Quick Answer
A sudden loss of atmosphere would cause surface pressure to collapse almost immediately. Humans without pressure protection would lose consciousness rapidly, exposed liquid water would begin boiling and then evaporating or freezing depending on local conditions, weather would cease as we know it, and the planet would receive much more radiation at the surface. Earth would still have gravity and oceans, but its surface environment would become hostile to complex life.
Earth's atmosphere is a physical system, not just the air we breathe. It supplies pressure, transports heat and water, filters part of the incoming radiation, supports weather and allows sound to travel. Removing it changes several basic conditions at the surface at once.
The first seconds
If the atmosphere disappeared instantly, surface pressure would fall toward the tiny residual pressure produced by water vapor and other escaping gases. Humans and animals exposed without protection would suffer from severe decompression and lack of oxygen. The problem would not be that the oxygen was simply 'sucked into space' in one dramatic blast; it would be the abrupt removal of the pressure and breathable gas surrounding the body.
Would the oceans boil?
Liquid water boils when its vapor pressure exceeds the surrounding pressure. With surface pressure near vacuum, water can boil at very low temperatures. At the same time, evaporation removes energy, causing remaining water to cool rapidly. So the oceans would not turn into steam in a single instant; they would undergo rapid boiling, evaporation, freezing and sublimation processes while the exposed surface evolved toward a new equilibrium.
Established scienceBoiling does not necessarily mean heating. Under very low pressure, water can boil while becoming colder because evaporation carries away latent heat.
The sky would go black
The familiar blue sky is produced largely by sunlight scattering from molecules in the atmosphere. Remove those molecules and the daytime sky viewed from the surface would become dark, much like the sky seen by astronauts in space. The Sun would remain intensely bright, but there would be almost no atmospheric scattering to illuminate the sky between the Sun and stars.
No weather, no ordinary sound
Weather is atmospheric motion, so ordinary wind, clouds, rain and storms would disappear. Sound also needs a medium. A person shouting next to you would not transmit the sound through open vacuum, although vibrations could still travel through solid ground or a sealed structure.
What happens to temperature?
The atmosphere currently redistributes heat and contributes to the greenhouse effect. Without it, the surface would experience enormous temperature contrasts between sunlight and darkness. The exact temperatures would depend on latitude, surface composition, ocean heat and the timescale of atmospheric loss, but the present climate system would be gone.
Could life survive underground?
Some life could survive if it already lived in protected environments with water and energy sources. Deep underground ecosystems, sealed habitats and microorganisms shielded by rock would be far better positioned than exposed terrestrial animals. Human survival would require pressurized habitats and a reliable supply of oxygen, water, food and energy.
Falls toward vacuum
Surface pressure
Cannot propagate
Open-air sound
The bottom line
An atmosphere-free Earth would still be a rocky planet with gravity and sunlight, but almost every surface condition that complex terrestrial life depends on would change. The atmosphere is not background scenery: it is part of Earth's life-support system.