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The Air Up There: Skew-T Examples
The Air Up There: Skew-T Examples
Radiosonde observations provide the condition of the atmosphere above the launch site (typically within 25 miles/40 km) at the time of launch. While they do not provide any direct forecast information, they do help explain why we experience different types of weather. Following sample soundings are typical for different weather conditions.
Snow
The atmosphere is very moist as indicated by the small amount of separation between the air temperature (red line) and the dew point (blue line). Even though the air temperature increases a few hundred feet above the ground (a temperature inversion) the air temperature, throughout the entire atmosphere, remains below freezing.
So, when precipitation begins, it will be in the form of snow and will remain frozen as snowflakes reaching the ground.
Ice pellets (Sleet)
As with the previous sounding, the atmosphere is very moist. So much so, the air temperature and dew point are the same from near 900 millibars (3,000 ft. / 1,000 m) to a little above 700 millibars (10,000 ft. / 3,000 m).
At the surface, an arctic cold front had moved south of the observation station with an air temperature well below freezing. The air temperature begins to decrease with height (which is normal) dropping from 23°F to 12°F (-5°C to -11°C).
However, the density of the arctic air is such that it lays close to the ground with its vertical extent fairly small, in this case only about 3,000 feet (1,000 meters) deep. Above 900 millibars (3,000 ft. / 1,000 m) the air becomes considerably warmer. This area is called an inversion, where temperature change with height is ‘inverted’ as it increases with height instead of typically decreasing with height. This inversion is often also referred as a ‘warm nose’.
Eventually, the temperature of the atmosphere will return to the typical decrease with height (near 800 mb) and will continue to cool until it falls to below freezing again (about 720 mb).
While there may be some precipitation forming as rain in the warm ‘nose’ region where the air temperature is above freezing, the vast majority of precipitation will form as snow in the colder below freezing air above the inversion.
As snow falls into the ‘warm nose’, it melts into a liquid drop/rain. Then the liquid drops fall back into the arctic air mass (near the ground) that is cold enough and deep enough for the liquid to freeze into ice pellets before reaching the ground.
Freezing Rain
The basic pattern for freezing rain is similar to ice pellets. The main difference is the cold, dense air near the surface is very shallow and/or the ‘warm nose’ is large or very warm or both. The melted snow does not have sufficient time to freeze before is reaches the ground.
Therefore, precipitation falls as rain but then it freezes upon contact with a surface such as a tree, power line, automobile or bridge. As a general rule, elevated surfaces will ice first because the ground cannot keep them warm, allowing them to cool to the air temperature quickly. This allows the rain to freeze to ice on contact with these surfaces.
Elevated surfaces may be capable of accumulating ice as soon as the air temperature falls below 32°F (0°C). Road surfaces in contact with the ground will generally begin to ice when the air temperature falls to 28°F (-2°C).
Hurricane
Inside hurricanes, the velocity of the air helps keep the air mixed. Therefore, other than the normal decrease with height, variations in temperature (and dew point) are fairly minimal.
Skew-T Plots
Skew-T Plots
As the radiosonde balloons ascends, it records the temperature and relative humidity at certain prescribed pressure levels (called the mandatory levels) and anytime a significant change occurs in the temperature, humidity, or wind.
Typically, a radiosonde observation is complete when the balloon, carrying the radiosonde, bursts and begins to descend. At that time the data is compiled into a series of five-digit groupings containing temperature, dew point depression and wind speed/direction for mandatory and significant levels. This data is plotted onto a skew-T.
The five-digit coded radiosonde observation is complicated to decode and plot onto a Skew-T diagram. As such, there are several private weather vendors and universities who have written programs to decode and plot (or redisplay the info in a tabular format) these observations. A simple Internet search for “atmospheric soundings” will provide you with several choices.
There are two basic lines plotted on a Skew-T from which we can derived much information. These represent the dew point which is calculated from the relative humidity (in blue, left line) and air temperature (in red, right line).
While it is generally true that the air temperature decreases with height, it is readily seen that this decrease is not uniform nor is it consistent. There may be several places where the air temperature remains the same or increases with height. These particular places are called ‘temperature inversions’ where the normal temperature decrease is ‘inverted’ and the temperature will increasewith height.
Another common characteristic of radiosonde soundings is the location of the tropopause. The tropopause is the boundary between the troposphere and stratosphere and is also indicated by a large temperature inversion.
