つつある details

understand + is in the process of

wakari tsutsu aru

is in the process of; is gradually becoming — a change still under way, viewed from outside

meaning: be gradually understanding

都市としの人口じんこうは年々ねんねん増えふえつつある。

toshi no jinkou wa nennen fue tsutsu aru.

The city's population is gradually increasing year by year. neutral

工事こうじは順調じゅんちょうに進すすみつつある。

kouji wa junchou ni susumitsutsu aru.

The construction is proceeding smoothly. neutral

Examples

is in the process of; is gradually becoming — a change or process still under way

都市としの人口じんこうは年々ねんねん増ふえつつある。

toshi no jinkou wa nennen fuetsutsuaru.

The city's population is gradually increasing year by year. neutral

工事こうじは順調じゅんちょうに進すすみつつある。

kouji wa junchou ni susumitsutsu aru.

The construction is proceeding smoothly. neutral

若者わかものの読書どくしょ離ばなれが、各国かっこくの調査ちょうさで明あきらかになりつつあります。

wakamono no dokushobanare ga, kakkoku no chousa de akiraka ni naritsutsu arimasu.

Surveys in various countries are making it increasingly clear that young people are drifting away from reading. formal

ている
つつある narrows ている's broad progressive/resultative range to a GRADUAL CHANGE still in motion (増えつつある, 回復しつつある) and skews formal/written throughout, where ている covers momentary action, habitual repetition, and resultative state alike, in any register from casual chat to keigo.

つつ
Same tail, unrelated grammar — つつ joins two actions or states within one clause (音楽を聞きつつ歩いた), while つつある fuses onto a single verb to mark a gradual change IN PROGRESS (増えつつある) and cannot take a second clause the way つつ does.