500 mb Heights and Vorticity
For all map analyses of 500 mb heights, the black contours indicate the geopotential height of the 500 millibar (mb) surface, in tens of meters. Low geopotential height (as compared to other nearby locations at the same latitude) indicates the presence of a storm (i.e., either developing or mature) or the presence of a trough at or right around the height of the mid-troposphere (i.e., between roughly 4 and 6 kilometers above the surface of the Earth). Relatively high geopotential height values indicate the presence of a ridge which is typically associated with relatively quiescent and non-eventful weather conditions. It is worth noting that the term geopotential height refers to a vertical coordinate position referenced to Earth’s mean sea-level and is effectively and adjustment made to geometric height using the variation of Earth’s gravitational force with respect to latitude and vertical position.
For all map analyses of vorticity, the color shaded contours indicate vorticity at 500 millibars (mb): Red for positive vorticity, blue for negative. Positive vorticity indicates the presence of a counterclockwise rotation of the winds in a given region. Negative vorticity indicates clockwise rotation of the winds, and/or lateral shear of the wind with stronger flow to the left of the direction of flow. Positive (or negative in the Southern Hemisphere) vorticity at 500 millibars (mb) is associated with cyclones or storms at upper levels, and will tend to coincide with troughs in the geopotential height field. Negative (positive in SH) vorticity is associated with calm weather and will tend to coincide with ridges in the geopotential height field.
Sea Level Pressure and 1000-500 Thickness
For all map analyses of sea-level pressure, the colored contour lines indicate sea level pressure in millibars. High pressure is colored in tints of red or yellow, low pressure is colored in tints of green or blue. Only the last 2 digits for sea-level pressure values are shown; while considering that sea level pressure is usually around 1000 millibars on average, so add 1000 to values in the range of 00-50, and add 900 to values in the range of 50-98. Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather.
For all map analyses of 1000-500 thickness, the shaded contours indicate the vertical distance, or thickness, between the 1000 millibar surface and the 500 millibar surface, measured in tens of meters. Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels. Thus, low values of thickness mean relatively cold air. The 540 line is highlighted in black, since this line is often used as a rule of thumb to indicate the division between rain and snow for regions of relatively lower terrain. When there is precipitation where the thickness is below 540 decameters, it is generally snow. If the thickness is above 540 decameters, it is usually rain (or sleet if the air next to the surface is below freezing). However, on some occasions, there can sometimes be exceptions to the 540 decameter contour rule depending on geography and time of the year.
700 mb Vertical Velocity
For all map analyses of 700 mb vertical velocity, the vertical velocity at 700mb (measured in millibars/hour) is shown to represent the strength of regions of either upward or downward motions in the atmosphere. Negative values indicate ascending air, and positive values denote sinking air. Ascending motion is associated with cloudiness, rain, and unsettled weather conditions. Large negative values of vertical velocity correspond to areas of heavy rainfall if moisture is available and there is sufficient atmospheric instability in place at a given time.
850 mb Temperature, Relative Humidity, and Winds
For all map analyses of 850 mb temperature, the colored contour lines indicate the air temperature at the 850 millibar level, in degrees Celsius. The 0 °C contour is highlighted, as this is also often used as a divider between rain and snow.
For all map analyses, of 850 mb relative humidity, the green shading indicates the relative humidity percentage at the 850 millibar level. High values indicate the availability of moisture. When areas of large upward vertical velocity are co-located with high moisture availability, heavy rainfall will be much more likely to occur.
For all map analyses of 850 mb winds, the streamlines indicate the direction of wind flow. Advection of moisture by the wind can be inferred by noticing the direction and rate at which moist areas appear to be blown at a given point in time. Similarly, temperature advection can be inferred by noticing whether the wind is blowing cold air toward a warmer region, or warm air toward a colder region.
200 mb Wind Streamlines and Isotachs
For all map analyses of 200 millibar (mb) wind streamlines and isotachs, the streamlines indicate the direction of flow of the wind, which is generally from west to east throughout most of the subtropics, mid- and high-latitudes on Earth.
Purple shading indicates the speed of the winds at the given pressure level, measured in terms of meters per second. The 200 millibar (mb) level is often near or right at the core of the jet stream, so the tracks of the jet streams can be seen very clearly via studying wind flow at that altitude on Earth.
Precipitable Water and Convective Available Potential Energy (CAPE)
The shaded contours indicate values of total precipitable water within a given vertical column region of the atmosphere. Hence, precipitable water is the total depth of liquid water that would materialize if all water vapor contained in a vertical column of air were “squeezed out” of a given column of the atmosphere. It also is very helpful for indicating the total humidity of the air above a given location and is a fairly good indicator of the amount of moisture which could potentially be made available to facilitate rainfall.
Convective available potential energy (CAPE) is a good indicator of the potential for strong thunderstorms and severe weather. It is essentially a measure of the amount of net energy available for convection to form, under the right circumstances. High values of CAPE indicate that most (but not necessarily all) conditions exist for strong thunderstorms to develop over some specified period of time. CAPE is drawn in various colored contour lines which are overlaid on top of the precipitable water shaded contours in most panels.
Lower-level, Mid-level, and Upper-Level Cloud Cover or Relative Humidity at 800 mb, 500 mb, and 300 mb
Light blue shaded areas show where low-level cloud cover exceeds 50%. For these map analyses, cloud cover is not available so relative humidity greater than 70% at 800mb is used as a proxy.
Light green shaded areas show where middle-level cloud cover exceeds 50%. For these map analyses, cloud cover is not available so relative humidity greater than 70% at 500mb is used as a proxy.
Pink shaded areas show where upper-level cloud cover exceeds 50%. For the analysis, cloud cover is not available so relative humidity greater than 70% at 300mb is used as a proxy.