The dew point line will be the most ‘wiggly’ as the radiosonde ascends through intervening pockets of moist and dry air. At each level on the Skew-T, the closer the dew point is to the temperature, the higher the relative humidity is at that level. The dew point will occasionally equal the air temperature and will be seen by the intersection of both lines.
The other piece of information plotted on a Skew-T is the wind speed and direction. This info obtained as the radiosonde is tracked using GPS during its ascent. The wind speed and direction is reported for the same mandatory pressure levels with additional required elevations above sea level and for any significant changes in speed or direction.
Across the country and around the world, radiosondes sample the atmosphere twice daily providing answers concerning the state of the upper air.
Skew-T Log-P Diagrams
Skew-T Log-P Diagrams
Once the radiosonde observation is plotted, the Skew-T will show the temperature, dew point, and wind speed/direction. From these basic values a wealth of information can be obtained concerning the meteorological condition of the upper air.
There are six basic set of fixed lines that comprise the skew-t diagram. (You can toggle on/off each of these lines on the image at bottom.)
- Temperature
- Temperature lines are drawn at a 45° angle with temperature values that increase from the upper left to the lower right corner of the chart. Early versions of this upper air chart were made with the temperature lines drawn in the vertical.
But in 1947, the modification of tilting the temperature lines 45° aids in analysis. This is where the name “Skew-T” comes from as the temperature lines are skewed at a 45° angle.
- Pressure
- Pressure lines are drawn in the horizontal. Distance between the lines increases from the bottom to the top of the chart (1050 millibars) to the top (100 millibars). This is due to the decrease in atmospheric density with increasing elevation (Learn more about atmospheric density).
Atmospheric pressure decreases logarithmically with increasing elevation. Therefore the heights of the various pressure levels are plotted as the “log” of the pressure…”Log-P” portion of the Skew-T Log-P diagram.
- Dry Adiabats
- Slightly curved, these lines increase in value (°C) from lower left to upper right. Dry adiabats represent the rate at which UN-saturated air cools as it rises. (Unsaturated air is air with a relative humidity lower than 100%.) As unsaturated air rises, it expands and cools with the temperature decreasing (or lapses) at a rate of 9.8°C per 1000 meters (5.5°F/1000 feet) until the relative humidity becomes 100% (the air becomes saturated). This rate is called the “dry adiabatic lapse rate” and these lines on the Skew-T represent that value.
- Moist (or Saturated) Adiabats
- These curved lines increase in value (°C) from left to right. Moist adiabats represent the rate at which saturated air cools (lapses) as it rises. When the air is at 100% relative humidity, further cooling causes water vapor to condense.In this condensation process, heat is released which then affects the rate of cooling and these lines represent that rate.
Near the surface, as saturated air rises, it expands and begins to cool at a rate of about 4°C per 1,000 meters (2.2°F/1,000 feet). As it continues to rise, the cooling rate decreases due to a decreasing amount of water vapor.
On the Skew-T the dry and wet adiabats become nearly parallel in the upper troposphere where the rate of cooling approaches that of dry adiabats, nearly 9.8°C/1,000 meters (5.5°F/1,000 feet).
Very cold air does not contain much water vapor. The reason is because as air cools, the temperature of the water vapor itself decreases leading to an increasing amount vapor condensing to a liquid (or deposits to a solid) state. This condensation and/or deposition decreases the amount of gaseous water remaining in the atmosphere.
With very cold air most of the water vapor has already condensed into a liquid or deposited into a solid. The end result is very cold air contains little water vapor and therefore cannot release much heat into the atmosphere.
Conversely, very warm air can contain large amounts of water vapor. Warming air means the temperature of the water vapor increases so evaporation and/or sublimation (changing from liquid or solid to a gas) also increases. This adds water vapor to the atmosphere.
In the tropics, the large amount of heat released by process of condensation from very moist air is one of the mechanisms for the formation of tropical cyclones and thunderstorms.
- Mixing Ratio
- In meteorology, mixing ratio is the mass of water vapor compared with the mass of dry air. It is expressed in grams per kilogram. Two mixing ratios can be learned from a Skew-T, the ordinary mixing ratio and the saturation mixing ratio. On a plotted radiosonde sounding, the mixing ratio at any given level is the amount of water vapor in the air where the dew point temperature line crosses the mixing ratio line.
The saturation mixing ratio is the maximum amount of water vapor that can be in the air at any given level and is found where the temperature line crosses the mixing ratio line.
