Givre

Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between −2 and −8 °C (28.4 and 17.6 °F).

Hard rime formations are difficult to shake off; they have a comb-like appearance, unlike soft rime, which looks feathery or spiky, or clear ice, which looks homogeneous and transparent.

Scientists at meteorologically extreme places such as Mount Washington in New Hampshire often have to break huge chunks of hard rime off weather equipment, in order to keep anemometers and other measuring instruments operating.

Rime ice can accumulate on the leading edges and control surfaces of aircraft operating in certain meteorological conditions.

Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. Of course there are also intermediate formations.

In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena) is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle (sometimes called mist - especially "Scotch mist"), rain, sleet, snow, graupel and hail. Precipitation occurs when a local portion of the atmosphere becomes saturated with water vapour, so that the water condenses and "precipitates". Thus, fog and mist (except when the terms are used to mean "drizzle") are not precipitation but suspensions, because the water vapour does not condense sufficiently to precipitate. Likewise, frost does not precipitate out of the atmosphere but rises from the ground; so it, too, is not precipitation. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air. Generally, precipitation will fall to the surface; an exception is Virga which evaporates before reaching the surface. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops. Unlike raindrops, snowflakes grow in a variety of different shapes and patterns, determined by the temperature and humidity characteristics of the air the snowflake moves through on its way to the ground. While snow and ice pellets require temperatures close to the ground to be near or below freezing, hail can occur during much warmer temperature regimes due to the process of its formation.

Ice is water frozen into the solid state. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions. The addition of other materials such as soil may further alter the appearance.

Ice appears in nature in forms of snowflakes, hail, icicles, glaciers, pack ice, and entire polar ice caps. It is an important component of the global climate, and plays an important role in the water cycle. Furthermore, ice has numerous cultural applications, from ice cooling of drinks to winter sports and the art of ice sculpting.

The molecules in solid ice may be arranged in different ways, called phases, depending on the temperature and pressure. Usually ice is the phase known as ice I_h, which is the most abundant of the varying solid phases on the Earth's surface. The most common phase transition to ice I_h occurs when liquid water is cooled below 0°C (273.15K, 32°F) at standard atmospheric pressure. It can also deposit from vapour with no intervening liquid phase, such as in the formation of frost.

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.

Weather is driven by density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in Earth's orbit affect the amount and distribution of solar energy received by the Earth and influence long-term climate and global climate change.