrometry is crucial in the field of atmospheric physics. By differentiating between these parameters and utilizing wet and dry bulb hygrometers, we can estimate and solve problems related to atmospheric moisture content.