- Wind Staff
- These staffs are for plotting the wind speed and direction as observed by the radiosonde.
Radiosondes
Radiosondes
Since the weather we experience is due to dynamic processes that take place throughout the atmosphere, we need to know what is taking place through the entire atmosphere. These observations are primarily taken with the aid of radiosondes.The radiosonde is a small, expendable instrument package that is suspended below balloon filled with either hydrogen or helium. As the radiosonde rises, sensors on the radiosonde measure values of pressure, temperature, and relative humidity.
These sensors are linked to a battery powered radio transmitter that sends the sensor measurements to a ground receiver. By tracking the position of the radiosonde in flight via GPS (global positioning system), information on wind speed and direction aloft is also obtained.
Worldwide, most radiosonde observations are taken at 00Z and 12Z daily (What is ‘Z’ time?). With worldwide coordination of these upper air observations we can get a good picture of the various pressure and wind patterns across the globe. For the United States mainland observation sites, the local times for these observations are centered around 6 AM and 6 PM daily.
Technically, a radiosonde observation provides onlypressure, temperature, and relative humidity data. When the position of a radiosonde is tracked (so that wind speed and direction can be determined) it is called a rawinsonde observation.Most stations around the world take rawinsonde observations. However, meteorologists and other data users frequently refer to a rawinsonde observation as a radiosonde observation.
The radiosonde flight can last in excess of two hours, and during this time the radiosonde can ascend to over 115,000 feet (35,000 m) and drift more than 125 miles (200 km) from the release point. During the flight, the radiosonde is exposed to temperatures as cold as -130°F (-92°C) and air pressure of only a few hundredths of what is found on the Earth’s surface.
When the balloon has expanded beyond its elastic limit (about 20 feet in diameter) and bursts, the radiosonde returns to Earth via a small parachute. This slows its descent, minimizing the danger to life and property.
If found, radiosondes can be reconditioned and used again saving the taxpayer some money. If you find a fallen NWSradiosonde, it is safe to handle. Cut the string to the burst balloon and place it in a trash receptacle.
Next, remove the plastic mailbag attached to the handle of the radiosonde and place the instrument inside the bag. Hand the package to your postal carrier. Postage is prepaid if the instrument is returned in the United States.Worldwide, there are about 1,300 upper-air stations. Observations are made by the NWS at 92 stations – 69 in the conterminous United States, 13 in Alaska, nine in the Pacific, and one in Puerto Rico.NWS supports the operation of 10 other stations in the Caribbean. Through international agreements data are exchanged between countries worldwide.
How Are Radiosonde Data Used?
Radiosonde observations are used over a broad spectrum of efforts including:
- Input for computer-based weather prediction models,
- Local severe storm, aviation, and marine forecasts,
- Weather and climate change research,
- Input for air pollution research, and
- Ground truth for satellite data.
Data from a radiosonde observation is plotted on a seemingly complicated chart called a “Skew-T” but provides is a wealth of information concerning the state of the atmosphere.
Stability/Instability
Stability/Instability
The following ball and bowl illustrations should help in understanding parcel stability and instability. The bowl represents the ‘state’ of the atmosphere. The red ball represents a “parcel” of air upon which an energy is applied to the ball to initiate motion.
Absolute Stability
With the ball inside of the bowl it will return to its initial position. In the atmosphere, if a parcel returns to its initial starting elevation then the atmosphere is considered to be absolutely stable.
Absolute Instability
With the bowl turn upside down, the ball now rests on the top of the bowl. When a force is applied to the ball it begins to move on its own without any additional force applied. When this occurs in our atmosphere it is considered absolutely unstable.
Neutral Stability
On a flat surface, it a force is applies to the ball it moves. Once the force is removed the ball stops and remains in its new position. In the atmosphere, the atmosphere this considered neutral stability.
Conditional Instability
The upside down glass bowl has a slight depression wherein the ball rests. If the force is not too great the ball will return to its initial position similar to absolute stability. However if the force is strong enough the ball will move up and out of the depression and continue to move on its own. This is one of the most common states of the atmosphere called conditional instability. The atmosphere is unstable if certain conditions are met otherwise it is stable.
To measure the state of the atmosphere over our heads (temperature profile, stability, moisture, wind, etc.) we use a device called a radiosonde.
