The “Lag of Season” – Why it Stays Hotter Longer into the Summer

"Lag of Season" – Why it Stays Hotter Longer into the Summer

As anyone who has ever stepped outside on a hot summer day knows, the air can take some time to heat up. The same is true for the atmosphere as a whole. Though the summer solstice marks the longest day of the year, it is not necessarily the hottest. That’s because it takes time for the atmosphere to absorb the Sun’s energy and convert it into heat. Similarly, midday is not necessarily the hottest time of day. That’s because it takes some time for the Sun’s rays to travel through the atmosphere and reach the ground. The same principle applies to the summer solstice: it can take some time for the atmosphere to heat up, even though the Sun is at its highest point in the sky.  Today we are going to talk about why it stays hotter longer into the summer or the “Lag of Season”.

From an Expert

It always sticks in my mind as an HVAC technician, but I’m reminded of it when KCRA weatherman Mark Finan talks about it every Summer. In an article on the channel’s weather blog, he said, “June is the month with the highest sun angle and the longest hours of daylight. By the time we get to the solstice (June 20 this year), we have 14 hours and 51 minutes of daylight. By August, we are seeing shorter days and a lower sun angle. By mid-August, we have lost about an hour and a half of daylight.

Despite the shorter days, August is a hotter month than June. The average high on June 1 is 85. On August 1, it is 94. June averages four days in the 100s, while August has 6. So why?

It’s called the lag of seasons…As we transition from Spring to Summer, the Sun gradually warms the surface of the Earth. The gradual warming lags behind the length of the day. So the hottest part of the year is roughly a month after the solstice. The opposite is true in the winter when you’ll find the coldest part of the season is often in January, a month or so after the Winter solstice.”

Lag of Season Explained

What I gather from this is that even though the Sun is at the same angle in the sky in June as it is in August, or May as it is in September, the Earth’s surface is cooler in the earlier summer months tater in the Summer. The winter cools the Earth’s surface, so less radiant heat is absorbed and retained in the atmosphere.

We notice this same phenomenon during the day as noon is not the hottest part of the day; it always falls around 4 pm to 5 pm. That’s the atmosphere heating up and trying to put on the brakes as night comes. It just takes a little longer to reject the day’s heat, which is why it can still be 90 degrees outside at bedtime.

This was a topic I’ve wanted to mention this topic in the Summer before, but I found it hard to explain on paper. Seasonal Lag, or as Mark Finan calls it, “Lag of Seasons,” makes it hotter in the later months of Summer than it is in the earlier months. It’s directly linked to the surface temperature of the Earth and its ability to retain heat as the warmer days upon the surface earth as well as its rivers, lakes, and strea

SEER Rating: How much will I save with a higher SEER Rating?


Most people know that higher SEER ratings for an air conditioner will save them money on their electric bills. But how much can you really expect to save with a higher SEER Rating? Let’s put some actual numbers behind this thought. So, stay tuned to find out how much savings you could see!

What is a SEER rating, and how does it work?

There are plenty of articles about what SEER ratings are for air conditioners and heat pumps, so I’m not going to dive into that topic. But, briefly, says, “The SEER measures air conditioning cooling efficiency, which is calculated by the cooling output for a typical cooling season divided by the total electric energy input during the same time frame. A SEER rating is a maximum efficiency rating, like the miles per gallon for your car.”

What are the current SEER ratings seen today?

14, 15, 16, 17, 18, 19, and 20. Some of the heat pump systems that have come out since 2020 offer up to 25 SEER as well.

As of 2022, most states in America were mandated to install systems with a SEER rating of 14. Some northern states are still able to squeak by with 13 SEER systems because the summers aren’t as long and intense as they are in southern states.

How much money can I save by upgrading to a higher SEER rating air conditioner or heat pump system in my home?

