The atmosphere of Venus supports decks of opaque clouds of sulfuric acid that cover the entire planet, preventing optical Earth-based and orbital observation of the surface.

In the Book II of Cosmotheoros, published in 1698, he writes:[10]I have often wonder’d that when I have viewed Venus at her nearest to the Earth, when she resembled an Half-moon, just beginning to have something like Horns, through a Telescope of 45 or 60 Foot long, she always appeared to me all over equally lucid, that I can’t say I observ’d so much as one spot in her, tho in Jupiter and Mars, which seem much less to us, they are very plainly perceived.

A large amount of the planet's hydrogen is theorised to have been lost to space,[14] with the remainder being mostly bound up in water vapour and sulfuric acid (H2SO4).

Strong evidence of significant hydrogen loss over the historical evolution of the planet is the very high D–H ratio measured in the Venusian atmosphere.

[18][19] On review, an interpolation error was discovered that resulted in multiple spurious spectroscopic lines, including the spectral feature of phosphine.

[25] In September 2024, the preliminary analysis of the JCMT-Venus data has confirmed the existence of phosphine in the atmosphere of Venus, with the concentration 300 ppb at altitude 55 km.

The winds are slow near the surface,[1] but at the top of the troposphere the temperature and pressure reaches Earth-like levels and clouds pick up speed to 100 m/s (360 km/h).

[30] The pressure found on Venus's surface is high enough that the carbon dioxide is technically no longer a gas, but a supercritical fluid.

This supercritical carbon dioxide forms a kind of sea, with 6.5% the density of water,[30] that covers the entire surface of Venus.

[6] The polar upper troposphere and mesosphere are extremely dynamic; large bright clouds may appear and disappear over the space of a few hours.

[40] Images from the Akatsuki orbiter revealed something similar to jet stream winds in the low and middle cloud region, which extends from 45 to 60 km in altitude.

[28] The upper boundary of the ionosphere (the ionopause) is located at altitudes 220–375 km and separates the plasma of the planetary origin from that of the induced magnetosphere.

[49] Due to the lack of the intrinsic magnetic field on Venus, the solar wind penetrates relatively deep into the planetary exosphere and causes substantial atmosphere loss.

In fact, due to the thick, highly reflective cloud cover, the total solar energy received by the surface of the planet is less than that of the Earth, despite its proximity to the Sun.

[59] It is theorized that early volcanic activity released sulfur into the atmosphere and the high temperatures prevented it from being trapped into solid compounds on the surface as it was on the Earth.

[69] However, the Venus Express findings are incompatible with data from the JAXA Akatsuki spacecraft which indicate a very low flash rate.

[72] The Pioneer Venus Orbiter (PVO) was equipped with an electric field detector specifically to detect lightning and the Venera 13 and 14 missions included a radio receiver and point discharge sensor to search for thunderstorms.

Other missions equipped with instruments that could search for lightning included Venera 9 which had a visible spectrometer; Pioneer which had a star sensor; and VEGA which had a photometer.

[83] In 2021, it was suggested the color of "unknown UV absorber" match that of "red oil" - a known substance comprising a mixed organic carbon compounds dissolved in concentrated sulfuric acid.

[84] In September 2020, research studies led by Cardiff University using the James Clerk Maxwell and ALMA radio telescopes noted the detection of phosphine in Venus's atmosphere that was not linked to any known abiotic method of production present, or possible under Venusian conditions.

[92] This longer timescale for the persistence of oceans is also supported by General Circulation Model simulations incorporating the thermal effects of clouds on an evolving Venusian hydrosphere.

[93] The early Earth during the Hadean eon is believed by most scientists to have had a Venus-like atmosphere, with roughly 100 bar of CO2 and a surface temperature of 230 °C, and possibly even sulfuric acid clouds, until about 4.0 billion years ago, by which time plate tectonics were in full force and together with the early water oceans, removed the CO2 and sulfur from the atmosphere.

[96][97] In 1940, Rupert Wildt calculated that the amount of CO2 in the Venusian atmosphere would raise surface temperature above the boiling point for water.

[100] The solar transit in 2004 enabled astronomers to gather a large amount of data useful not only in determining the composition of the upper atmosphere of Venus, but also in refining techniques used in searching for extrasolar planets.

[100][101] The Venus Express spacecraft formerly in orbit around the planet probed deeper into the atmosphere using infrared imaging spectroscopy in the 1–5 μm spectral range.

In 2002, the NASA contractor Global Aerospace proposed a balloon that would be capable of staying in the upper atmosphere for hundreds of Earth days as opposed to two.

[53] In addition to this, the slightly lower gravity, high air pressure and slow rotation allowing for perpetual solar power make this part of the planet ideal for exploration.

[112] It has been proposed to fly together with Venera-D an inflatable aircraft designed by Northrop Grumman, called Venus Atmospheric Maneuverable Platform (VAMP).

In June 2018, NASA also awarded a contract to Black Swift Technologies for a concept study of a Venus glider that would exploit wind shear for lift and speed.

The mission consists of a descent sphere that will plunge through the planet's thick atmosphere, measuring noble gases and other elements to understand Venus' climate change.