the 5 fundamentals of weather
Weather seems to be one of those areas where a lot of pilots memorize talking points and regurgitate them by rote, but don’t actually have a firm conceptual understanding of the basic physics at play. My goal with this article is to provide some basic rules that are easy to wrap one’s head around and then apply in order to reason through weather theory. All of this is in the FAA Weather Handbook, which is a well written document, if a bit long-form to remember. Here are the five rules in a list, then broken down with some examples:
- Air is a bad conductor (ABC)
- Coriolis (C)
- Phase changes and inertia of water (∆)
- Solar heating (H)
- Ideal Gas Law (PV=nRT)
Air is a Bad Conductor/Air Parcel Theory (ABC rule)
Simply put, air doesn’t conduct heat well. If you hold your finger directly over a candle, it’ll warm up quickly. Move it an inch to the side, out of the heat plume, and it’s nice and cool. Go on a walk by the lake on a warm day and you might get the occasional cool blob of air, then another warm one right after it. These are examples of air not conducting heat, but retaining whatever temperature is in that blob and not mixing with adjacent blobs. Many forms of insulation, like Styrofoam, are just blobs of air held in with bubbles of plastic.
A common visual for how blobs interact is the air parcel theory, where you imagine that the air is made up of small parcels that, due to their lack of conductivity, move among one another without mixing very much. A hot air balloon is an example of harnessing this effect: a thin fabric holds in a parcel of warm air that’s rising, thus lifting the basket with it. If the balloon is less dense than the surrounding air (usually because the air is warm), it will keep rising.
A common visual for how blobs interact is the air parcel theory, where you imagine that the air is made up of small parcels that, due to their lack of conductivity, move among one another without mixing very much. A hot air balloon is an example of harnessing this effect: a thin fabric holds in a parcel of warm air that’s rising, thus lifting the basket with it. If the balloon is less dense than the surrounding air (usually because the air is warm), it will keep rising.
Coriolis
We live on a spinning ball, which means that the air moving above the spinning ball will behave peculiarly when seen from the surface. If there’s a high and a low pressure zone, you’d expect the air to move directly from H to L due to the pressure gradient force (PGF). It does actually do that, the only problem is that the Earth sneaks away underneath it. At the equator, the surface is moving eastward in excess of 1000 miles an hour, and this decreases to 0 at the poles. If you imagine a zone of high pressure at the equator and a low just north of it, the air will start moving directly north. The issue is that this northerly landmass may only be rotating at 950 MPH, so from the perspective of the airmass, the land will regress to the west at 50 MPH. The corollary then is that when seen from the vantage point of the ground, the south-to-north breeze is drifting off to the east at 50 MPH. If we continue further north, the rotation keeps getting slower, so the eastward deflection continues. Looking at the initial northward vector, the easterly addition means the wind is “off right.” Looking at a north-to-south breeze in the northern hemisphere, the faster eastward moving southern area will “sneak off” to the east underneath the airmass. From the vantage point to that landmass, the wind is now off path to the west, or off right again.
East/west is a bit trickier to conceptualize: off north of the equator, a point east of us will rotate through a circumpolar circle, which we see as an arc to the left. Consequently, a breeze going west-to-east will be off right when seen from the surface. Similarly, if we face westward on our circumpolar arc, the point we look toward will move left, so again the breeze will “drift right” from the surface’s perspective.
The net takeaway from these four examples is that wind in the northern hemisphere will deflect to the right when viewed from the reference frame of the ground. Performing the same thought experiment in the southern hemisphere yields a leftward deflection.
At a weather pattern level, and when viewed from the perspective of the ground, the Coriolis force will, as previously demonstrated, deflect air to the right. As a parcel of air moves along, it will be subject to a continued PGF and Coriolis force. These act on the parcel to result in a spiraling pattern out from the high and diagonally towards the low. As it nears the low, the PGF acts at a greater angle toward the low, drawing the parcel inward in a counterclockwise pattern.
At a weather pattern level, and when viewed from the perspective of the ground, the Coriolis force will, as previously demonstrated, deflect air to the right. As a parcel of air moves along, it will be subject to a continued PGF and Coriolis force. These act on the parcel to result in a spiraling pattern out from the high and diagonally towards the low. As it nears the low, the PGF acts at a greater angle toward the low, drawing the parcel inward in a counterclockwise pattern.
Near the ground, obstructions slow the air, thus weakening the Coriolis-induced rightward deflection. Thus, the PGF has a larger impact on the wind direction at lower altitudes, while higher up the Coriolis effect is more dominant. This means that winds at higher altitude usually come from a more rightward direction. An easy memory aid for this is CCC, "Coriolis Clockwise Climb," meaning that as we climb the wind will come from a more clockwise direction. There are areas around mountains or near the jet stream where this doesn't apply, but more often than not it's true. Knowledge of this can help us pick better altitudes for headwinds and tailwinds. In the below example, the northbound aircraft should cruise at 6,000 feet to get a bit of a push from a quartering tailwind, while the southbound aircraft should be at 8,000 to avoid the quartering headwind.
