Why is it hotter in december than june in the southern hemisphere?

Earth

The 2019 June solstice has come and gone. But, for us in the Northern Hemisphere, the hottest weather of the year is still to come. The phenomenon of the hottest weather following the summer solstice by a month or two is called the lag of the seasons.

You can understand it if you’ve ever visited a beach in June. On Northern Hemisphere beaches around now, you’ll notice how cold the ocean feels. Or think about mountaintops in June. Ice and snow still blanket the ground on some high mountains. The sun has to melt the ice – and warm the oceans – before we feel the most sweltering summer heat.

That’s why the hot weather lags behind the year’s longest day and highest sun.

By August, ocean water on that same beach will be much warmer. And the snow line will have crept up the mountaintops. That’s why the hottest weather comes some months after the year’s longest day. The land and oceans simply need those extra months to warm up – a scientist might say to store heat – after the cold of winter.

In the Southern Hemisphere now, the same phenomenon is occurring, but, there, the lag of seasons is delaying the year’s coldest weather. The June solstice, for the Southern Hemisphere, is the winter solstice. The coldest weather comes in July and August because the land and oceans in that part of the world take some extra weeks to give up their stored heat.

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Bottom line: The solstice marks the height of the sun, but the hottest weather comes a month or two later. That’s because the land and oceans have to warm up, too, before the truly hot summer heat can begin. This phenomenon is called the lag of the seasons.

Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.

Throughout the course of the year, most places on Earth goes through four noticeable seasons: summer, autumn (fall), winter and spring, each lasting for about 3 months. The seasons experienced by the northern and southern hemisphere always differ by six months – when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and so on.

Seasons are a direct consequence of the Earth’s tilted rotation axis, which makes an angle of about 23.5 degrees to a line drawn perpendicular to the plane of the ecliptic. The direction of the Earth’s axis stays nearly fixed throughout one orbit, so that at different parts of the orbit one hemisphere ‘leans’ towards the Sun (summer), while the other ‘leans’ away (winter). Six months later, the Earth is leaning in the opposite direction.

The Earth’s tilt causes the Southern Hemisphere (SH) to lean towards the Sun during SH summer. Meanwhile, it is winter in the Northern Hemisphere (NH) which leans away from the Sun. Six months later, the situation is reversed.

For locations north or south of the equator, the main feature accompanying each season is a change in temperature caused by the varying amount of sunlight that falls on each hemisphere of the Earth throughout its annual orbit. The hemisphere tilted towards the Sun will experience longer hours of sunlight, and more direct sunlight.

left: Winter in the Northern Hemisphere – the amount of sunlight falling on the Northern Hemisphere is much less than than… right: …the amount of sunlight falling on the Southern Hemisphere during Southern Hemisphere summer.

As the Sun is higher in the sky during summer, the sunlight reaching the surface is more concentrated. In winter, the Sun is lower in the sky, and sunlight is spread out over a larger area. During spring and autumn, both hemispheres receive about the same amount of sunlight.

At the equator, the temperature variation is much smaller throughout the year, and it is common to consider just two seasons: dry and wet (or monsoon). For observers right at the north pole and the south pole, there are only two seasons – an almost six-month long winter night followed by an almost six-month long summer day! Within the Arctic circle and the Antarctic Circle (latitudes 66.5 degrees north and south), there will be at least one polar day (24 hours of continuous daylight, sometime called the ‘midnight sun’) and one polar night (24 continuous hours of darkness).

The date of the start of the seasons is often chosen to start on the dates of the solstices (summer and winter) and equinoxes (autumn and spring). Alternatively, the start of a new season may be associated with the first day of the month (December, March, June and September) in which a solstice or equinox occurs.

The Earth’s changing distance from the Sun due to the Earth’s elliptical orbit is sometimes thought to cause the seasons. This is incorrect! The Earth’s distance from the Sun varies by about 3% from closest (perihelion distance = 147.09 million km) to furthest approach (aphelion distance = 152.10 million km). This small change in distance cannot account for the temperature differences between summer and winter, and cannot explain how it can be winter in one hemisphere and summer in the other hemisphere.