The ‘Parcel’ Theory
The ‘Parcel’ Theory
It is common knowledge that warm air rises. It is normally assumed that is because warm air is lighter than cooler air. While that is true there is a more fundamental process that takes place for the cause of rising warm air.
Warm air rises primarily due its lower densityas compared to cooler air. As the temperature increases, the density of the air decreases. But even air that is of a lower density will not begin to rise by itself.Isaac Newton’s first law of physics is that the velocity of an object will remain constant unless another force is exerted on that object. The more common way of saying this is ‘an object at rest tends to stay at rest and an object in motion tends to stay in motion’.
This is why decreasing the density of air alone is not sufficient enough to cause air to rise. There must be another force exerting on the less dense air for it to begin its upward motion.
That force is ‘gravity’. Gravity’s role is its pull of cooler, denser air toward the earth’s surface. As the more dense air reaches the earth’s surface it spreads and undercuts the less dense air which, in turn, forces the less dense air into motion causing it to rise.
This is how hot air ballooning works. A flame is used to heat the air inside of the balloon making it less dense. Outside of the balloon, the cooler air, being more dense, is pulled towards the earth’s surface by gravity. The cooler air undercuts the warmer, less dense air trapped inside the balloon causing it to lift.
This is why thunderstorms often form along weather fronts. A front represents the boundary where cooler, more dense air undercuts less dense, warmer air forcing it up into the atmosphere forming the storms.
In meteorology, we often treat ‘pockets of air’ in a similar way to ballooning. We call these pockets of air “parcels”. A parcel is a bubble of air of no definite size that we generally assume it retains its shape and general characteristics as it rises or sinks in the atmosphere.
The theory behind the “parcel” has several assumptions.
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In a stable atmosphere, the rising parcel becomes cooler than the surrounding environment slowing or ending its rise (left image). In an unstable atmosphere, the temperature of the parcel is higher than the surrounding environment and as such remains buoyant and will continue to rise (right image).
In both cases the parcel’s rate of cooling remains fixed. Therefore, stability/instability is based upon the vertical temperature profile of the atmosphere.We generally assume the ratio of moist air to dry air in the parcel remains constant as it rises (or sinks) in the atmosphere.
- We also assume there is no outside source of heating added to the parcel.
- Any parcel that is UNsaturated (relative humidity less than 100%) will cool (or lapses) at a rate of of 9.8°C per 1,000 meters (5.5°F/1,000 feet) until the relative humidity becomes 100% (the air becomes saturated).
- Any saturated parcel (parcel with 100% relative humidity) cools at a slower rate. This is because the process of water vapor condensing into a liquid releases heat. The released heat that is added to the atmosphere slows the rate of cooling.
Because of many different influences on a parcel of rising air most, if not all, of the assumptions will not be 100% true at all times. However, the ‘parcel theory’, while an over-simplification of real world processes in the atmosphere, is a good way of thinking about how the atmosphere produces the weather.
Buoyancy: Positive and Negative Energy
The reason for looking at parcels is to help determine the stability of the atmosphere. As an unsaturated parcel rises it will cool at the fixed rate of 9.8°C per 1,000 meters (5.5°F/1,000 feet).
If the temperature of the rising parcel decreases to less than the surrounding atmosphere (due to its cooling) the parcel will become denser than the surrounding environment and gravity will slow, or even reverse, the rise. This is called negative energy and means the atmosphere at that level is ‘stable’.
If the temperature of the rising parcel remains higher than the surrounding atmosphere (despite its cooling), the parcel, being less dense than the surrounding environment, will continue to rise. This is called positive energy and means the atmosphere at that level is ‘unstable’.
Introduction to the Upper Air
Introduction to the Upper Air
What we experience as weather at ground level is the end result of what takes place over our head. Therefore to determine the forecast, and therefore the impacts, of weather we will need to determine the weather patterns in the upper air before looking at the surface weather.
The term “upper air” refers to the earth’s atmosphere above about 5,000 feet (1,500 meters). It is from the upper air where we get our rain and drought, wind and calm, heat and cold at the earth’s surface.
The map (above) was a picture the state of the atmosphere for a particular time at about 18,000 feet in altitude. The lines represent the locations of various higher and lower pressure regions in the upper atmosphere.
While upper air maps like this (and following information) can be rather complicated, this is the “meat and potatoes”‘ for the meteorologist.
All weather forecasts stem from our understanding of the upper air where weather patterns such as ridges, troughs, upper air disturbances and upper-lows occur and where they are moving.
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