How can the atmosphere be more stable

The lack of vertical mixing leads to a "stratified" atmosphere, where many atmospheric variables are separated into layers instead of being well-mixed. The stratification of the atmosphere when stable leads to, for instance, pollution episodes and drastic changes in wind speed and direction over short vertical distance.

An example of a stratified and stable environment can be seen in Figure 6 to the left. Another atmospheric consequence of a stable and stratified atmosphere involves the process of cloud formation. Assuming that there is sufficient moisture present in the atmosphere, stratiform clouds can form in a stable environment. This can only occur if the stable air is forced upward either through the convergence of air into a low pressure center or through the orographic lifting.

An example of each of these processes can be found below in Figure 7 and Figure 8. Please install the Macromedia Flash Player in order to view this demonstration. Figure 7: This figure shows how air converges into the center of a low pressure system and is forced to rise. Figure 8: The demonstration above shows how air can be forced to rise due to a change in topography. As mentioned above, a temperature inversion is the most stable environmental profile possible.

This type of atmospheric temperature profile can occur pretty much anywhere throughout the atmosphere, aloft or near the ground- each having a different impact on the weather. At the surface, temperature inversions can form during both the daytime and the nighttime, depending on the conditions and location.

The more common type of inversion is the nocturnal temperature inversion that occurs due to the radiational cooling of the Earth's surface after the sun sets. A temperature inversion can also form over cool bodies of water during the daytime in the summer. Temperature inversions also occur aloft in the stratosphere the layer directly above the troposphere.

Inversions in this layer of the atmosphere limit the vertical development of thunderstorms and their anvils hence the name, stratosphere.

The last type of temperature inversion is called a subsidence inversion. This type of temperature inversion is a result of the sinking and warming of air parcels. In this case, however, the top of the air parcel descends and warms more than the bottom of the air parcel. This type of temperature inversion is generally found to the east of high pressure systems well above the ground and well below the stratosphere.

Houze's Cloud Atlas. One can tell the difference between clouds composed of primarily water and clouds composed of ice by looking at the edges of the cloud. If the cloud has distinct, sharp edges, it is most likely a water cloud. A rising parcel of unsaturated air ends up cooler and denser than the surroundings.

A parcel of saturated air, which cools at a slower rate, ends up warmer than the air around it. The condition for instability is that the air must be saturated. The condition for instability in that case is the direction of the initial push that you give to the rock.

We'll leave the environmental lapse rate the same for the last and most instructive example. In this case the parcel of air starts out unsaturated. It becomes saturated when lifted to 1. From that point on upward the rising parcel will cool at the moist adiabatic rate.

Initially the rising parcel is colder and denser than the surrounding air. If the parcel is lifted to 3 km it has the same temperature as the air around it. 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.

The effects of moisture change the lapse rate of the air parcel and, therefore, affects stability. However, the concepts are still the same and we still compare the air parcel temperature to the environmental temperature. We have just one added complication to worry about—we need to know whether the air parcel is dry or moist.

Some definitions are included below, which take into account both dry and moist adiabatic lapse rates. The atmosphere is said to be absolutely stable if the environmental lapse rate is less than the moist adiabatic lapse rate.

This means that a rising air parcel will always cool at a faster rate than the environment, even after it reaches saturation. If an air parcel is cooler at all levels, then it will not be able to rise, even after it becomes saturated when latent heating will counteract some cooling. The atmosphere is said to be absolutely unstable if the environmental lapse rate is greater than the dry adiabatic lapse rate.

This means that a rising air parcel will always cool at a slower rate than the environment, even when it is unsaturated. This means that it will be warmer and less dense than the environment, and allowed to rise. The atmosphere is said to be conditionally unstable if the environmental lapse rate is between the moist and dry adiabatic lapse rates.

This means that the buoyancy the ability of an air parcel to rise of an air parcel depends on whether or not it is saturated. In a conditionally unstable atmosphere, an air parcel will resist vertical motion when it is unsaturated, because it will cool faster than the environment at the dry adiabatic lapse rate. If it is forced to rise and is able to become saturated, however, it will cool at the moist adiabatic lapse rate. In this case, it will cool slower than the environment, become warmer than the environment, and will rise.

Around Hawaii, the atmosphere is almost always conditionally unstable, meaning that the environmental lapse rate lies somewhere between the dry and moist adiabatic lapse rates. For this reason, Hawaii almost always has convective clouds. Convective clouds are clouds where the edges are bumpy and cumuliform, like cauliflower. The clouds are convective because the atmosphere is stable to dry lifting and unstable to moist lifting.

Once the air is saturated, instability sets in and vertical motion takes off. This is especially common as air is lifted over our mountainous islands. 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.