Sporadic E
Sporadic-E, sometimes called E-skip, is so named because it occurs in the E-layer of the ionosphere, and because it's well, sporadic.  
Sporadic E is generally classified into three types by, Mid-latitude, Equatorial and Auroral. Here equatorial means to within approximately 10 degrees of the Geomagnetic equator.  
Methodology:  The mechanisms of Sporadic E: Equatorial sporadic E is closely associated with the electrojet, an intense electric current within the E layer that travels around the geomagnetic equator during daytime.  
Here in the contiguous United States we generally experience mid-latitude sporadic E which reaches a maximum in the summer.  
During the summer months, ionized particles in the E-layer can clump together, becoming dense enough to reflect radio signals back to earth.  
The E region of the ionosphere extends from about 90-160km above the surface.  
In the daytime the concentration of free electrons can reach 100,000 per cubic centimetre, around one tenth of the F-layer concentration.  
Sporadic E is a sporadic concentration of the E layer ionization into thin layers of high ionization density that are able to reflect radio waves at much higher frequencies than normal.  
These thin layers are in the form of patches, "clouds" of maybe 100km in horizontal extent but only a few km thick.  
This is why sporadic E propagation is so variable, especially at higher VHF frequencies.  
For a signal to propagate via Es a cloud of sufficient ionization must exist at the mid-path point.  
Just like the F layer, a sporadic E cloud will support a maximum frequency, above this frequency signals will not be reflected to earth.  
Because of the limited size of the cloud the areas where propagation occurs are also limited.  
Unlike F-layer propagation, mid-latitude sporadic E does not appear to be correlated with the solar cycle.  
F-layer propagation at HF is caused by the presence of free electrons.  
In the E layer ions are also important, principally O+, O++, NO+, Fe+, Mg+.
The ionization is caused by solar radiation.  
The mechanisms of mid-latitude (our) Sporadic E: How ions become concentrated into thin layers at mid latitudes is still under debate, probably the most accepted current theory is that it is caused by ionospheric wind shear.  
At the height of the E-layer, the majority of the gas is still unionized; it is also very sparse and would probably be considered a fairly hard vacuum on earth.  
The ions are relatively heavy compared to the free electrons and their motion is controlled by that of the neutral gas as well as by the magnetic field.  
Because the overall gas density is low, metallic ions (Fe+,Mg+) can remain ionized for a long enough period to be concentrated into sporadic E clouds under the influence of the winds and the earth's magnetic field.  
Ions moving faster will experience a greater electromotive force due to the magnetic field and will therefore migrate vertically at different rates than ions moving more slowly.
Prediction:   Prediction of Sporadic E is difficult because of its random nature.  
Live MUF short term predictions are a good resource. 
Longer term prediction relies upon statistical analysis of MUF dependent parameters. 
These long term statistics help us predict the probability that a Sporadic E event will occur at a future time and place. 
We can also predict how strong signals are likely to be, so we might say something like there is a 1% probability that there will be a 15 minute sporadic E opening in June where 144MHz signals will be strong enough to make contact between two given stations 1500 miles apart, or that the 50MHz band will be open between the Los Angeles and New York at mid-day for 20% of days during the summer. 
In the mid latitudes between the equatorial regions and Polar Regions, sporadic E is found to occur mainly in summer.  
In the northern hemisphere, the months of May to August yield the highest number of openings with a peak in June, and as a result it is found that the two meter openings generally only occur in June and July.  
Surprisingly, a small peak of sporadic E occurrences is also noticed in December, although these openings rarely affect the higher VHF bands. 
The time of day also has a significant effect on sporadic E.  
There are two main peaks that occur.  
The first is around midday and there is another at around 7 pm.  
It is found that there are fewer occurrences in the afternoon and particularly in the morning.  
There are also very few at night.  
Real time data from beacons in your area are of enormous value.  
An FM receiver can perform a service similar to the beacons.   Reception of distant FM broadcast transmitters can indicate a Sporadic E event. 
Utilization:
50MHz and below:  Sporadic E openings on the 50 MHz, 70 MHz (for those who have access to this band), and 28 MHz amateur radio bands tend to be similar to a normal E layer opening on that band.  
The sporadic E ionization tends to affect these frequencies for much longer and therefore the operating techniques tend to follow those normally used for HF band contacts. 
144 MHz and above:  Sporadic E openings on the 144 MHz amateur radio band tend to be infrequent and short lived.  
Activity tends to increase very rapidly as news of the opening spreads via the Internet, social media and various other means.  
Contest style contacts are normally made, with exchanges consisting of just the callsigns, locator and report. 
The Technician's test teaches us that Sporadic E is the propagation type "most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands".
Because the cloud providing the Sporadic E is moving it is often necessary to adjust your antenna heading to follow the cloud. 
The higher the frequency, the shorter the opening is likely to be, so get your QSL and let the next guy have a shot.