The average cost of electricity in the summer here in Sacramento, CA, is around $0.247 per kWh. So, between 14 SEER and 20 SEER, you’ll save about 6-7% when you go from 14 to 15 to 16 all they way to 20 SEER. Take a look at these charts I made for you, considering a 3-ton system (36,000 btu’s) and you have a house around 1500 sq ft that is fairly well insulated in the attic and exterior walls:

SEER Rating How much will I save with a higher SEER Rating

I created a calculator with some formulas that customers can use to play around with these numbers too. You can find it here on and I’ll leave a link to it in the description area of this video. Just give me a few weeks to get it posted on my website, since I just created it in July 2022.

Compared to your older 10 SEER system

Compared to your older 10 SEER system

A lot of systems that have been running since let’s say, 1995 to 2005 are probably running around 10 SEER. They may have started around 12 SEER, but systems lose efficiency through wear and tear over the years. So, most people agree their system is running around 10 SEER now as they are looking to buy a new system. Since 2010, the lowest SEER system we can install is 14 SEER. In 2023, it’s going up to 15 and 16 SEER-2, so we’ll update this blog when we get there. And yes, I meant to say SEER-2. Just the industry leaders and EPA mandating we get rid of the 14 SEER systems in order to make the 15 and 16 SEER systems the lowest SEER systems that we can install for customers.

Compared to your older 10 SEER system

So, with 14 SEER being the lowest, customers will also be offered options for higher-rated systems, like 15, 16, 17, 18, 19, and 20 – and yes, some systems are even higher. Let’s look at how much you might save by upgrading from your 10 SEER system to a modern 14, 16, 18, or 20 SEER system:

Compared to your older 10 SEER system (2)

So, you can see how upgrading your older 10 SEER system to pretty much anything today will save you money. Even $250 in savings each summer just by switching from an older system to a standard single-stage 14 SEER system sounds great! Upgrading to the 18 and 20 systems can save you double that each summer.  

Comparing the modern systems to each other

Now let’s compare 14 to 16, 16 to 18, and 18 to 20 SEER systems. Once again, these are numbers based on fairly well-insulated homes where the system is installed correctly, and the size of the system is right.

Comparing the modern systems to each other
Comparing the modern systems to each other

If you were given some options from your HVAC salesman – something like a 14 SEER, 16 SEER, an 18 SEER, and a 20 SEER system you might be thinking, “Well, what kind of savings would I get if I buy a 16 SEER over a 14 SEER system if the 16 SEER costs $1500.00 more?”

From the chart, you’d save about $79 a summer with a 16 over a 14 (At 24¢ per kWh)—a 13% savings. Over ten years, that’s $793, and assuming your system lasts a full 20 years, which is a good long life, you’d be looking at about $1587 in savings.

You can mess around with the SEER Comparison Calculator to get a good idea of what you’ll save here in California based of the average rate of 24¢ per kWh at summer rates.

What are the testing requirements to calculate SEER?

Real quick, the industry’s standard for testing includes a high-speed test at 95°F outdoor temp and another at 82°F. Two-stage and variable speed systems add a low-speed test at 82°F. Each of these tests is done for 30 minutes and is performed three times to get an average. I always thought they just calculated these numbers at high-speed, including the variable speed units that typically run at their lower speeds for longer periods of time.

There are way more considerations when calculating these SEER ratings. Some of the terms and testing they use are:

  • Voltage tolerance
  • Low temperature
  • Insulation efficiency
  • Condensate disposal
  • Maximum operation (115°F)
  • Extra high maximum operation (125°F)
  • Wet bulb
  • Dry bulb
  • Piping length
  • A proper refrigerant charge
  • Proper installation

Even just looking at some of the equations these guys use to determine the SEER ratings is crazy. Look at these!

the SEER ratings

And that’s just one calculation and the considerations for determining it. As I scroll down the page on the AHRI document, I have to use the roller to scroll 5 or 6 more times down the page to get to the end of this section!

In conclusion… how much will I save with a higher SEER Rating?

I thought this was a pretty interesting topic. So often, when people ask Google what SEER rating is, they’re just told – it’s a measurement of how much cooling effect they can get for the electricity used to cool the house. Like miles per gallon on a car. And it’s correct. But today, I wanted to put some numbers behind how much you can actually save when choosing between today’s modern air conditioners.

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