The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases

dSolar radiation at the frequencies of visible light largely passes through the atmosphere to warm the planetary surface, which then emits this energy at the lower frequencies ofinfrared thermal radiation. Infrared radiation is absorbed by greenhouse gases, which in turn re-radiate much of the energy to the surface and lower atmosphere. The mechanism is named after the effect of solar radiation passing through glass and warming a greenhouse, but the way it retains heat is fundamentally different as a greenhouse works by reducing airflow, isolating the warm air inside the structure so that heat is not lost by convection.^]

If an ideal thermally conductive blackbody were the same distance from the Sun as the Earth is, it would have a temperature of about 5.3 °C. However, since the Earth reflects about 30%^] of the incoming sunlight, this idealized planet's effective temperature (the temperature of a blackbody that would emit the same amount of radiation) would be about −18 °C.^[ The surface temperature of this hypothetical planet is 33 °C below Earth's actual surface temperature of approximately 14 °C.^[] The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect.^[

Earth’s natural greenhouse effect is critical to supporting life. However, human activities, primarily the burning of fossil fuels and clearing of forests, have intensified the natural greenhouse effect, causing global warming.

वायु में बहुत से तत्त्व होते हैं जो पौधों और पशुओं (मानव समेत) का स्वास्थ्य ख़राब कर सकते हैं या नजर ख़राब कर सकते हैं यह प्राकृतिक प्रक्रियाओं तथा मानव गतिविधियों दोनों से उत्पन्न होते हैं। वायु में प्राकृतिक रूप से नहीं पाए जाने वाले तत्व या अधिक सांद्रता के साथ या सामान्य से अलग तत्वों को प्रदूषक कहा जाता है।

प्रदूषकों को प्राथमिक या द्वितीयक के रूप में वर्गीकृत किया जा सकता है। प्राथमिक प्रदूषक वे तत्व हैं जो सीधे एक प्रक्रिया से उत्सर्जित हुए हैं जैसे ज्वालामुखी विस्फोट से राख, मोटर गाड़ी से कार्बन मोनो ऑक्साइड गैस, कारखानों से निकलने वाली सल्फर डाइऑक्साइड गैस.

द्वितीयक प्रदूषक सीधे उत्सर्जित नहीं होते हैं। बल्कि जब प्राथमिक प्रदूषक आपस में क्रिया या प्रतिक्रिया करते हैं जब वे वायु में बनते हैं। द्वितीयक प्रदूषक का एक महत्वपूर्ण उदाहरण है जमीनी स्तर की ओज़ोन- बहुत से द्वितीयक प्रदूषकों में एक जो प्रकाश-रसायनिक धूम कोहरा बनाती है।

ध्यान रखें कि कुछ प्रदूषक प्राथमिक और द्वितीयक दोनों हो सकते हैं, यानि वे सीधे भी उत्सर्जित हो सकते हैं और अन्य प्राथमिक प्रदूषकों से बन सकते हैं।

मानव गतिविधियों से उत्पन्न प्रमुख प्राथमिक प्रदूषकों में शामिल हैं:

• सल्फर ऑक्साइड (Sulfur oxide)(SO_x ) विशेष रूप से सल्फर डाइऑक्साइड कोयले और तेल के जलने से उत्सर्जित होती है।
• उच्च तापमान पर दहन से नाइट्रोजन ऑक्साइड (Nitrogen oxide)s (NOं_x) विशेष रूप से नाइट्रोजन डाइऑक्साइड (nitrogen dioxide) से उत्सर्जित होते हैं। प्लूम (plume) नगरों में निचली हवा की धुंध या ऊपर भूरे रंग के रूप में देखी जा सकती है।

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Music is an art form whose medium is sound. Generally, a song is considered the smallest standalone work of music, especially when involving singing. The common elements of music are pitch (which governs melody and harmony), rhythm (and its associated concepts tempo, meter, and articulation), dynamics, and the sonic qualities of timbre and texture. The word derives from Greek μουσική (; "art of the Muses").^[1] In its most general form the activities describing music as an art form include the production of works of music, the criticism of music, the study of the history of music, and the aesthetic dissemination of music.

The creation, performance, significance, and even the definition of music vary according to culture and social context. Music ranges from strictly organized compositions (and their recreation in performance), through improvisational music to aleatoric forms. Music can be divided into genres and subgenres, although the dividing lines and relationships between music genres are often subtle, sometimes open to personal interpretation, and occasionally controversial. Within the arts, music may be classified as a performing art, a fine art, and auditory art. It may also be divided among art music and folk music. There is also a strong connection between music and mathematics.^[2] Music may be played and heard live, may be part of a dramatic work or film, or may be recorded.

To many people in many cultures, music is an important part of their way of life. Ancient Greek and Indian philosophers defined music as tones ordered horizontally as melodies and vertically as harmonies. Common sayings such as "the harmony of the spheres" and "it is music to my ears" point to the notion that music is often ordered and pleasant to listen to. However, 20th-century composer John Cage thought that any sound can be music, saying, for example, "There is no noise, only sound

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