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This page first made public: Sep 15, 2009

One of the most common and persistent scientific misconceptions is that Earth's seasons are caused by Earth's distance from the sun. A closely related and perhaps more common misconception is that the equator is warmer than the poles because the equator is significantly closer to the sun than are the poles (i.e. the equator "bulges out" toward the sun). Even professional geoscientists sometimes hold the latter misconception. It is, for example, stated as fact in one of the (to remain nameless) "Geology Underfoot" series of guidebooks. Many people combine the two misconceptions. For example, many people know that the southern hemisphere experiences winter while the northern hemisphere experiences summer (and visa versa), but they explain this phenomenon by erroneously stating that the northern hemisphere is closer to the sun in June than it is in December because Earth's tilt toward the sun in June makes the northern hemisphere "bulge out" toward the sun.

The guided-discovery activity described here helps students confront and overcome both of these common misconceptions as it guides students toward an understanding of how and why the angle of incident sunlight determines the intensity of the solar energy that strikes the ground and hence how the angle of incident sunlight can be used to explain both seasonal and latitudinal differences in temperature. Along the way, this activity helps students visualize the true dimensions of the solar system and the various objects within it. This seemingly unrelated topic is included in this activity because an accurate perception of the scale of the solar system helps students understand that (1) Earth's equator is not significantly closer to the sun than are its poles, and (2) all sunrays intercepted by Earth are essentially parallel to each other, whether they strike the equatorial or polar regions -- a concept that is essential for understanding how and why the angle of incident sunlight varies systematically with latitude and season.

• Visualize the true-scale proportions of the solar system -- the sizes of objects and distances between them.
• Clearly and fully explain why it is warmer at the equator than it is at the poles.
• Show how the tilt of Earth's axis and Earth's revolution around the sun cause seasonal variations in temperature by causing seasonal variations in day length and maximum daily solar altitude.

This activity has been extensively tested, revised and retested for more than 15 years in the Concepts in Earth and Space Sciences course for future teachers at California State University, Chico. It should work well in Earth science and astronomy courses from middle school to college level. Because this activity is designed to guide students toward discovery of the concepts, it works best if complete it before reading any explanations of or receiving any direct instruction about the causes of the seasons or why it is warmer at the equator than it is at the poles.

• Student Handout (Microsoft Word 522kB Sep5 09) for the Activity on the Seasons and Why it's Warmer at the Equator than the Poles
• Grid (Acrobat (PDF) 240kB Sep5 09) to be printed on an overhead transparency.
• Other materials needed (in chronological order):
  • Per Table:
    • Flashlight
    • Collection of spherical objects to represent celestial bodies as follows:
      

    • Light fixture with a single unshaded frosted light bulb.
    • Pencil inserted into one 3 inch diameter white Polystyrene ball. An ordinary Styrofoam ball will not do; the ball must be opaque. Suggested source: Molecular Model Enterprises, 116 Swift St., P.O. Box 250, Edgerton, WI 53334, (608)884-9877. Prices are under $1 each.
  • Per classroom
    • Yellow or white exercise ball -- 65 cm (25.5 inch) to 75 cm (29.5 inch) diameter, to represent the sun, which has a diameter of 1.4 million km. (The ideal scale model size would be 70 cm or 27.5 inches.)
    • Exercise ball stand
    • 10 cm Pilates toning ball to represent Proxima Centauri, the nearest star
    • Outdoor space at least 120 m long
    • Tape measure (optional; students can also measure distances by counting paces)
    • 3 ring stands
    • Overhead projector
    • Large piece of white stiff (foam backed) poster board
    • Large globe on a stand; 30" - 36" diameter is best.

General Comments: This activity can be completed in two hours if students give it a superficial treatment, but three to four hours are required for an in-depth exploration, including the confrontation of any misconceptions and the construction of a full understanding of the concepts.

As students work through these activities, encourage them to engage in lively conversations, continually brainstorming, questioning, and checking for consistency. In my experience, students are often inconsistent -- gently call them on this. For example, one student adamantly insisted -- despite the protestations of her team members -- that the equator was not significantly closer to the sun than were the poles. Yet, she then attributed the temperature differences between the equator and the poles to differences in their distances to the sun. I pointed out that she was contradicting herself, which caused her to engage in some deep thinking which culminated in an "Aha!" moment that she absolutely delighted in.

