Radar School › Lesson 0
OPTIONALHow a Storm Works
You will know what an updraft is, why a few keep going when most stop, and why the storms that last are the ones that keep their rain out of their own way. After this, every time Radar School says updraft, inflow, shear or organized, you will be picturing something.
Radar School teaches you to read what a storm looks like. This page is the short version of what a storm is. Nothing here is a prerequisite in the sense that you cannot start without it. It is a prerequisite in the sense that Lesson 3 is about to tell you a sharp edge means a strong updraft, and that sentence is worth more if you already know what an updraft is doing.
Warm air rises, and that is most of it
Warm air is less dense than the air around it, so it floats upward, the same way a cork floats up through water. As warm air climbs, there is less air above pressing down on it, so it expands, and expanding costs energy, so it then cools. Every rising parcel of air is getting colder the whole way up, without anything having to cool it.
As rising air cools, it can eventually reach the point where water vapor begins condensing into droplets. That is a cloud. It is also why the clouds on an ordinary afternoon all have their bases at nearly the same height: they are all finding the same ceiling.
Keep the air rising and the droplets keep growing, bumping into each other and merging, until they are too heavy for the rising air to hold up. Then they fall. That is precipitation, and it is the thing your radar can see.
The two engines
A thunderstorm is two columns of moving air, and almost everything in this course is evidence of one or the other.
The updraft is the column going up. It is fed by warm moist air drawn in near the ground, which is called the inflow. A strong updraft is what makes a storm dangerous, because it is what holds heavy things up long enough for them to get heavier.
The downdraft is the column coming down. Falling rain drags air with it, and some of that rain evaporates on the way, which cools the air and makes it heavier still. So the downdraft is a slug of cold dense air arriving at the ground and spreading out in all directions. That spreading edge is called the gust front, and it is the wind you feel a few minutes before the rain hits.
Why some rising air keeps going
Most rising air does not get far. A parcel rises a little, cools, finds itself the same temperature as its surroundings, and stops. That is every harmless puffy cloud you have ever seen.
It only keeps going if it stays warmer than the air around it. The rising parcel is cooling the whole way up. But if the surrounding air gets colder with height even faster, then the parcel is still the warm one everywhere it arrives, so it keeps floating, and it can keep floating for forty thousand feet.
The word for an atmosphere set up that way is unstable. Instability is the whole difference between a fair weather cumulus and a storm with a top in the stratosphere.
Moisture helps power the updraft
Here is the part that is not obvious. When water vapor condenses it releases heat, and that heat goes into the parcel it condensed in. So the moisture a storm carries up is one thing that helps keep that parcel warmer than its surroundings. Take the moisture away and the parcel cools off and quits.
Which means moisture is not simply the stuff that eventually falls on you. It is the fuel supply. A blazing hot dry afternoon produces nothing at all, and a slightly cooler afternoon with a soggy air mass produces a bad evening. The number that tracks this is the dew point, and it is one of the four things a surface observation shows you.
Something has to give it the first shove
Instability and moisture are potential. They sit there being potential all day. Something still has to lift the air far enough to get the process started, and that is called lift.
Sun heating the ground unevenly can provide lift. Terrain can too, when air is forced up a hill or mountain. Boundaries are a different mechanism. A boundary is a zone in the atmosphere where two different air masses meet. Denser air can wedge underneath lighter air and force it upward along that boundary.
A cold front is the famous one. There are others, and they matter just as much. An outflow boundary is the leading edge of the cold air an earlier storm dumped, still travelling long after that storm is gone. A sea breeze is one. A dryline is one.
Storms fire along boundaries because the boundary is doing the lifting, all at once, along its whole length. That is why so much of what you will see is arranged in lines rather than scattered at random. A line of storms is usually a boundary you are looking at sideways.
Wind shear, and why it is the whole ballgame
Wind shear is wind changing with height. Faster higher up, or blowing from a different direction higher up, or both. It sounds like a detail. It is one of the biggest factors in whether a storm quickly falls apart or stays organized for hours.
With no shear, the storm stands straight up, and much of its precipitation and cold downdraft can fall back into the updraft and the inflow feeding it. A storm interfering with itself like that usually puts itself out in under an hour.
With shear, the storm leans. The updraft goes up and downwind, and the rain falls out of the leaning side, landing beside the updraft rather than on it. The inflow keeps arriving warm and undisturbed. Now the thing can run for hours, and travel a hundred miles doing it.
With enough wind shear, especially when the wind changes direction with height, some storms can develop a rotating updraft. A storm with a persistent rotating updraft is a supercell. Supercells produce many of the strongest tornadoes, although tornadoes can occur with other kinds of storms too. The important connection here is that shear helps create the environment where organized rotation can develop.
Where does the spin come from? Wind that changes with height sets the air between the layers rolling, the way a pencil rolls between two palms moving at different speeds. All that rolling air is lying on its side near the ground, invisible. When an updraft draws it in and hoists it, the roll gets stood up on end, and a horizontal roll stood on end is a rotating column. The spin was in the wind the whole time. The updraft just tips it upright.
Build a storm
You have now met the controls that matter. Set them yourself and watch what the atmosphere builds. Try strong instability with the shear at zero first, and watch what the storm does to its own inflow. Then bring the shear up and see what changes.
That collapse with the shear at zero is the one to carry with you: a storm with no shear tends to end up sitting under its own rain, and the cold air pouring out of it can shut off the warm air feeding it. It is not that the storm ran out of energy. It is that it stepped on its own supply line.
What organized actually means
A storm is organized when its structure lets the updraft keep being fed instead of quickly being cut off by its own rain and cold outflow. Keeping the updraft and downdraft separated is a big part of that.
The shapes you will meet in Lesson 3 are different ways storms organize themselves, from loose clusters to lines to rotating supercells. Better organization often means a storm can last longer and hold a clearer structure on radar.
To recap
Warm, moist air gets lifted along a boundary. If it becomes warmer and less dense than the air around it, it keeps rising. Moisture condenses, releases heat, and a storm builds.
Eventually precipitation falls, helping pull colder air downward. With little wind shear, that downdraft can fall back into the storm's inflow and choke it off.
With stronger wind shear, the storm tilts. Rain falls away from the updraft, warm inflow keeps feeding it, and the storm can stay organized much longer. With enough shear, the updraft can begin to rotate.
That is the movie playing behind every radar image in the next ten lessons.