Activity 6: Read and interpret mid-latitude cyclones on a weather map and a satellite image Use the hints above to help you answer these questions. Refer to Figures 2.24, and 2.25 a and 2.25 b on page 82 . 1. Describe and explain the weather conditions (maximum temperature, cloud cover and winds) being experienced in the area on the synoptic weather map in Figure 2.25a behind the cold front labelled C. Give examples from the weather map. 2. Describe how you expect the weather in Port Elizabeth to change over the next two days. Explain each point you make. 3. Sketch the area shown on the satellite image (Figure 2.24). Draw the (8) position of the cold front. 4. Study Figure 2.25b, which shows recordings of weather conditions at Cape Town on three consecutive days as a cold front moved over the area. Describe and explain the changes in weather conditions that took place over the three days. Use headings: 'Temperature', 'Cloud cover, (16) 'Winds' and 'Rain'. 5. Explain why cold-frontal systems occur more frequently over South (4) (3) Africa during the colder months of the year. (2) 6. Draw a vertical cross section through an active cold front. Clearly indicate the following: warm air, cold air, nature of slope, cumulonimbus (6) clouds and rain.

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### **Activity 6: Read and Interpret Mid-Latitude Cyclones on a Weather Map and a Satellite Image**

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#### **1. Weather Conditions Behind the Cold Front (Figure 2.25a)**

**Description of Weather Conditions Behind Cold Front C:**

- **Maximum Temperature:**
  - **Lower Temperatures:** Behind the cold front labeled **C**, temperatures are generally cooler compared to the warm sector ahead of the front. For example, if the warm sector showed temperatures around 25°C, areas behind the cold front might experience temperatures dropping to around 15°C.

- **Cloud Cover:**
  - **Clear to Partly Cloudy Skies:** The passage of a cold front often leads to clearing skies after its passage. Behind Front C, the weather map likely indicates reduced cloud cover, transitioning from overcast conditions to mostly clear or partly cloudy skies.

- **Winds:**
  - **Shift in Wind Direction and Increase in Speed:** Winds behind the cold front typically shift from a southerly or southwesterly direction to a northerly or northwesterly direction. Additionally, wind speeds may increase due to the pressure gradient associated with the cold front’s passage.

**Examples from the Weather Map:**

- **Isobars and Wind Indicators:** Tight isobar spacing behind Front C indicates higher wind speeds. If wind barbs show winds shifting from southwest to northwest, this reflects the typical post-front wind shift.
  
- **Temperature Gradient:** A noticeable drop in temperature labels or shaded areas behind Front C compared to the warm side ahead.

- **Cloud Symbols:** Fewer cloud symbols or transition from thick to scattered clouds behind the front.

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#### **2. Expected Weather Changes in Port Elizabeth Over the Next Two Days**

**Day 1: Passage of the Cold Front**

- **Temperature:** A noticeable drop as the cold front moves through. Expect highs to decrease from, for example, 22°C to around 16°C.
  
- **Cloud Cover:** Initially cloudy with possible rain or thunderstorms as the front approaches, clearing up to partly cloudy skies post-passage.
  
- **Winds:** Winds shift direction (e.g., from southeast to northwest) and may become stronger.
  
- **Rainfall:** Likely precipitation during the front’s passage, possibly brief but intense.

**Day 2: Post-Frontal Conditions**

- **Temperature:** Cooler, stable temperatures around 15°C.
  
- **Cloud Cover:** Mostly clear skies with some lingering high clouds.
  
- **Winds:** Steady northwest winds maintaining the cooler air mass.
  
- **Rainfall:** Minimal to no precipitation expected as the atmosphere stabilizes.

**Explanation:**

The initial impact of the cold front brings abrupt weather changes, including temperature drops and precipitation. Once the front has passed, the region settles into cooler and more stable conditions until the next weather system approaches.

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#### **3. Sketching the Satellite Image and Position of the Cold Front (Figure 2.24)**

*Since a sketch cannot be provided in this format, here is a description to guide your drawing:*

- **Satellite Image Overview:**
  - **Cloud Patterns:** Display the typical arrangement of clouds ahead of and behind a cold front. Ahead of the front, expect extensive stratiform clouds, possibly cumulus or cumulonimbus if precipitation is present.
  
