4.3 Weather Models
4.3.4 Skew-T Log-P Diagram
Figure 18 and Figure 19 show a Skew-T log-P diagram [97]. This diagram is used to predict weather information such as saturation, stability, and wind shear based on visual analysis. It shows the logarithmic
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FI GURE 18 Skew-T log-P diagram as a means to calculate properties of the moist air with a pressure range of 20 kPa to 100 kPa [97].
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FIGU RE 19 Example of the use of a Skew-T log-P diagram as a means to calculate properties of the moist air with a pressure range from 20 kPa to 100 kPa [97].
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pressure (ordinate) versus the temperature (abscissa) for dry adiabats and saturated scenarios. The far-left ordinate shows pressure decreasing logarithmically; the far-right ordinate shows the altitude increasing linearly.
The temperature increases to the right. Isotherms are solid-straight-right- oblique (i.e., skewed) lines, crossing the vertical lines at a 30-degree angle, corresponding to the decrease in temperature with increasing altitude and pressure. Isobars are solid-straight horizontal lines. Two main groups of isopleths are moist and dry adiabats. The moist adiabats are dashed-left- oblique lines, crossing the vertical lines at angles that vary between 5 and 30 degrees, concaving downward in lower temperatures and curving upward in higher temperatures. The dry adiabats are solid-left-oblique lines, crossing the vertical lines at 30-degree to 40-degree angles, concaving downward. Dry and moist adiabats converge to parallel lines at colder temperatures and higher altitudes. Isohumes are dotted-straight-right- oblique lines with a 20-degree inclination. Isotherms and dry adiabats in this diagram are almost perpendicular to one another. The Skew-T log-P diagram is similar to the tephigram. Using this diagram, it is possible to calculate the Lifting Condensation Level (LCL), Convective Condensation Level (CCL), Level of Free Convection (LFC), Equilibrium Level (EL), Convective Available Potential Energy (CAPE), and Convective Inhibition (CIN). A weatherperson plots the dry-bulb temperature (T) and dew- point (Td) temperatures on the diagram and, based on their variation with altitude, predicts the aforementioned factors as a function of the altitude.
Any prediction is to start from the current elevation on the diagram and is identified by the (T, Td). Dry properties (e.g., dry adiabats) are to start from the dry-bulb temperature while the saturated properties (e.g., moist adiabats) are to start from the dew-point temperature.
For instance, to predict the LCL, which is the pressure level that an air parcel needs to reach for it to rise in dry adiabatic fashion in order to become saturated, you may follow the dry adiabats from the dry-bulb temperature, follow a saturation mixing ratio (isohumes) from the dew-point temperature, and intersect the two—where the LCL is located. In this location, the air parcel’s lapse rate changes from the dry adiabats to the moist adiabats.
CCL is the pressure level of a parcel when it is heated to the convective temperature (a temperature that the parcel needs to reach to freely rise), and it rises to form cumulus clouds. The intersection of the environment temperature and the line of saturation mixing ratio (isohumes) passing from the dew-point temperature is the CCL. To obtain the convective temperature, you are to warm the parcel in a dry adiabatic process, starting from the CCL until you cross the current isobar. The intersection of the
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isobar and the dry adiabatic line identifies the convective temperature.
The LCL and CCL are used to determine the height of the cloud base, with the former mainly used for non-convective clouds and the latter used for convective clouds—the cloud bases are usually between the two condensation levels.
LFC is the pressure level of an air parcel that it is to reach so that its temperature equalizes to that of the environment, and it is obtained by intersecting the moist adiabats at the LCL with the environment temperature. Note that not all weather conditions result in the LFC—this is seen where the moist adiabats does not intersect the dry-environmental temperatures, and this is when the temperature is stable enough not to decrease sharply with increasing altitude. Additionally, having the LFC during the day does not guarantee also having one during the night in more stable conditions.
EL only exists where the LFC exists. This is where the moist adiabats intersect the air parcel environmental temperature again. This is the level at which the temperature of the air parcel is the same as that of its environment, and above this level the parcel is colder and denser. This identifies the top boundary of the anvil clouds, where they look as if they are creating an umbrella for their high momentum and not because the air above is buoyant.
CAPE is the area in between the environmental temperature and temperature of the parcel as it undergoes a moist adiabatic process.
Therefore, its lower boundary is the LFC, and the upper boundary is the EL. CAPE determines the positive buoyancy of an air parcel relative to its environment and therefore identifies the strength of an updraft.
CIN measures the negative buoyancy—resistance to convection—with its upper boundary being the LFC and its lower boundary being the level of the environment temperature—namely the surface. Early morning has the highest CIN, and it decreases during the day with the warming up of the surface.
Examples for determining the LCL (670 hPa), CCL (540 hPa), LFC (320 hPa), and EL (255 hPa) are shown in Figure 19. The convective temperature is 5 °C. The CAPE and CIN are indicated by blue and gray- filled areas.