When is air unstable

The wet adiabatic lapse rate is not a constant but averages out to be about 3. This lapse rate is the dry adiabatic rate modified by the latent heat of condensation. Latent heat of Vaporization. For every gram of water that evaporates, there are calories of cooling. A calorie is the amount of heat needed to 1 g. For every gram of water that condenses, there are calories of warming. Thermals are columns of spontaneously rising absolutely unstable dry air.

The following explanations relate to the Handout on Instability Absolutely Stable. In the first situation the air at sea level is 75 degrees. As ones goes up in elevation note, you are "going up" through a horizontally moving atmosphere , one experiences a average decrease in temperature of 3 degrees per ft.

This is the environmental lapse rate. The temperature at ft in the atmospheric environment is 71 degrees. When a single air parcel in this environment is forced to rise from the ground for example from sealevel up a mountain , it cools at 5. This is the dry adiabatic lapse rate. In this case, the air parcel starts with a temperature 75 degrees, but by the time it rises to the top of the mountain at feet elevation, it will cool to Note that the air parcel now at feet has a temperature of So the air parcel is 1.

Since in this case the air parcel is colder than its surroundings, it is relatively dense. After being forced to the top of the mountain, the air parcel will sink back down to its initial elevation. The forced lifting from the terrain creates clouds and rain right over the mountains! In scientific terms, the initial lifting of the stable low level dry air by the terrain causes the air to adiabatically expand and reach saturation, at which point the environment is unstable to moist lifting and convection is the result.

There are many different types of thermodynamic diagrams, but the main one we will discuss are Skew-T Log-P diagrams, so-named because the isotherms lines of equal temperature, T on the diagram are slanted skewed and the isobars lines of equal pressure, P on the diagram are in log space. Here we will focus on how to read and utilize Skew-T Log-P diagrams often shortened to Skew-T diagram to determine parcel buoyancy and atmospheric stability.

You can see the vertical environmental temperature profile T plotted as the black jagged line on the right. The dew point temperature T d with height is plotted with the black jagged line on the left.

The vertical axis is air pressure in hPa, decreasing with height, so higher heights are toward the top of the chart. When the T and T d lines are close together, the environment has a high relative humidity and the air is closer to saturation.

In this particular sounding, there is a lot of moisture near the surface, but dries out in the mid-levels. Radiosonde balloons are launched twice a day 00Z and 12Z from many locations around the world. The latitude and longitude for the station is given in the top of the list on the right where station latitude SLAT is given as The station elevation SELV is 30 m.

The horizontal lines on a Skew-T are isobars, or lines of equal air pressure. You will typically see them given in hPa, but the lines in the above figure are in kPa.

The isobars have larger spaces as you get toward the top of the diagram because they are logarithmic with height. The evenly-spaced solid lines that slant up and to the right are isotherms, or lines of equal temperature T. This allows colder temperatures to be plotted on the diagram.

The dashed lines that run up and to the right are isohumes, or lines of constant mixing ratio. If you use a Skew-T where these lines are not dashed or color-coded, remember that these are spaced more closely together than isotherms and are more steep. They also do not line up with the temperature labels on the x-axis. The evenly-spaced curved solid lines that run from bottom right to top left are dry adiabats, and depict the dry adiabatic lapse rate 9. The dry adiabatic lapse rate is considered a constant, but you can see here that over large changes in temperature and pressure, it varies a little.

The dry adiabats always curve upward from right to left in a concave way. The uneven, dashed, lines that curve up and to the left are the moist adiabats. The moist adiabatic lapse rate varies with both temperature and moisture content, but is close to the dry adiabatic lapse rate at high altitudes due to cold temperatures and small moisture content. These lines are parallel to the dry adiabats higher up on the Skew-T Log-P diagram.

Here is a complete Skew-T Log-P diagram. All of the lines look confusing and complicated when combined, but each represents a constant change in one variable. On this Skew-T diagram, all of the same lines are there.

Horizontal blue lines are isobars, slanted blue lines are isotherms, slanted purple lines are isohumes, the green lines are the dry adiabats, and the blue curved lines are the moist adiabats. The T right and T d left black lines are close together and sometimes overlap in the lowest hPa of the atmosphere because the lower levels are incredibly moist, and a deep cloud layer extended up to nearly 6 km altitude.

When plotting a sounding on a Skew-T diagram, you may have a selection of data similar to the example given below. You will likely have pressure, temperature T , and a dew point temperature T d with altitude.

In order to plot the sounding, it is easiest to start by finding the pressure level and then move to the right to plot the temperature and dew point temperature. Pay careful attention to the fact that the isotherms are skewed.

Rotate the axis in your mind when you plot your temperature and dew point. Once you have plotted all of your temperatures and dew points, you will have a vertical temperature and humidity profile of the atmosphere. Now that we plotted the sounding, it is useful to know how a rising air parcel will behave when placed in this environment. Is the atmosphere stable, unstable, or conditionally unstable? We can determine this by estimating the rate at which a rising parcel will cool and drawing a parcel path upward.

A rising air parcel will cool at the dry adiabatic lapse rate until it is saturated, after which it will cool at the moist adiabatic lapse rate. How do we know when a parcel will be saturated? The Lifting Condensation Level LCL is the level at which the water vapor in an air parcel that is lifted dry adiabatically will be saturated.

If lifted above 3 km the parcel air finds itself warmer and less than the air outside. If lifted just a little bit beyond 3 km altitude the parcel will be able to continue to rise on its own. The atmosphere is conditionally unstable in this case. A rising parcel must first of all become saturated. Then it must be lifted to and just above the level of free convection. The value of the environmental lapse rate is one of the main factors that determines whether the atmosphere will be stable or unstable.

The ground and the air above it cool during the night. The atmosphere is usually most stable early in the morning. A temperature inversion represents an extremely stable situation. Rising parcels always cool with increasing altitude at either the dry or moist rate. In an inversion the surrounding air gets warmer and warmer with altitude. The difference between the cold parcel air and the warmer surroudings gets larger and larger with increasing altitude.

Sunlight warms the ground and the air next to it during the day. This steepens the environmental lapse rate and makes the atmosphere more unstable. Cooling air above the ground has the same effect. When the air is colder and sinks, we call this a stable air mass. It could mean a steady rain or snowfall but no rapid changes. Skip to content. Watch Live. I Bridge Shutdown. Back to School. School Day Forecast. Special Reports.

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