Flying instrument approaches, you should expect the breeze to weaken and clock left for the above reasons. If you know the power settings and required crab corrections, this will make your life a lot easier.
Phase changes and thermal inertia of water
Water is one of the most thermally-inert substances known to humankind. This means that any temperature change requires lots of heat energy to be added or removed. Phase changes especially consume enormous amounts of energy. It takes 1 cal/g/degree to heat water, and 540 cal/g to change it from a liquid to a gas. To illustrate this effect, you can take a small pot of water and put it on a stove. It will take a few minutes to make the water boil, and then a very long time for the water to completely boil off. It’s the same reason you often feel a chill when you get out of the shower or a pool: the liquid water is absorbing energy from your skin to evaporate it. Sweat, or your dog sticking out its tongue when panting, are natural ways that we harness this phenomenon to thermoregulate. When water condenses from a gas to a liquid, this absorbed energy is released again. If you ever sit outside on a warm, humid day with a cold beverage, you may see condensation forming on the outside of your glass. That water is releasing its heat energy into your drink as it condenses. These phase changes of water absorb or release huge amounts of energy, which have a profound effect on the weather.
Solar heating
The sun puts out a bunch of radiation that hits the ground, ocean, and clouds above the surface. Near the equator at midday, the insolation is vertical, so a given amount gets concentrated on a small surface. In mornings and evenings, or further away from the equator, the insolation angle gets shallower, so the same radiation is spread out over a greater area. The second factor that plays a major role is the albedo of the surface, which is the fraction of the inbound radiation that is reflected. Ice has a high albedo while a lump of charcoal is low. The third factor is thermal inertia: some substances require less heat energy than others to change their temperature a given amount. For example, dark soil on a plowed field will heat up faster than a lake next to it.
Ideal gas law (PV=nRT)
Fog forms over the wing of an A340 as the lower-pressure air cools down past the dewpoint, illustrating the relationship between pressure and temperature.
How many of you, when told about adiabatic lapse rate, thought “Why is the air cooler up there, shouldn’t it be warmer because warm air rises?” The answer comes from the ideal gas law, PV=nRT. n and R are constants that matter to chemists and thermodynamicists, so we can set those aside and worry about PV=T, where P is pressure, V is volume, and T is temperature. If you put a sealed, empty soda bottle out in the sun, it will warm up. Squeeze the side of it and you’ll notice that there’s more pressure within the container. We’ve held volume constant, so as temperature rose, so did pressure. If we go back to our example of the hot air balloon from earlier, the open base of the balloon lets the pressure equalize, so as the temperature goes up, the volume of the gas expands out the bottom, which is why we now have a balloon with reduced-density air that rises. Take a bike pump and push the handle down while touching the bottom of the cylinder. As the pressure goes up and volume decreases, the temperature climbs. Next, turn on a vacuum cleaner and hold your finger in front of it: the air feels cooler because it has a lower pressure than the ambient air. Airplanes will form fog on humid days when the low-pressure air over the wing cools past the dewpoint. These are a few illustrations of the concept. Returning to the question of the cooler air higher up: the reason this works is that the air parcels are stacked up on top of one another in the atmosphere, so the ones lower down are subject to more pressure from the mass of the air parcels above, and hence higher temperature (think bike pump). When you take a parcel and lift it, the increasing volume gives you a pressure and temperature drop.
Putting it all together: applied examples
Sea breeze: solar heating warms land more than the adjacent water (differential thermal inertia). The warm land heats the adjacent air, which then expands and becomes less dense (PV=nRT). This less dense air rises, which creates a low pressure zone inland, allowing the cooler air over the water to move inland.
Tradewinds: the water closer to the equator is warmer because it gets more concentrated solar radiation. Consequently, the equatorial air heats up more than the tropical (~20 degrees latitude) air, and rises. This low pressure draws the air from the tropics toward the equator. On its way to the equator, the air deflects to the right (Coriolis), leading to a northeasterly wind (in the northern hemisphere) that we refer to as the tradewinds. When humid tradewinds strike a mountain (e.g. in the Hawai’ian Islands), they get pushed up the side of it and cool (PV=nRT). When the temperature and dewpoint converge, the water starts to condense and rain out on the windward side of the islands, which gives us the lush green jungles on the windward side and the drier wind shadows on the leeward side.
