SAAT ini di Eropa dan wilayah utara bumi tengah musim dingin.Salah satu fenomena menarik saat musim dingin adalah salju. Menjadi unik karena kristal-kristal es yang lembut dan putih seperti kapas ini hanya hadir secara alami di negeri empat musim atau di tempat-tempat yang sangat tinggi seperti puncak gunung Jayawijaya di Papua. Kenapa salju secara alami tidak bisa hadir di wilayah tropis seperti negeri kita?

Proses pembentukan salju

Untuk menjawab itu, bisa kita mulai dari proses terjadinya salju. Berawal dari uap air yang berkumpul di atmosfer Bumi, kumpulan uap air mendingin sampai pada titik kondensasi (yaitu temperatur di mana gas berubah bentuk menjadi cair atau padat), kemudian menggumpal membentuk awan. Pada saat awal pembentukan awan, massanya jauh lebih kecil daripada massa udara sehingga awan tersebut mengapung di udara – persis seperti kayu balok yang mengapung di atas permukaan air. Namun, setelah kumpulan uap terus bertambah dan bergabung ke dalam awan tersebut, massanya juga bertambah, sehingga pada suatu ketika udara tidak sanggup lagi menahannya. Awan tersebut pecah dan partikel air pun jatuh ke Bumi.

1. Galileo Thermometer

A Galileo thermometer, Galilean thermometer (named after Italian physicist Galileo Galilei), or thermoscope is a thermometer made of a sealed glass cylinder containing a clear liquid. Suspended in the liquid are a number of weights. Commonly those weights are themselves sealed glass bulbs containing coloured liquid for an attractive effect. As the liquid in the cylinder changes temperature its density changes and those bulbs which are free to move, rise or fall to reach a position where their density is either equal to that of the surrounding liquid or where they are brought to a halt by other bulbs. If the bulbs differ in density by a very small amount and are ordered such that the least dense is at the top and most dense at the bottom, they can form a temperature scale.

The temperature is typically read from an engraved metal disc on each bulb. Usually a gap would separate the top bulbs from the bottom bulbs and then the temperature would be between the tag readings on either side of the gap. If a bulb is free-floating in the gap, then its tag reading would be closest to the ambient temperature. To achieve this requires manufacturing the weights to a tolerance of less than 1/1000 of a gram (1 milligram).

The Galileo thermometer works due to the principle of buoyancy. Buoyancy determines whether objects float or sink in a liquid, and is responsible for the fact that even boats made of steel can float (of course, a solid bar of steel by itself will sink). The only factor that determines whether a large object will float or sink in a particular liquid relates the object's density to the density of the liquid in which it is placed. Small objects, such as a pin (device), can float through surface tension. If the object's mass is greater than the mass of liquid displaced, the object will sink. If the object's mass is less than the mass of liquid displaced, the object will float.

2. Termometer Air Raksa