Wind, Weather, and You (Part 2)

Under Pressure

Pressure and wind are one in the same. Without a difference in pressure there would be no wind. Water is a special molecule. It's many special properties help build the oceans, feeds the rainforest, and makes up part of every organism on earth. Today we will look at how both these forces work to define the weather around you.

So what are high and low pressure systems?

High Pressure is a larger collection of air molecules in a single air mass compared to surrounding air masses
Low Pressure is a smaller collection of air molecules in a single air mass compared to surrounding air masses


Say we are standing on the equator, right below the sun at its zenith (the place it shines directly 90* down to earth). The suns rays travel through space warming up any matter it hits. Fortunately for most of space it has a nominal amount of matter. When it finally hits something like earth it heats up the mass that then radiates heat away from it into the atmosphere. Furthermore, as you increase in altitude towards space relative pressure decreases as the molecules spread out. This results in a gradient from 90* F at the equator, to -30* F at 30,000 ft, and -454* F in dead space.

That single ray of sunshine heats up your spot on the equator, heating up the air mass at the surface. We know heat rises away from the surface, as it rises it comes in contact with cooler air. This cools off the air mass as it rises and final hits a temperature equilibrium. This new colder air aloft naturally wants to sink back to the surface. The space at the surface has lost air molecules as they rose, while the air aloft has gained air molecules from the new cooled air. There are less molecules at the surface resulting in lower pressure, and more molecules aloft resulting in high pressure.


Air wants to move from high pressure to low pressure
 
So in our singular example at the equator, air will move upwards from heating, then back down to replace the lower pressure created at the surface. This cyclical system is known as a convection current. The same physics used in cooking in your oven are used to cook you on the beach.

The opposite of this effect happens at the poles where there is no heating from the sun. This cold air naturally wants to sink and collects at the surface, creating a high pressure system on the ground and a weak low above it.

Generally all high or low pressure systems are paired with the opposite system aloft.

The uneven heating properties of water and land

Water has a higher specific heat than land

Specific heat is the amount of energy a mass must absorb before rising 1* C. This means certain materials heat up faster than others (this is relative to molecular structure, composition, etc). Metal has a low specific heat requiring very little energy to heat up, compared to better insulators like water which heat up and cool off slowly. So water will take a longer time to heat up, while land will heat up and cool off much faster.

What this also means is a humid pocket of air will react differently than a dry one. The water in the air column will absorb more of the energy resulting in a slower heating of the air mass, and in reverse it will take longer to cool off.

Hot air holds more water molecules than cold air

The result of heating a substance increases its energy and movement of its atoms. This increase in movement creates larger space between each atom. Colder air has less energy and moves less, causing less space between atoms. In the same volume of air, water can occupy more space in the hot air than in the colder. This is why we refer to relative humidity so often in weather.

Relative Humidity- The percentage of water actually in the air compared to how much water it could hold.

Once humidity reaches 100% the water has to condensate out causing either rain, clouds, or fog (clouds at the surface).


That's great, but what can all this explain?
We can in fact predict many weather patterns and phenomenon with these.
 
1. Why it's windy near the ocean and why they shift direction at night. As the sun heats up the land and the ocean, the land heats up much quicker creating a low at the surface, while the cooler ocean creates a high at the surface(remember each one has its own corresponding H or L pressure aloft). The air escaping upwards on land is replaced by air from the ocean creating offshore winds.

At night the opposite occurs, the land cools faster than the ocean creating a H at the surface and the still (relatively) hot ocean creates a L at the surface. The wind moves from H to L and shifts the wind direction 180 onshore.

Let's add the second half of that pretty picture now

2. Why the southern hemisphere is relatively cooler than the northern hemisphere. The southern hemisphere contains less land mass than the southern hemisphere. The ocean is much harder to heat up and stays cooler, the result is a cooler overall climate as there's less land to heat up the atmosphere.

Using the equator as a guide and ignoring Antartica, most of the land on Earth is focused in the northern hemisphere.

3. Why it snows in Hawai'i. In dry conditions you can expect the thermocline (temperature gradient) in the atmosphere to decrease 5.4* F for ever 1000ft altitude and 3.3*/1000ft in high humidity and rain. So an island that is 80*F at the ocean (0 ft altitude) can expect to be 74.6*F at 1000ft, 69.2*F at 2000ft, and somewhere between 5.48*F  or 34.46*F at the summit of Hawaii's tallest volcano Mauna Kea (13,800ft). Now these numbers are not perfect, for one it's never been less than 12*F at Mauna Kea. This is due to the many complications arising from its proximity to the ocean, wind, humidity, relative climate, etc. But on a smaller scale you can guess the temperature of any mountain top from where you're standing. This same effect keeps permanent snow caps (glaciers) on the tallest mountains in the nation and world.

The big island of Hawai'i

4. Why its harder to get below freezing at the beach and harder to get above 100*F. Large bodies of water like the ocean take a lot of energy to heat up and cool down and are thus always lag behind the land in temperature. This means that the air at the ocean surface is going to be warmer than the land when its cold and colder than the land when its hot. High humidity in the air acts the same way and takes longer to heat up and cool off. So land right on the coast takes more energy to heat up and is regularly cooled by the relatively cooler offshore wind. This is why Snowbirds migrate to the southern coast in winter! The difference in temperature from the coast to inland can be as extreme as 20 degrees. (I wish I had a good picture of this from the 1st of February when it was 70* where I am in coastal Mississippi and 50* in central Mississippi.

5. Much, much, more, which will be covered in Part 3