Weather: Understanding Some Basic Facts
Weather: Understanding Some Basic Facts
Weather occurs every day and at every location. So, how much do you really understand about the weather? Let us discuss some facts, and delve into the mystery of weather.
Why Does It Rain, Snow, Sleet, Or Hail?
This question can be answered in two ways. The answer can address why a cloud forms, and what it contains. Or, the answer can address what happens inside a cloud.
Why a cloud forms is simple, the atmosphere has more water than it can hold. Air can hold more water at warmer temperatures and at higher pressures. As air rises it cools in response to the reduction in pressure. Once condensation occurs we have a cloud.
It is not as simple as reaching one hundred percent humidity, since water will not spontaneously condense at one hundred percent humidity unless it has something to condense on. Dirty air, especially air with electrically charged particles, can have condensation with less water vapor than clean air. The one hundred percent relative humidity refers to equilibrium between liquid and gaseous water over a flat surface, and clean air in the middle troposphere can go well past one hundred percent humidity without condensation. So, it helps if there is something for the cloud to form on.
What causes the air to rise could be any of several factors. One is that the air must go over a hill or a mountain. Another is air must go over heavier, colder air behind a cold front. Air treats cold domes of air like mountains that move. And, another mechanism is that the air is heated making it lighter, and gravity pushes heavier air in as it rises out of the way.
Precipitation occurs from two mechanisms. There is convective precipitation and non-convective precipitation.
Convective precipitation occurs when the air rises, and reaches the point where condensation occurs. Then, as the air rises more, the cloud components grow, since the colder air under lower pressure can hold less water as vapor. Once the air rises to where the liquid freezes, the process is enhanced, since ice is supersaturated where liquid water is not. The ice grows until gravity can bring it down from the top region of the cloud.
If the ice is large enough to make it to the ground as ice, we have hail. If it melts on the way down, we get rain, which is the usual result. If the rain freezes we get sleet, which requires a colder layer of air below where the liquid drops exist. Snow from convective clouds is rare, but can occur. For snow, the condensation must at first be directly into the ice phase, allowing a snowflake to form. A liquid drop cannot freeze into a flake structure.
The cloud from which convective precipitation occurs is called cumulonimbus.
Non-convective precipitation occurs as a slow rain from a layer cloud called nimbostratus. This is a more complicated mechanism. Once the air becomes saturated for water, droplets grow slowly, since there is no mechanism to cause the air to not become unsaturated once enough water vapor is removed. So, what is really necessary is both ice and water exist. Water can remain liquid below the normal freezing point because, as with the humidity definition above, it does not have a flat surface. The greater the curvature of the surface, the colder the liquid water can exist as liquid, so small droplets can remain liquid well below what we call freezing. If there is both ice and super-cooled water present the ice can be supersaturated, and grow from the water vapor. The water vapor can then be resupplied by evaporation of the liquid water, which acts as a reservoir. This allows the snowflake to grow heavy enough to start the journey down. If it reaches the ground as ice, it is snow, if it melts on the way down it is rain, and if it melts and refreezes it is sleet. Often the snow does not fully melt, and we get an ice encapsulated white granule. This is still called sleet.
Warm rain is rain from a cloud that does not contain ice. It can occur when a convective cloud did not reach the point of having freezing take place, but the cause of the rising air stops. Typically, this happens when heated air loses its source of heat as the sun sets. Some drops may be heavy enough to fall to the ground once the rising currents fail. This is often a short rainfall, and does not occur often. Ice in a cloud is a main requirement for most precipitation.
The cumulonimbus cloud photograph below is our own image.
Light is scattered as it passes through a cloud, so the greater the path the more light is scattered. For this reason thicker clouds often are dark and considered to indicate bad weather. But, a setting sun can send light through a long but thin cloud and make the same appearance. And, a thunderstorm can be brilliant white if the sun is illuminating a side.
To determine if a cloud is producing look for rain streaks, those dark bands that reach the ground. Another indicator is if the base of the cloud is smooth in appearance. Precipitation and cloud components react with light in about the same way, so if you cannot identify a structure at the base of the cloud, the precipitation is masking the cloud base. Smooth gray clouds are more likely to be producing than dark ragged clouds.
What Makes the Wind Blow?
Wind blows because warmer air is lighter than colder air. As the heavier air moves in to move the warmer air, we experience wind. The best way to determine how fast, and in what direction, wind will blow is to observe the barometric pressure. Air moves from higher pressure to lower pressure, and its speed is determined by the rate of how fast the pressure drops over distance. The air does not rush directly towards the lower pressure, but is deflected to the right in the northern hemisphere, and the left in the southern hemisphere due to the earth’s rotation.
What Determines If It Is Hot Or Cold?
The jet stream sets the boundary between warmer and cooler air, with the cooler air being on the side of the jet stream that includes the pole. As the jet stream moves, fronts or boundaries of different air temperatures move along.
Does a Rainbow Really Mean the Storm Is Passed?
A rainbow acts like a prism and splits up sunlight inside a water drop or droplet. The light reflects internally, and actually has the colors reverse inside the droplets. What we need is the sun to reflect back to us from the cloud. Since most people live in the area of the westerlies, our weather moves from west to east. A storm is most likely in the afternoon due to thermal heating, and normally travels to the east. If the storm is east of us in the late afternoon the sun lights up the back of the storm, and may produce a rainbow, hence we often see a rainbow after the storm has passed. This does not mean a morning rainbow, or one in the tropics, would indicate the storm is past.
A double rainbow requires two reflections inside the droplets with two color crossovers to produce a secondary bow. A secondary bow always has the colors reversed compared to the primary bow.
The double rainbow image below is our own image.
What Causes the Sky to Be Blue? Why Is the Sun Red at Sunrise and at Sunset?
Our atmosphere scatters blue light most efficiently, so in whatever direction we look some scattered sunlight is coming towards us. Red and orange scatter less efficiently, so when the sun has a greater path through the atmosphere, such as at sunrise and sunset, the red and orange components of sunlight remain after other colors have been scattered away.
Deeper blue skies indicate more water vapor, and unusually red sunsets occur in response to dust in the sky, such as after a volcanic eruption.