  - **Cold Front Position:**
    - **Sharp Boundary Line:** Draw a line with triangles pointing in the direction of movement to represent the cold front's position.
    - **Orientation:** The front should extend from the cyclone’s center, typically moving from northwest to southeast or another relevant direction based on the specific map.

- **Additional Features:**
  - **High and Low Clouds:** Illustrate high cirrus clouds ahead of the front and lower cumulonimbus clouds along the front’s boundary.
  
  - **Precipitation Indicators:** Show areas of precipitation adjacent to the front where clouds are denser.

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#### **4. Weather Condition Changes in Cape Town Over Three Days (Figure 2.25b)**

**Temperature:**

- **Day 1 (Before Front Passage):**
  - **Warmer Temperatures:** Higher daytime temperatures, possibly around 20°C.
  
- **Day 2 (During Front Passage):**
  - **Significant Drop:** Temperatures decrease sharply to around 12°C as the cold air mass moves in.
  
- **Day 3 (After Front Passage):**
  - **Stable Lower Temperatures:** Temperatures remain cooler, fluctuating between 10°C and 15°C.

**Cloud Cover:**

- **Day 1:**
  - **Scattered Clouds:** Partial cloud cover with some sunny intervals.
  
- **Day 2:**
  - **Overcast Conditions:** Thick cloud cover due to the approaching cold front, increasing precipitation likelihood.
  
- **Day 3:**
  - **Clearing Skies:** Gradual dissipation of clouds leading to partly cloudy or mostly clear skies.

**Winds:**

- **Day 1:**
  - **Mild Winds:** Southeast winds at moderate speeds.
  
- **Day 2:**
  - **Increased Winds:** Winds become stronger and shift to the northwest as the front passes.
  
- **Day 3:**
  - **Stable Winds:** Northwest winds continue but stabilize at a steady speed.

**Rain:**

- **Day 1:**
  - **Light Rain Showers:** Occasional light precipitation as the front approaches.
  
- **Day 2:**
  - **Heavy Rainfall:** Intense and prolonged rain during the front’s passage, possibly thunderstorms.
  
- **Day 3:**
  - **Diminishing Rain:** Rain ceases as the atmosphere stabilizes post-front passage.

**Explanation:**

The transition through a cold front typically involves significant weather changes. Initially, as the front approaches, temperatures are warmer with scattered clouds and light rain. During the front's passage, there is a sharp temperature drop, increased cloudiness, strong winds, and heavy precipitation. Afterward, cooler and more stable weather conditions prevail with clearer skies and reduced wind variability.

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#### **5. Frequency of Cold Frontal Systems Over South Africa During Colder Months**

**Explanation:**

Cold-frontal systems occur more frequently over South Africa during the colder months (typically winter) due to the following reasons:

- **Jet Stream Position:** During colder months, the jet stream shifts southward, bringing more frequent interactions between polar air masses and the warmer air over South Africa. This enhances the formation of mid-latitude cyclones and associated cold fronts.

- **Temperature Gradients:** Increased temperature contrasts between the cold polar air and the warmer subtropical air create stronger pressure gradients, promoting the development and movement of cold fronts.

- **Increased Cyclonic Activity:** Winter is characterized by more active synoptic weather patterns, including low-pressure systems that generate cold fronts as they move from west to east across the continent.

- **Energy Dynamics:** The greater temperature differential in colder months provides more energy for the development and propagation of cold-frontal systems, making their occurrence more frequent.

**Summary:**

The alignment of atmospheric dynamics during colder months, including the position of the jet stream and stronger temperature contrasts, leads to an increased frequency of cold-front systems over South Africa during this period.

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#### **6. Vertical Cross-Section Through an Active Cold Front**

*As a sketch cannot be provided here, the following description outlines how to draw the vertical cross-section:*

**Components to Include:**

- **Warm Air Mass (Ahead of the Front):**
  - **Position:** Represented on the right side of the diagram.
  - **Characteristics:** Warm, moist air shown as a gentle slope rising upwards towards the cold front.

- **Cold Air Mass (Behind the Front):**
  - **Position:** On the left side of the diagram.
  - **Characteristics:** Cold, denser air depicted as a steeper slope beneath the warm air.

