Instructions and discussion can be found HERE
The maps and other images can be found HERE
Instructions and discussion can be found HERE
The maps and other images can be found HERE
Instructions and discussion can be found HERE
The maps and other images can be found HERE
The 500mb Chart: This is perhaps the most important weather map of them all. The 500mb level lies at about the halfway point in the atmosphere -- half the atmosphere is below, and half is above. The 500mb level varies from about 4800 geopotential meters (in polar regions) to about 6000 geopotential meters (in tropical regions). This is about 18,000 feet above mean sea level. For most places on the earth, this is comfortably above the planetary boundary layer.
The first thing to look for on the 500mb chart is the troughs and ridges. There are various ways to identify the troughts and ridges. The isoheight analysis will show the major troughs and ridges. Look for the maximum cyclonic curvature of the isoheight lines to identify troughs. Look for the maximum anti-cyclonic curvature of the lines to identify ridges. Note that ridges are sometimes not as well defined as troughs.
Also use the wind field to identify troughs and ridges. Again, look for cyclonic curvature of the winds to identify troughs, and anti-cyclonic curvature to identify ridges. The wind field can help to locate a trough or ridge when the placement is in doubt by just looking at the height field. The winds are also important for identifying small short-wave troughs. These are troughs that are smaller in scale, perhaps only 100 miles in wavelength. Because there is so little upper air data, these troughs can be hard to identify. But when they are imbedded in the jet stream (see below) they can be very important. These small features can be easier to identify at the 700mb level. So use both the 700 and 500mb charts to look for short-wave troughs. Also look at the actual observed winds at the ballon sounding stations. Sometimes these will show the turning in the wind more clearly than the computer analysis.
By old tradition, the troughs are labeled with a dashed red line, and the ridges by a zig-zag blue line. More recent practice is to label the troughs by a solid thick black line, and the ridges by a solid zig-zag black line.
Another important field to look at on the 500mb chart is the wind speed. Lines of equal wind speed are called Isotachs. If you look at an analysis of wind speed at the 500mb level, you will see that areas of higher winds tend to form bands extending from west to east. The main band of higher wind speeds is the jet stream. When there is more than one main band of higher wind, this is called split flow. Elongated areas of wind maximums, as identified by the isotach analysis, are called jets and are often labeled with a thick arrow than runs through the maximum wind area. By old tradition, the jet maxima are drawn in blue at the 500mb level. Localized intense maxima in the jets are sometimes called jet streaks.
The maximum wind speed in the jet stream is normally above the 500mb level, and is found by examining the 200mb or 300mb chart. But the main polar jets begin to be visible on the 500mb chart and it is important to identify them, particularly the way they are interactiing with troughs and ridges.
Another important variable to examine at the 500mb is something called vorticity. Vorticity is a measure of spin in the atmosphere. In the field of fluid mechanics, the localized spin of a fluid is called the curl. Meteorologists look at the component of the curl that is expressed as horizontal rotation, and they call it vorticity. This will be discussed in the lecture in the near future.
The term relative vorticity is used to refer to the rotation of the air relative to the moving earth's surface. The term absolute vorticity is used when the calculation of vorticity includes the earth's rotation. Vorticity is difficult to calculate by hand, so we look at computer analyses of this field. Vorticity units are radians per second. The values are usually multiplied by 10 to the 5th power, ie the values are plotted as radians per 100,000 seconds.
The Air Force Training Manual for weather analysis provides a lot of useful information on weather map analysis.
A complete set of GFS analysis and forecast maps for the late September 2010 heavy rain event can be found at this LINK
Analyzing fronts requires looking at every meteorological variable.
Pressure: Pressure usually drops ahead of fronts and rises after the front passes. With a strong cold front, this can be dramatic. In some cases, the pressure will rise slowly ahead of a front, but more rapidly after the front passes. If the isobars are drawn on the map, the front will usually be found in a "trough" of lower pressure. It may be appropriate to show the isobar contour lines "kinked" at the front.
Temperature: Fronts usually seperate air masses of different temperature. Within an air mass, the isotherms will be widely spaced, indicating gradual changes in temperature over distance. Near a front, the isotherms will be closely spaced, indicating a rapid change in temperature. The front is usually located on the warm edge of the closely spaced (also referred to as packed) isotherms.
Moisture: Fronts usually seperate air masses having different amounts of moisture. When the moisture is the only significant change across a front, it is sometimes called a "dryline".
Winds: The winds typically shift speed and/or direction when a front passes. On the surface map, winds will usually be different in direction on one side of a front compared to the other.
Pressure is defined as force/area. Think of cutting something with a knife. If you attempt to cut with the flat side of the blade, it will not cut. If you apply the same force on the knife, using the sharp side of the blade, it cuts easily. This is because the area of the sharp edge is much less, and thus the pressure along that edge is much higher.
The official unit of pressure is the pascal. This is one Newton per square Meter. (To put the Newton in perspective, one Newton is about the force of Earth's gravity on an object with a mass of about 102 grams. Such as, perhaps, a small apple.)
It used to be there was no official SI pressure unit in the metric system, and meteorologists adopted the Bar. This is also a metric unit of pressure, where 1 Bar equals 100,000 pascals. It turns out that one Bar is close to the typical pressure of the atmosphere near sea level.
The millibar (1000th of a bar), abbreviated mb, became the most commonly used unit of pressure in meteorology. In these units, the standard atmosphere is 1013.25mb. We still see mb used very widely in meteorology, especially in the US. However, the use of the hectoPascal, abbreviated hPa, is growing. hPa is exactly the same as the mb.
Inches of mercury is older usage, abbreviated inHg, and is the pressure required to lift a column of mercury the specified number of inches when under the influence of earth's gravity. 1 inHg equals 3,386.389 pascals at 0C and at standard gravity. The standard atmosphere, 1013.25hPa, is 29.92inHg.
In Clim301 Lab, we will be using primarily mb and/or hPa.
In meteorology, the 24 hour clock is used. The time zone normally used is Universal Time, or UTC. This is often labeled as "Z" time.
Noon in Universal Time is approximately the time that the sun is directly overhead at the zero'th meridian, which is anchored at Greenwich, England. Local time in the US is earlier than UTC.
By convention, midnight UTC -- 00Z -- is the start of the new day, not the end of the old day. Thus, 00Z Thursday is the borderline between Wednesday and Thursday.
Eastern Daylight Time (EDT) is four hours earlier than UTC.
Eastern Standard Time (EST) is five hours earlier than UTC.
Examples:
00Z Thursday is 8pm Wednesday EDT
00Z Thursday is 7pm Wednesday EST
12:00UTC Friday is 7am Friday EST
18:32Z Sunday is 2:32pm Sunday EDT
Note that Z and UTC are used interchangeably.
Sometimes seen on older charts is the use of GMT, or Greenwich Mean Time. It is the same as Z or UTC.