Unit 5 : Activity 5 : Latitude and Climate : Reason for the Seasons : Seasonal Delay : Reference Page : Unit Exam

Unit 1: How to Point to a Star
Unit 2: Where on Earth Are You?
Unit 3: Earth's Rotation and the Sun's Apparent Motion
Unit 4: Yearly Changes in the Sky
Unit 5: Seasons and Climate
Unit 6: Sundials
Unit 7: Navigation
Unit 8: Ancient Astronomy
Unit 9: Constellations



Seasonal Delay

The Altitude of the Sun isn't the Whole Story

Why do we call the vernal equinox the first day of spring?  We also call the summer solstice, which is the longest day of the year, the first day of summer.  And, the shortest day of the year, the winter solstice, is called the first day of winter.  It seems like it would make more sense to call these days mid-spring, mid-summer, and mid-winter.  So why is that not the case?

In places where the difference between the length of day and night is greater between summer and winter (such as very far north Alaska, Norway, etc), the solstices are called midsummer and midwinter.  But where the temperature variation is more noticeable than the variation in the length of the day, like in Iowa or the lower United States, we use the terms "the first day of summer," etc.

It actually does make some sense.  The coldest day of the year is not generally close to the shortest day, however, but comes a couple of weeks to a month later. Our coldest months are December, January, and February, even though the winter solstice is in December.  So there is a delay, or lag between the shortest day (the day with the least amount of sunlight during the year) and the coldest time of the year.

If we get more direct sunlight (and more sunlight in general due to the longer days) at the summer solstice than on any other day of the year in the Northern Hemisphere, why isn't June 21 (the summer solstice) the hottest day of the year?

On this plot above, the number 1 on the X-axis (the bottom axis) equals "January."  The number 2 = February, and so on.  Notice the chart goes to 24, meaning that it plots temperature curves for 2 full years.  Since the plot has the mean temperature for all of January (the average temperature), we can say the plot point at 1 or 13 best represents what the temperature might be on January 15.

Using the numbering scale on the X-axis as we are, the longest day of the year, June 21, shows up on the graph in the first year as a little over 6.  To be exact it shows up as 6.2.  In the second year, June 21 is the number 18.2.  The peak temperature occurs at number 7 in the plot for the first year, yet the longest day in the year is at 6.2.  This difference indicates that there is a seasonal delay of 0.8 months.

The reason for this delay between maximum heating and maximum heat (temperature) is that it takes time to warm up the ground.  Similarly, in the winter, heat stored in the ground gradually leaks out, giving us our coldest weather in January and not in December, when the shortest day occurs.

While latitude is the main factor in determining your climate as we saw in Unit 4.1, there are a couple of other important factors that affect your local climate as well.


1) Since water has a large heat capacity, people who live near large lakes or oceans tend to have daily and seasonal temperature variations that are (larger or smaller?) than those we experience here in Ames, Iowa where there are no large lakes or oceans nearby.

Larger temperature variations
Smaller temperature variations


Proximity of Water

Below are plots of the temperature averages for the month of January taken over roughly a hundred years for both Rome, Italy and Des Moines, Iowa.

Clearly it is much warmer in Rome, than in Des Moines.  However, Rome and Des Moines have the same latitude, 42N, so it isn't a latitude difference.  It turns out that water is much better at storing energy than dry soil.  So, if you are surrounded by water (as Italy is) then the water stores energy from the summer and releases it throughout the winter, which keeps things warmer (and delays the minimum temperature in the winter as well).

So, let's look at another example.  Why isn't Reykjavik, Iceland colder in the winter than Des Moines, Iowa?  We can see by the plots below that, on average, Reykjavik is warmer in January than Des Moines.  Please note that in the plots, the X-axis is again the temperature average for January.

In this case we see that water isn't only good at storing heat (as we saw in the Rome / Des Moines example above), it is also good at moving heat around the planet.  There is a current of warm water that is heated in the Gulf of Mexico and then flows NE to Iceland.  This "Gulf Stream" carries a lot of heat to Iceland, keeping it from getting too cold in the winter time.

Yet another question we can ask is why isn't the Des Moines weather in January always about the same?  As you can see by the plots above, the average temperature in January can vary from year to year.

To provide the answer, we can look at another example.  This time, we want to ask why Winnipeg, Canada is cooler than Des Moines in July.  Winnipeg gets more sunlight and neither Des Moines nor Winnipeg has much water around them, yet, as you can see by the charts below, Winnipeg is cooler in the summer.

The answer is that arctic air keeps Winnipeg cool in the summer.

These same cool arctic air masses slide down over Minnesota and northern Iowa.  There is often a 10F difference in temperature between Des Moines and Minneapolis, even though Minneapolis is only 3 farther north.  Des Moines weather is very variable in part because the cool air from the arctic and the warm air from the Gulf of Mexico often meet right around here over the middle of the state.  This collision often creates impressive storms.

There is much more that could be said about water, air, and weather -- but this concludes the required core material for this course!

Once you have completed the End of Unit 5 Assignment, go back to the North Star Introduction and review the requirements for the rest of the units in this course.

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