When studying or modeling Venus' climate system, it is important to recognize what exactly constitutes a "climate". In lecture, climate was defined as "the statistical summation of the weather," and a climate system said to be comprised of many different components or variables. The most relevant of these variables (based on mankind's limited understanding of Venus) are its atmosphere, land surface, and astronomical positioning (orbit). The sun, which is an external force on the climate system, also affects Venus' climate. Based on several scientific exploration missions, we now know much more about Venus' climate than ever before.
In our solar system, no planet more closely resembles Earth than does Venus. Not only is it located in an orbit very near our own, it is also roughly the same size (with its radius of 6,051.8 km, only 326 km smaller than Earth's) and is composed of many of the same materials. Venus' planetary density, volume, and gravity are also close to Earth's, though they are slightly lower than our own. These facts have caused researchers to develop the theory that millions of years ago, both Venus and Earth formed from the same nebula in space, and had Venus ended up in Earth's location, life would most likely have existed there much as it does here. These similarities have given many people cause to call Venus Earth's "sister planet". A few decades ago, in the early 1960's, the United States and the then Soviet Union began exploring Venus with many different space missions, and the more we learned about our "sister," the more we realized that Venus is almost nothing like the planet we inhabit.
Venus has a thick layer of constant cloud cover that reflects 85% of all the sunlight reaching the planet. This not only makes Venus the brightest object in the night sky (after the sun and moon), but also makes observational study very difficult, as we are unable to observe the surface from space.