Weather Watch

What weighs on you

The strong invisible force of gravity accelerates trillions of air molecules toward Earth’s surface, producing what we feel as air pressure.

Imagine a 1-square-inch column of air extending from to the top of the atmosphere down to sea level — it would weigh about 14.7 pounds. All the air surrounding Earth weighs about 5,600 trillion tons!

Normally, we don’t notice it because our bodies maintain an internal pressure that balances the external pressure.

But swift changes in elevation can cause us to detect atmospheric pressure changes.

One local example can be found driving over the Cuesta Grade, which is 1,522 feet above sea level.

Maybe you have felt your ears “pop.” This is caused by your inner ear trying to equalize the pressure from the outside air.

In the summer, professor Grace Neff hosted a group of fifth-grade science teachers from San Luis Obispo County at the Central Coast Science Project on the Cal Poly campus. The goal of this workshop was to introduce innovative ways to teach science to fifth-graders.

Cal Poly professor Bob Echols and I had the honor to teach about weather. We used real-life, hold-in-your-hand experiments: a cloud in the bottle, fire cylinders, and my favorite, the lightning-fast crush can, all designed to teach about air pressure.

A good way to get your arms around the concept of air pressure is to perform the lightning-fast crush can experiment.

All you need is an empty aluminum can (an Orange Crush soda can is the best), a hotplate, a pair of kitchen tongs, safety glasses, a 2- or 3-quart glass casserole dish filled with ice water (the colder the better) and gloves.

Note: This experiment requires the use of a hotplate, and children should not perform this experiment without adult supervision. 1. Put on your safety glasses.

2. Fill the aluminum can with about a quarter-inch of water.

3. Heat the can on a hotplate. Please don’t use your bare hands to touch the hot cans or hotplate. Use gloves.

4. When the water at the bottom of the aluminum can boils, the water changes states, from liquid to gas (water vapor). You should be able to hear the water boil and see water vapor steam up. Wait for a few minutes to allow the water vapor to completely purge the air from the can.

5. Using the tongs, quickly and carefully invert the can so that the top of the can is submerged in the glass casserole dish of ice water. Try to have the dish immediately beside the hotplate so that the transfer can be completed in one fluid motion.

6. CRACK! The can will collapse instantly.

The water vapor (gas) that was inside the can, which took up all the interior space, instantly condenses back to its liquid phase and turns into just a few drops of water.

These few drops of liquid water take much less space than the water vapor did, which reduces the pressure inside the can.

Water from the casserole dish is forced in to fill this vacuum, but before it does, air pressure on the walls implodes the can.

The surface area of a 12-ounce aluminum can is about 48 square inches. If you were able to produce a perfect vacuum in the aluminum can at the time of condensation, the atmosphere would apply a total weight of about 700 pounds on it! Of course, this far exceeds what the can is able to sustain.

7. Crush more cans.

8. Make sure you turn off the hotplate after you’re done and do not touch a warm hotplate without gloves.

9. Disposal. Aluminum cans should be recycled.

This week’s forecast

A 1,031-millibar area of high pressure over the Great Basin will produce moderate to fresh (13- to 24-mph) and at times gusty northeasterly (offshore) winds this morning.

These offshore winds will produce dry and clear skies and above-normal temperatures because of compressional heating. In fact, many areas in San Luis Obispo County will range between the mid-70s and the low 80s this afternoon.

A cooling trend will develop Monday as the winds turn out of the northwest (onshore) ahead of an approaching cold front from the Pacific. This onshore flow will allow the marine layer to develop along the coastline by Monday evening.

This cold front will sweep across the Central Coast on Tuesday morning, bringing a chance of a few light rain showers along the northwesterly slopes of the mountains of San Luis Obispo County. The main effect of this cold front will be to set up a steep pressure gradient along the California coast.

This pressure gradient will produce moderate gale- to fresh gale-force (32- to 46-mph) northwesterly winds along the coast and much cooler conditions Tuesday afternoon and night.

A ridge of high pressure will build over California and the Great Basin on Wednesday. This high-pressure ridge will produce clear and warmer conditions with plenty of sunshine Wednesday into Thursday.

A persistent onshore flow may allow the marine layer to redevelop along the Central Coast on Friday.

At this time, it appears that there will be increasing chances for unsettled weather next Sunday through the following week.

Despite this rain season’s near-record-breaking dry conditions, the percent of capacity of the reservoirs and lakes of San Luis Obispo County is about the same as it was last year.

Today’s Surf Report:

A 3- to 4-foot northwesterly (290-degree deep-water) swell (with an 11- to 18-second period) is forecast along our coastline today, increasing to 4 to 6 feet (with a 14- to 16-second period) Monday morning.

Strong- to gale-force (25- to 38-mph) northwesterly winds will generate a 5- to 7-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 14-second period) along the Pecho Coast on Monday afternoon through Tuesday morning.

Moderate gale- to fresh gale-force (32- to 46-mph) northwesterly winds will generate an 8- to 10-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 13-second period) along the coast Tuesday afternoon into Tuesday night.

A 6- to 8-foot northwesterly swell (with an 8- to 11-second period) is forecast along the coastline Wednesday, decreasing to 4 to 6 feet (with an 11- to 17-second period) Thursday.

This northwesterly swell will further lower to 3 to 5 feet (with an 11- to 15-second period) Friday into next Saturday.

Preliminary extended surf analysis

The longer-range models indicate a moderate-energy northwesterly swell should arrive along the Pecho Coast on March 12.

Seawater temperatures

Intake seawater temperatures will range between 50 and 52 degrees through Wednesday.

Thank you, Phyllis Madonna! Your annual Musical Revue and Fashion Show has raised nearly $3 million over the past 25 years for the Women’s Shelter Program of San Luis Obispo County.

Your efforts have provided many women and their children with a safe and warm home for healing and recovery. No doubt, you have saved lives.

As you may know, the Pacific Gas and Electric Company Bright Minds scholarship application closed Feb. 17 at midnight. PG&E will notify the winners and finalists in late May.

John Lindsey is a media relations representative for Pacific Gas and Electric Company. He is also a local weather expert and has lived along the Central Coast for more than 22 years. If you have a question, send him an email at