Skew T Flashcards
Precipitable Water (PW)
Gives the amount of moisture in the atmosphere, reported in inches.
Indicates how much precipitation will be associated with severe weather for a fixed location.
Contributors to “water loading”, which prevents updrafts within a cloud (hail formation).
Greater than 1.75 in. is water loaded
Less than 0.75 in. Is fairly dry sounding
Relative Humidity (RH)
A ratio of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated/
Maximum Temperature (MAXT)
Find temp at 150mb and bring it down to the surface using the dry adiabatic rate.
L57
The 700mb to 500mb lapse rate in unties of C/km
Lifted Condensation Level (LCL)
The height at which a parcel of air becomes saturated when it is lifted at the drug adiabatic lapse rate of 9.8 C/km (~10)
To find the LCL follow the temp up the dry adiabatic line until it crosses the saturation mixing ratio of the surface dew point.
Wet Bulb Zero (WB0/WBZ)
The value at which the sounding is at zero degrees Celsius due to evaporative cooling. Given as a pressure.
Lower WBZ heights indicate that the low level atmosphere is often too cool and stable to support large hail.
Lifted Index (LI)
The temperature difference between the environmental and parcel temperatures at the 500mb level.
Used in warmer temps/months
Showalter Index (SI)
The difference between the temp at 500mb and the temp of a parcel at 500mb when lifted from the 850mb level.
Used in colder temps/months
Total Totals (TT)
Purely focuses on instability. Has two components: the Vertical totals(VT) and the cross tables(CT).
VT is the static stability or lapse rate between 850mb and 300mb.
CT is the 850mb dew point.
K-index (KI)
“An attempt” to quantify the overall lapse rate and moisture content in the lower half of the troposphere and the depth of the moist layer.
Favors non-serve convection and should NOT be used in mountainous regions.
Relative Humidity (RH) values
0-40% = very low 41-60% = low 61-80% = moderate 81-100% = moist
L57 values
Less than 5.5 = stable
5.5 to 9 = Conditionally unstable
9 or greater = incredibly unstable
Lifted Index (LI) values
0 or greater = stable
- 1 to -4 = marginal instability
- 5 to -7 = large instability
- 8 to -10 = extreme instability
- 11 or less = ridiculous instability
Total Totals (TT) values
<44 = convection not likely 44-50 = convection likely 51-52 = isolated severe storms 53-56 = widely scattered severe storms Greater than 66 = scattered severe storms
K-Index (KI) values
<15 = convection not likely 15-25 = small potential for convection 26-39 = moderate potent for convection \+40 = high potential for convection
Severe Weather Threat Index (SW/SWEAT)
Combines thermodynamics and winds values.
Formula includes: low level moisture, instability, low level jet, upper level jet and warm air advection.
If the TT is less than 49, then the term equation is set to zero
Severe Weather Threat Index (SW/SWEAT) values
150-300 = slight severe 300-400 = severe storms possible \+400 = tornado severe storms possible
Energy index (EI)
Used to assess a severe storms potential by comparing heat and moisture characteristics between 850mb and 500mb.
An environment that is warm and moist in the low levels under a layer that is cool and dry is convectively unstable.
Convective Available Potential Energy (CAPE)
The difference between the air temp and the moist adiabat of a rising parcel is in indicator of the buoyancy of the atmosphere.
Measured in J/kg.
Convective Available Potential Energy (CAPE) values
0 = stable 0-1000 = marginally unstable 1000-2500 = moderately unstable 2500-3500 = very unstable \+3500 = extremely unstable
Convective Inhibition (CIN)
The negative area on a sounding or negative CAPE. Represents the amount of energy required to overcome the negative atmosphere.
A large cap or dry PBL will lead to high values of CIN and stability, inhibiting the growth of CBs.
Two most common cases:
- From sun heating the sfc of the atmosphere & adds energy to the air
- Along a boundary such as a front where the air is forced up into the unstable portion of the atmosphere
Hydrolapse
Rapid increase and decrease of dew point with height
Shear Value (SHR)
Value is found by finding the change in wind speed front the surface to 3000m (then dividing by 3000), represents positive wind shear
Shear Value (SHR) values
0-3 = weak 4-5 = moderate 6-8 = large \+9 = very large
Helicity Value (HEL)
Stream-wise vorticity available for a storm. SWV is a function of low level inflow and horizontal velocity generated by speed shear with height or directional shear with height in the PBL.
“Amount of turning within a storm”
Helicity Value (HEL) values
150-300 = possible supercell 300-400 = supercell severe storms \+400 = tornado supercells possible
Storm-relative Helicity (SRH)
Measure of the potential for cyclonic updraft rotation in right-moving supercells and is calculated for 1-3km above the ground.
> values > tornado risk
Energy Helicity Index (EHI)
Combines CAPE and SRH
>1 = supercells likely 1-5 = F2, F3 tornado possible >5 = F4, F5 tornado possible
Bulk Richardson Number (BRN)
An index that assesses the balance between CAPE and wind shear
Less than 45 = supercells possible
11-19 = optimum for severe storms
Less than 10 = environment too sheared
Potential Temperature
Temperature found by lifting or descending a parcel to 1000mb
Equivalent Potential Temperature
Temperature of a parcel after all latent heat energy is released in a parcel then brought to the 1000mb level.
Convective Condensation Level (CCL)
The height at which a parcel of air when heated from below will rise adiabatically until it is saturated.
Is always greater than or equal to the LCL
To find the CCL start with the surface dew point and follow the saturation mixing ratio line up until it crosses the temperature curve.
Level of Free Convection (LFC)
The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely without any additional force acting on it.
To find the LFC, being at the LCL and follow the moist adiabat up to where it intersects the temperature line.