For all map analyses of 500 mb heights, the black contours indicate the geopotential height of the 500 millibar (mb) surface, in tens of meters. Low geopotential height (as compared to other nearby locations at the same latitude) indicates the presence of a storm (i.e., either developing or mature) or the presence of a trough at or right around the height of the mid-troposphere (i.e., between roughly 4 and 6 kilometers above the surface of the Earth). Relatively high geopotential height values indicate the presence of a ridge which is typically associated with relatively quiescent and non-eventful weather conditions. It is worth noting that the term geopotential height refers to a vertical coordinate position referenced to Earth’s mean sea-level and is effectively and adjustment made to geometric height using the variation of Earth’s gravitational force with respect to latitude and vertical position.
For all map analyses of vorticity, the color shaded contours indicate vorticity at 500 millibars (mb): Red for positive vorticity, blue for negative. Positive vorticity indicates the presence of a counterclockwise rotation of the winds in a given region. Negative vorticity indicates clockwise rotation of the winds, and/or lateral shear of the wind with stronger flow to the left of the direction of flow. Positive (or negative in the Southern Hemisphere) vorticity at 500 millibars (mb) is associated with cyclones or storms at upper levels, and will tend to coincide with troughs in the geopotential height field. Negative (positive in SH) vorticity is associated with calm weather and will tend to coincide with ridges in the geopotential height field.
Sea Level Pressure and 1000-500 Thickness
For all map analyses of sea-level pressure, the colored contour lines indicate sea level pressure in millibars. High pressure is colored in tints of red or yellow, low pressure is colored in tints of green or blue. Only the last 2 digits for sea-level pressure values are shown; while considering that sea level pressure is usually around 1000 millibars on average, so add 1000 to values in the range of 00-50, and add 900 to values in the range of 50-98. Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather.
For all map analyses of 1000-500 thickness, the shaded contours indicate the vertical distance, or thickness, between the 1000 millibar surface and the 500 millibar surface, measured in tens of meters. Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels. Thus, low values of thickness mean relatively cold air. The 540 line is highlighted in black, since this line is often used as a rule of thumb to indicate the division between rain and snow for regions of relatively lower terrain. When there is precipitation where the thickness is below 540 decameters, it is generally snow. If the thickness is above 540 decameters, it is usually rain (or sleet if the air next to the surface is below freezing). However, on some occasions, there can sometimes be exceptions to the 540 decameter contour rule depending on geography and time of the year.
700 mb Vertical Velocity
For all map analyses of 700 mb vertical velocity, the vertical velocity at 700mb (measured in millibars/hour) is shown to represent the strength of regions of either upward or downward motions in the atmosphere. Negative values indicate ascending air, and positive values denote sinking air. Ascending motion is associated with cloudiness, rain, and unsettled weather conditions. Large negative values of vertical velocity correspond to areas of heavy rainfall if moisture is available and there is sufficient atmospheric instability in place at a given time.
850 mb Temperature, Relative Humidity, and Winds
For all map analyses of 850 mb temperature, the colored contour lines indicate the air temperature at the 850 millibar level, in degrees Celsius. The 0 °C contour is highlighted, as this is also often used as a divider between rain and snow.
For all map analyses, of 850 mb relative humidity, the green shading indicates the relative humidity percentage at the 850 millibar level. High values indicate the availability of moisture. When areas of large upward vertical velocity are co-located with high moisture availability, heavy rainfall will be much more likely to occur.
For all map analyses of 850 mb winds, the streamlines indicate the direction of wind flow. Advection of moisture by the wind can be inferred by noticing the direction and rate at which moist areas appear to be blown at a given point in time. Similarly, temperature advection can be inferred by noticing whether the wind is blowing cold air toward a warmer region, or warm air toward a colder region.
200 mb Wind Streamlines and Isotachs
For all map analyses of 200 millibar (mb) wind streamlines and isotachs, the streamlines indicate the direction of flow of the wind, which is generally from west to east throughout most of the subtropics, mid- and high-latitudes on Earth.
Purple shading indicates the speed of the winds at the given pressure level, measured in terms of meters per second. The 200 millibar (mb) level is often near or right at the core of the jet stream, so the tracks of the jet streams can be seen very clearly via studying wind flow at that altitude on Earth.
Precipitable Water and Convective Available Potential Energy (CAPE)
The shaded contours indicate values of total precipitable water within a given vertical column region of the atmosphere. Hence, precipitable water is the total depth of liquid water that would materialize if all water vapor contained in a vertical column of air were “squeezed out” of a given column of the atmosphere. It also is very helpful for indicating the total humidity of the air above a given location and is a fairly good indicator of the amount of moisture which could potentially be made available to facilitate rainfall.
Convective available potential energy (CAPE) is a good indicator of the potential for strong thunderstorms and severe weather. It is essentially a measure of the amount of net energy available for convection to form, under the right circumstances. High values of CAPE indicate that most (but not necessarily all) conditions exist for strong thunderstorms to develop over some specified period of time. CAPE is drawn in various colored contour lines which are overlaid on top of the precipitable water shaded contours in most panels.
Lower-level, Mid-level, and Upper-Level Cloud Cover or Relative Humidity at 800 mb, 500 mb, and 300 mb
Light blue shaded areas show where low-level cloud cover exceeds 50%. For these map analyses, cloud cover is not available so relative humidity greater than 70% at 800mb is used as a proxy.
Light green shaded areas show where middle-level cloud cover exceeds 50%. For these map analyses, cloud cover is not available so relative humidity greater than 70% at 500mb is used as a proxy.
Pink shaded areas show where upper-level cloud cover exceeds 50%. For the analysis, cloud cover is not available so relative humidity greater than 70% at 300mb is used as a proxy.