Cold-front thunderstorm: let’s say you have a warm, humid blob of air that’s barely stable. Next to it, you have a mass of colder air that pushes into the area. These don’t mix (ABC, 1), so instead the cold one pushes under the warm one because it’s denser (PV=nRT, 2). The upward displacement cools the warm air as it expands (more PV=nRT), until it reaches the dewpoint, at which point the water starts to condense, releasing heat (phase change, 3). This heat is released directly into that air parcel and not into the adjacent slightly less humid air (ABC), so the air parcel remains less dense than the surrounding air (PV=nRT), and keeps rising (4). As it rises, the pressure further decreases, so the condensation>heating>rising cycle continues in the cumulus stage (5). At some point, enough water has condensed that it starts combining to form larger droplets that merge (6). At some point, the larger droplets fall faster than the updraft. As a concentration of droplets falls into air lower down, it cools that air, some of which then becomes denser and starts to descend (7). Thus begins the mature phase of the thunderstorm, where the cold columns with precipitation descend next to warm columns that are still rising, all the while not mixing (ABC). Some droplets end up switching columns and making a few round trips, which is how they turn into big hailstones as they freeze near the tops. As more condensation energy is dissipated out the top of the storm, more water precipitates through the column, and we end up with predominantly downdrafts and the storm begins to dissipate (8). The net effect of the thunderstorm is to convectively redistribute energy higher in the atmosphere using water as an energy storage medium, thus restoring cooler surface temperatures, warmer temperatures aloft, and a more stable lapse rate.
Tradewinds: the water closer to the equator is warmer because it gets more concentrated solar radiation. Consequently, the equatorial air heats up more than the tropical (~20 degrees latitude) air, and rises. This low pressure draws the air from the tropics toward the equator. On its way to the equator, the air deflects to the right (Coriolis), leading to a northeasterly wind (in the northern hemisphere) that we refer to as the tradewinds. When humid tradewinds strike a mountain (e.g. in the Hawai’ian Islands), they get pushed up the side of it and cool (PV=nRT). When the temperature and dewpoint converge, the water starts to condense and rain out on the windward side of the islands, which gives us the lush green jungles on the windward side and the drier wind shadows on the leeward side.
Cold-front thunderstorm: let’s say you have a warm, humid blob of air that’s barely stable. Next to it, you have a mass of colder air that pushes into the area. These don’t mix (ABC, 1), so instead the cold one pushes under the warm one because it’s denser (PV=nRT, 2). The upward displacement cools the warm air as it expands (more PV=nRT), until it reaches the dewpoint, at which point the water starts to condense, releasing heat (phase change, 3). This heat is released directly into that air parcel and not into the adjacent slightly less humid air (ABC), so the air parcel remains less dense than the surrounding air (PV=nRT), and keeps rising (4). As it rises, the pressure further decreases, so the condensation>heating>rising cycle continues in the cumulus stage (5). At some point, enough water has condensed that it starts combining to form larger droplets that merge (6). At some point, the larger droplets fall faster than the updraft. As a concentration of droplets falls into air lower down, it cools that air, some of which then becomes denser and starts to descend (7). Thus begins the mature phase of the thunderstorm, where the cold columns with precipitation descend next to warm columns that are still rising, all the while not mixing (ABC). Some droplets end up switching columns and making a few round trips, which is how they turn into big hailstones as they freeze near the tops. As more condensation energy is dissipated out the top of the storm, more water precipitates through the column, and we end up with predominantly downdrafts and the storm begins to dissipate (8). The net effect of the thunderstorm is to convectively redistribute energy higher in the atmosphere using water as an energy storage medium, thus restoring cooler surface temperatures, warmer temperatures aloft, and a more stable lapse rate.
memory aids
The stratiform clouds on the left in the distance are probably in much smoother air than the lumpy ones just under the flap track canoe.
If I’ve done an adequate job of convincing you that weather is a product of these five rules, let’s think of a way to remember them. I’m a big fan of memory jingles, so I came up with “Hot gas boils, pack it and spin it,” broken out as follows:
Characteristics of stable and unstable air memory aid:
Unstable air: Bumpy (turbulence) lumpy (cumuliform clouds) grumpy (showery/unpredictable precipitation)
Stable: 4 Ss: stratiform (clouds), steady (precip), smooth (air), shitty (visibility)
Throw any of the common ACS weather questions over, and I’ll think about which rules to apply to give a good "Explain how ..." answer. I encourage you to apply these as well before any evaluation event. 5 rules are easier to memorize than 25 rehearsed answers plus a hope that your evaluator doesn’t double click. Hint: rehearsing the FAA weather handbook line verbatim is a good way to get your evaluator to ask you to explain, so it's probably better to understand and explain than to memorize. If you have better memory aids, by all means send me them and I’ll add them here if you like.
- Hot (differential heating, H)
- Gas (Gas law, Pv=nRT)
- Boils (Phase changes and heat inertia of water, ∆)
- Pack it (Air parcel theory/ABC)
- Spin it (Coriolis, C)
Characteristics of stable and unstable air memory aid:
Unstable air: Bumpy (turbulence) lumpy (cumuliform clouds) grumpy (showery/unpredictable precipitation)
Stable: 4 Ss: stratiform (clouds), steady (precip), smooth (air), shitty (visibility)
Throw any of the common ACS weather questions over, and I’ll think about which rules to apply to give a good "Explain how ..." answer. I encourage you to apply these as well before any evaluation event. 5 rules are easier to memorize than 25 rehearsed answers plus a hope that your evaluator doesn’t double click. Hint: rehearsing the FAA weather handbook line verbatim is a good way to get your evaluator to ask you to explain, so it's probably better to understand and explain than to memorize. If you have better memory aids, by all means send me them and I’ll add them here if you like.