The Rules of Formation

5 12 2009

What is rain?
Rain is formed by water vapor molecules as they rise in the atmosphere (by a force such as a low pressure system, cold front, etc). As the molecules rise, they cool and condense and merge together to form a droplet of liquid water. As the droplet gets larger, it gains weight and drops to the ground.

What is Freezing Rain?
Freezing rain is much like sleet – however, the layer of colder air which refreezes the liquid is shallow and very close to ground level. When the liquid passes through this layer, it freezes. The layer can even be shallow enough to where the rain drop will already be splattered on the ground before it freezes.

What is sleet?
Sleet is formed as snowflakes are but travels through a layer of warmer air midway down and melts. When the liquid refreezes, it turns into a tiny bead of ice by the time it hits the ground.

How atmospheric layer temperatures determine precipitation type.

What is Snow?
Snow is formed when liquid vapor molecules freeze and gain weight (ice is heavier than vapor) and fall. Individual snow crystals have 6 sides – many snow crystals form or merge together to form elegant flakes of snow. Their size is determined by the amount of liquid water coating the flakes. In order for snow to form and successfully fall to the ground, temperatures where they form all the way to ground level must remain above freezing.

Dry Snow vs. Wet Snow – The Difference
Dry Snow is formed when temperatures in the troposphere are well below freezing. This is commonplace in the northern US during the winter and more often in Canada. Dry snow can be identified easily – the snowflakes are very small and don’t stick together very well. Try making a large snowball from dry snow and it will fall apart. Wet snow, on the other hand, forms when temperatures in the troposphere are at or just below freezing or there is an unbalanced temperature range in that particular atmospheric layer. Wet snow flakes tend to be larger than dry snow flakes and stick together very well. Wet snow is the best type of snow to use to build snow-men (or snow-women, I won’t discriminate). Wet snow is formed when snowflakes fall and partially melt. This forms a thin layer of liquid water on the snow flake – cold enough to prevent the total melting of the flake but warm enough to not freeze itself. This tends to make the flake sticky. For this reason, wet snow is usually larger. Snowflakes stick together, if wet, as they are falling. This can lead to some snowflakes to appear to look like little snowballs falling from the sky. Some snowflakes have been recorded to reach a half-dollar in size or even larger. The only problem with measuring such immensely sized snowflakes are that they crumble when they hit the ground- eye witness reports have shown flakes to be the size of baseballs or even softballs but it remains unproven. Flakes of great size are infrequent and usually only form when there is a gentle breeze. No wind at all will force the flakes to break apart due to changes in air pressure and resistance. A hard breeze will have the same effect – a shearing effect more or less, but a light wind will keep the flake floating down more slowly but be gentle enough to not shred the flake.

Guide to Collapsing Raindrops

Why are some raindrops large while others are small?
Raindrops merge and grow larger infinitely – all the way until they are on the ground (and even after, research flooding/flash flooding). As they fall, they grow larger until some force breaks them apart – usually air resistance. As the raindrop grows, the resistance from the air forces it to ‘parachute’ itself. The raindrop begins to form a dome shape which eventually ‘pops’ and the raindrop breaks into several smaller raindrops. Small raindrops form from rainclouds that are very high in the atmosphere. They have a longer amount of time to gain speed, merge and break apart. Thus, this is how mist, sprinkles and other tiny drops are formed. Large raindrops are formed from storm clouds which are closer to the ground. They don’t have very far to fall and thus less air resistance to move through. Winds are also a variable in this sense. If winds are strong, the drops will not fall vertically, but at an angle. This slows the drop’s descent somewhat, leaving a little less shearing stress on the drop itself, although strong enough wind can shred the drop itself.

Hope you enjoyed the article and it helped you to better understand why it pours one precipitation sometimes and others at other times. It’s easy to get confused with sleet, snow and freezing rain – especially in our area. Feel free to post any comments, suggestions, questions or concerns. I will respond as soon as I get a chance.

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Shared Beliefs

9 08 2009

I was just browsing around and came across a good post by a well known, albeit local, personality. Dan Satterfield is but a television meteorologist to some but an inspiration for others. Since 1995, I’ve been watching his forecasts and enjoying severe weather coverage with his excitable antics. The post I am mentioning here came from his blog, Wild Wild Weather Journal. It’s a good and startling read when you contrast today with tomorrow (figuratively).

