The phrase "are sometimes observed" is correct and usable in written English.
It can be used when discussing occurrences or phenomena that do not happen consistently but can be noted at certain times or under specific conditions.
Example: "In nature, certain behaviors are sometimes observed in animals during mating season."
Alternatives: "are occasionally seen" or "can sometimes be noted".
These smaller objects are sometimes observed by using radar.
Nevertheless, regular flow patterns with periods of 20 s and more are sometimes observed.
In particular, significant differences in the critical shear required to stabilize turbulence are sometimes observed.
Also, increases in foF2 prior to intense storms are sometimes observed.
Large feeding flocks of this nighthawk are sometimes observed on warm, cloudy days, after rain, and at dusk.
When building these structures, defects including lack-of-fusion (LOF) at interlayer boundaries and intralayer porosity are sometimes observed.
In seasonal influenza, gastrointestinal symptoms such as diarrhea, vomiting, and abdominal pain are sometimes observed, especially among young children.
We report here that strong anomalous reversals of the equatorial electrojet are sometimes observed and quasi-horizontal plasma waves have been detected for the first time during such an event.
Commonly, since a CCD sensor is sensitive to radiation and high intensity spots due to the direct detection of radiation sources, environmental radiation and cosmic rays are sometimes observed with a long exposure time.
In contrast, layered composite fluids are sometimes observed in confined systems of rather chaotic initial states, for example, lattes formed by pouring espresso into a glass of warm milk.
Surface reflections are sometimes observed in the same frequency range as the diffuse echoes, suggesting that small isolated holes exist in the nightside ionosphere, possibly similar to those that occur on the nightside of Venus.