Notes on Lab Activity #1: Scale Model of the Solar System

• I recommend doing the outdoor activity as a whole class with student volunteers to represent each planet; choose the tallest person in the class to pace off the distances. When you get to "Earth," have the students place the moon at the appropriate distance from Earth and point out that, coincidentally, the moon and sun look the same size as seen from Earth.
  
• In the table at the bottom of page 2, fill in appropriate landmarks in your local area that are located at the model distances.
  
• Questions 1 and 2 on page 3 typically generate a great deal of discussion. The students may need a fair amount of prodding and questioning to get to the correct answers.
  

Notes on Lab Activity #2: Why is it Warmer at the Equator Than it is at the Poles?

• Place the globe on a counter so that it is tilted to the right or left (i.e. Spring or Fall position) with the Pacific Ocean facing the overhead projector -- having a relatively featureless part of the globe face the projector helps students see the grid projected on it more clearly, without being distracted by the details of complex shorelines or borders.
  

Notes on Lab Activity on the Causes of the Seasons

• Before students begin this activity, explain to them that each group will be asked to formulate an initial hypothesis to explain the causes of the seasons. The students will then try to use this hypothesis to explain a series of facts. If their hypothesis is not up to the task, they must discard and reformulate, modify, or add to their initial hypothesis until it can provide a satisfactory explanation of the facts.
  

At the end of this activity, I typically assess student learning by having student groups present their answers to the rest of the class. I divide the different parts of this activity among the student groups, assigning each group to prepare illustrations and orally present their part to the rest of the class. Each presentation is then followed by a whole-class discussion.

I assign this Homework Assignment on the Causes of the Seasons (Microsoft Word 58kB Sep5 09) as assessment, but also as a way for students to consolidate the understandings that they have built during the guided-discovery activity. Students often benefit from a formal traditional presentation of concepts that they have previously struggled to grasp while working through a hands-on guided-discovery activity. When the traditional presentation follows guided discovery, it can be very meaningful, building students' confidence in their freshly hatched ideas and organizing their discoveries into a logical and elegant construct. Without some kind of traditional presentation of the concepts, students are often so unsure of themselves that they feel lost. Yet, in my classes, students rarely complete (let alone comprehend the material presented in) a "Read Chapter 3" type of assignment, even when it's followed by a quiz. But the vast majority of students will complete an assignment like the one presented here, which requires students to extract and record specific information from the textbook. Higher-order thinking it's not, but it is a helpful incentive to encourage students to open their textbooks and actually comprehend what is written there.

I give students practice answering questions in low-stakes ConcepTests (using clickers) or on-line practice quizzes before asking such questions on high-stakes exams. Here are some sample questions:

Why is it hotter at the equator than it is at the poles?

a. Because the equator is closer to the sun.

b. Because the sun's rays travel through more atmosphere at the equator.

c. Because the sun's energy is more spread out at the equator.

d. Because the sun's rays hit the earth's surface at a higher angle at the equator.

e. Because the sun is always directly overhead at the equator.

If the Earth's axis only had a 5° tilt, how would the seasons in Chico be different from how they are now?

a. The seasons would be shorter.

b. The transitions between seasons would be more abrupt.

c. The contrast in temperature between summer and winter wouldn't be as great.

d. Summer days would be longer than they are now and winter days would be shorter than they are now.

e. All of the above.

On exams, I ask students open-ended essay questions (Microsoft Word 4.3MB Sep5 09) that require them to synthesize the concepts gained from this lab activity.

Gould, A., Willard, C. and Pompea, S., 2004, The Real Reasons for Seasons: Sun-Earth Connections: GEMS (Great Explorations in Math and Science), Lawrence Hall of Science, University of California, Berkeley.

This teacher's guide features many engaging hands-on activities. Although it was written for teachers of grades 6-8, it is also useful for high school teachers and college professors.

This video was produced by the folks at the Private Universe Project who studied common misconceptions about the seasons and the phases of the moon. To view this video, scroll to the bottom of the page and click on the "VoD" icon. You will have to first register with the Annenburg Media Center (learner.org) to see the video, but registration is free and it grants you access to many other videos for educators.