Dan’s Wild Wild Weather Blog
Dan’s “Climate Change In Your Backyard” Blogpost

Gravity Waves – A Closer Look

13 04 2009

Gravity Waves
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media (e.g. the atmosphere and the ocean) which has the restoring force of gravity or buoyancy.

When a fluid element is displaced on an interface or internally to a region with a different density, gravity tries to restore the parcel toward equilibrium resulting in an oscillation about the equilibrium state or wave orbit. Gravity waves on an air-sea interface are called surface gravity waves or surface waves while internal gravity waves are called internal waves. Wind-generated waves on the water surface are examples of gravity waves, and tsunamis and ocean tides are others.

Wind-generated gravity waves on the free surface of the Earth’s ponds, lakes, seas and oceans have a period of between 0.3 and 30 seconds (3 Hz to 0.033 Hz). Shorter waves are also affected by surface tension and are called gravity-capillary waves and (if hardly influenced by gravity) capillary waves. Alternatively, so-called infragravity waves — which are due to subharmonic nonlinear wave interaction with the wind waves — have periods longer than the accompanying wind-generated waves.

Atmospheric Dynamics on Earth
Since the fluid is a continuous medium, a traveling disturbance will result. In the earth’s atmosphere, gravity waves are important for transferring momentum from the troposphere to the mesosphere. Gravity waves are generated in the troposphere by frontal systems or by airflow over mountains. At first waves propagate through the atmosphere without affecting its mean velocity. But as the waves reach more rarefied air at higher altitudes, their amplitude increases, and nonlinear effects cause the waves to break, transferring their momentum to the mean flow.

This process plays a key role in controlling the dynamics of the middle atmosphere.

The clouds in gravity waves can look like Altostratus undulatus clouds, and are sometimes confused with them, but the formation mechanism is different.

Quantitive Description
The phase speed c of a linear gravity wave with wavenumber k is given by the formula

where g is the acceleration due to gravity. When surface tension is important, this is modified to

where g is the acceleration due to gravity, σ is the surface tension coefficient, ρ is the density, and k is the wavenumber of the disturbance.
Since c = ω / k is the phase speed in terms of the frequency ω and the wavenumber, the gravity wave frequency can be expressed as

The group velocity of a wave (that is, the speed at which a wave packet travels) is given by

and thus for a gravity wave,

The group velocity is one half the phase velocity. A wave in which the group and phase velocities differ is called dispersive.

The Generation of Waves by Wind
Wind waves, as their name suggests, are generated by wind transferring energy from the atmosphere to the ocean’s surface, and capillary-gravity waves play an essential role in this effect. There are two distinct mechanisms involved, called after their proponents, Phillips and Miles.

In the work of Phillips[1], the ocean surface is imagined to be initially flat (‘glassy’), and a turbulent wind blows over the surface. When a flow is turbulent, one observes a randomly fluctuating velocity field superimposed on a mean flow (contrast with a laminar flow, in which the fluid motion is ordered and smooth). The fluctuating velocity field gives rise to fluctuating stresses (both tangential and normal) that act on the air-water interface. The normal stress, or fluctuating pressure acts as a forcing term (much like ‘pushing’ is a forcing term for a swing). If the frequency and wavenumber of this forcing term match a mode of vibration of the capillary-gravity wave (as derived above), then there is a resonance, and the wave grows in amplitude. As with other resonance effects, the amplitude of this wave grows linearly with time.

The air-water interface is now endowed with a surface roughness due to the capillary-gravity waves, and a second phase of wave growth takes place. A wave established on the surface either spontaneously as described above, or in laboratory conditions, interacts with the turbulent mean flow in a manner described by Miles[2]. This is the so-called critical-layer mechanism. A critical layer forms at a height where the wave speed c equals the mean turbulent flow U. As the flow is turbulent, its mean profile is logarithmic, and its second derivative is thus negative. This is precisely the condition for the mean flow to impart is energy to the interface through the critical layer. This supply of energy to the interface is destabilizing and causes the amplitude of the wave on the interface to grow in time. As in other examples of linear instability, the growth rate of the disturbance in this phase is exponential in time.

This Miles-Phillips Mechanism process can continue until an equilibrium is reached, or until the wind stops transferring energy to the waves (i.e. blowing them along) or when they run out of ocean distance, also known as fetch length.

This article was not my own. I do not take credit for this article, in whole or part. Article was derived from a collaborative effort from Wikipedia.