Chapter 13 G.C. Leong’s  Certificate Physical and Human Geography

Chapter 13: Weather 

This chapter distinguishes between Weather (short-term, daily state of the atmosphere) and Climate (the average of weather conditions over 30+ years). To study weather, we must measure its individual elements using specific instruments. 

1. The Elements of Weather & Their Instruments 

To remember these, think of the “5 Main Ingredients” of the atmosphere: 

• Temperature: Measured by a Thermometer.
  • Details: Usually kept in a Stevenson Screen (a white wooden box with louvred sides to allow free air but block direct sunlight).
• Pressure: Measured by a Barometer.
  • Details: High pressure usually means “Fair” weather; Low pressure usually means “Stormy” weather.
• Wind: Measured by an Anemometer (Speed) and a Wind Vane (Direction).
  • Details: Wind is always named after the direction it blows from (e.g., an East wind blows from East to West).
• Humidity: Measured by a Hygrometer (or Sling Psychrometer).
  • Details: It measures the amount of water vapor in the air.
• Precipitation (Rain): Measured by a Rain Gauge.
  • Details: Must be placed in an open area away from trees or buildings to get an accurate “catch.” 

2. Important Concepts to Know 

• Isotherms: Lines on a map joining places with the same Temperature.
• Isobars: Lines on a map joining places with the same Pressure.
• Isohyets: Lines on a map joining places with the same Rainfall.
• Lapse Rate: The rate at which temperature decreases as you go higher. The standard is $6.5\text{°C}$6.5°C for every $1,000\text{meters}$1,000 meters of ascent. 

10 Revision Questions with Mnemonics 

Q1: What is the difference between Weather and Climate? 

• Answer: Weather is Temporary (daily); Climate is Permanent (average over 30 years).
• Mnemonic: “Weather is what you Wear today; Climate is the Closet you own.” 

Q2: What instrument measures Air Pressure? 

• Answer: Barometer.
• Mnemonic: “Barometer measures the Burden of air.” 

Q3: What is a Stevenson Screen used for? 

• Answer: To provide a standardized environment for thermometers, protecting them from direct sun and ground heat.
• Mnemonic: “Stevenson Shelters the Sensors.” 

Q4: If a wind is called a “Southerly,” which way is it blowing? 

• Answer: Toward the North (It comes from the South).
• Mnemonic: “From is the Fact.” (Winds are named from where they start). 

Q5: What do we call lines on a map that connect points of equal rainfall? 

• Answer: Isohyets.
• Mnemonic: “Hyet sounds like Humid” (Rain/Water). 

Q6: What is “Relative Humidity”? 

• Answer: The ratio of water vapor actually in the air compared to the maximum it could hold at that temperature.
• Mnemonic: “Relative is a Ratio.” 

Q7: Which instrument measures wind speed? 

• Answer: Anemometer.
• Mnemonic: “Anemometer measures Air speed.” 

Q8: Why are Stevenson Screens painted white? 

• Answer: To reflect sunlight (insolation) so the thermometer inside doesn’t get artificially hot.
• Mnemonic: “White reflects Wattage (heat).” 

Q9: What is the “Normal Lapse Rate”? 

• Answer: Temperature drops as you go up.
• Mnemonic: “Higher is Harshly cold.” 

Q10: What does a “Barograph” do? 

• Answer: It is a self-recording barometer that draws a continuous line showing pressure changes over time.
• Mnemonic: “Graph means it Generates a drawing.” 

Quick Summary Table: Map Lines (The “Iso” Family)