The Veda mantras to bring rain are also found in the Atharva Veda (AV).

The Suktas, AV 4.15 and AV 7.18, are entirely dedicated to rain.

Samikshayantu tavishA prudAnvo (1) ApAm rasA oshadhimihi sachantAm (2) Varshasya sargo mahayantu bhumim (3) Prithag jAyantAm oshadhayo viswrupAh (4)

....

By the grace of mighty ones and auspicious donors (1), let the essence (rasa) in the waters come to the herbs (2). Let the gushes of rain gladden the earth (3); Let all forms of (medicinal) plants be born here and there.

AV 4.15.2

In these Suktas, the Marut Gods, who are the helpers of Indra, are asked to propel the clouds to yield rain.

Some further mantras are as follows:

Apo viddhut abhram varpa sam vo avantu sudhAnava (1) UtsA ajagarA uta (2)

.....

Let the waters, lightning, cloud, rain favour lavish givers (1), also favour the fountains and serpents (ajagarA).

AV 4.15.9

....

Pra navasva prithivi bhindri idam divyam navah (1) Udro divyasya no dhAtarishAno vishvA datim (2)

..........

Earth is asked to split the cloud of heaven. (1) and release the water in the bag.

AV 7.18.2

Yet another mantra (giving only the translation) is:

Open and raise up (udacha) the great covering sheath and spribkle abundantly along with the lightning and wind. Let thew sacrifice be extended (tanvata) and released in amny ways. Let the growths of earth (oshadhayah) become full of delight.

AV 4.15.16

Also, we should not doubt the fact that Yajnas (if properly done) can bring rains. As the scriptures clearly say that it can:

Manu Smriti 3.76. An oblation duly thrown into the fire, reaches the sun; from the sun comes rain, from rain food, therefrom the living creatures (derive their subsistence).

In Veda mantras God Indra has been appealed to release rain in times of draught and to stop downpour in times of excessive rainfall.

Even, in these times, there have been people who have displayed those powers at times of real need alone.

One such person was Sri Ganapati Muni (1878-1936), an adept in Vedas and Tantras and the first disciple of Sri Ramana Maharshi.

Now, those who want empirical evidences of experiments comprising of Yajnas bringing out rain in times when it is needed and stopping it when it has been excessive downpour, can read the book Vedic Metereology by Ravi PrakAsh Arya.

This book records over 400 such experiments, performed in various parts of India, over a period some 40 years in this century.

Several of these experiments have been successful and they have the following key features:

(1) Specification of the appropriate atmospheric conditions as relevant to each season; for instance, wind has to blow from a certain direction in a particular season.

(2) Performance of a prticular type of Yajna accompanied by chanting of Veda Mantras.

(3) Spiritual attainment of the person performing the Yjana. Absolute faith in the efficacy of the procedure is essential to its success. There can be no guarantee of the rain pouring down or its stopping, should an unbeleliever follow the procedure casually with meeting its prerequisites.

So, it is not as easy as chanting few mantras or performing few Yajnas and rain will start pouring down, but its not impossible too.