- **Slope of the Front:**
  - **Depiction:** The cold front should be shown as a boundary with a steep upward tilt, indicating that cold air is undercutting the warm air.

- **Cumulonimbus Clouds:**
  - **Location:** Draw tall, dense cumulonimbus cloud tops along the cold front boundary where the warm air is being forced to rise over the cold air mass.

- **Rain:**
  - **Illustration:** Rain precipitation should be shown descending from the cumulonimbus clouds towards the ground in the region of the warm air being lifted.

**Steps to Draw:**

1. **Draw the Vertical Axis:** Label the vertical axis as altitude or height.

2. **Illustrate Air Masses:**
   - **Warm Air:** Sloping gently upwards on the right side.
   - **Cold Air:** Sloping steeply upwards on the left side.

3. **Mark the Cold Front Boundary:** A sharp line dividing the two air masses, with triangles pointing in the direction of movement.

4. **Add Cumulonimbus Clouds:** Positioned along the slope where warm air is ascending.

5. **Depict Rainfall:** Lines indicating rain descending from the cloud base into the warm air region.

**Explanation:**

This vertical cross-section illustrates the dynamics of an active cold front, where cold, dense air advances beneath warm, moist air. The steep slope causes the warm air to rise rapidly, leading to the formation of cumulonimbus clouds and associated precipitation along the front.

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*By understanding these concepts and visual representations, you can effectively interpret mid-latitude cyclones, cold fronts, and their associated weather patterns on synoptic weather maps and satellite images.*
**Activity 6: Interpreting Mid-Latitude Cyclones on Weather Maps and Satellite Images** --- 1. **Weather Conditions Behind Cold Front C (Figure 2.25a):** - **Temperature:** Cooler temperatures (e.g., from 25°C to 15°C). - **Cloud Cover:** Fewer clouds, transitioning from overcast to partly cloudy skies. - **Winds:** Wind direction shifts from southwest to northwest, and wind speeds increase. 2. **Expected Weather Changes in Port Elizabeth Over Two Days:** - **Day 1:** Temperature drops from 22°C to 16°C, cloudy with possible rain, winds shift to northwest, and precipitation occurs. - **Day 2:** Cooler temperatures around 15°C, mostly clear skies, steady northwest winds, and minimal rainfall. 3. **Satellite Image Sketch:** - Draw a satellite image showing cloud patterns ahead and behind the cold front. - Indicate the position of the cold front with a line of triangles pointing in the direction of movement. 4. **Weather Condition Changes in Cape Town Over Three Days (Figure 2.25b):** - **Temperature:** Day 1: 20°C; Day 2: 12°C; Day 3: 10-15°C. - **Cloud Cover:** Day 1: Scattered; Day 2: Overcast; Day 3: Clearing. - **Winds:** Day 1: Southeast winds; Day 2: Strong northwest winds; Day 3: Steady northwest winds. - **Rain:** Day 1: Light showers; Day 2: Heavy rainfall; Day 3: Diminishing rain. 5. **Frequency of Cold Fronts in South Africa During Colder Months:** - Cold fronts occur more frequently in winter due to: - Southward shift of the jet stream. - Increased temperature contrasts between polar and subtropical air masses. - More active synoptic weather patterns. - Greater energy for front development. 6. **Vertical Cross-Section Through an Active Cold Front:** - **Warm Air Mass:** Gently sloping on the right, warm and moist. - **Cold Air Mass:** Steeply sloping on the left, cold and dense. - **Cold Front Boundary:** Sharp line with triangles pointing towards the warm air. - **Cumulonimbus Clouds:** Tall, dense clouds along the front where warm air rises. - **Rainfall:** Descending rain from clouds into the warm air region. **Summary:** Understanding mid-latitude cyclones involves analyzing weather maps and satellite images to interpret temperature changes, cloud cover, wind shifts, and precipitation patterns associated with cold fronts. Cold fronts are more common in South Africa during colder months due to atmospheric dynamics that enhance their formation and movement. A vertical cross-section illustrates the vertical structure of a cold front, showing how warm and cold air masses interact and the formation of clouds and precipitation.

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