Applied Atmospheric Dynamics

Question: How do I read a surface station report from a map?
Answer: This is discussed in Section 1.2.3 of the book, and a sample station model is shown in Figure 1.4.

The basic station model provides information on the temperature, dew point temperature, sea level pressure, wind speed and direction, cloud cover, and current significant weather. The top left number is temperature, plotted in ^oF in the U.S. and in ^oC elsewhere around the world. At the bottom left is the dew point temperature (T_d) in the same units. At top right is the coded sea level pressure, described in the next Question. The cloud cover (center circle) is indicated by the number of quarter circles that are filled in. The significant weather is given as an icon between the temperature and the dew point temperature - some examples are shown in Figure 1.4.

The wind speed symbols are additive, and wind is generally reported in units of knots (1 kt = 0.51 ms^-1). A short wind barb indicates a wind speed of 5 kts, a long barb indicates a wind speed of 10 kts, and a pennant indicates a wind speed of 50 kts. The actual wind may be within ± 2 kts of the plotted wind speed. The wind direction is given by the angle of the line that anchors the barbs; the direction the line points is the direction the wind is coming from.
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Question: How do I find sea level pressure from the surface observations?
Answer: If the sea level pressure code is less than 500, divide by 10 then add 1000. For example, if the sea level pressure code is 283, first divide by ten to get 28.3, then add 1000 to get the final result of 1028.3 hPa. If the sea level pressure code is greater than 500, divide by 10 then add 900. For example, if the sea level pressure is 992, first divide by 10 to get 99.2, then add 900 to obtain the final result of 999.2 hPa.
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Question: How do I decode a US state name abbreviation?
Answer: A list of US states and their abbreviations is given here.
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Question: How do I work out the name and location of a station code?
Answer: A list of US stations and their abbreviations is given here. More generally, meteorological stations may be identified in three ways:

a station index number, assigned by the World Meteorological Organization (WMO),

a four letter location indicator, assigned by the International Civil Aviation Organization (ICAO) or,

in the US, a three letter location indicator assigned by the U. S. Federal Aviation Administration (FAA).

At least one of these type of identifiers is associated with each station around the world. A large number of stations have both an index number and a location indicator assigned to them. On this CD ROM we use a three letter identifier. So, for example, "PGA" is Page Municipal Airport in Arizona. More information is available from the US National Weather Service, and a complete listing (which you can search using "Find in this Page" in your browser) of all stations around the globe in order of ICAO station indicator is here. Alternatively, you can search for specific stations in the United States and around the globe at US NWS Station Information web page.

In all cases, it is very useful to combine this information with a detailed map from an atlas or an online service such as Google Earth or Mapquest.
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Question: How do I read an upper air station report from a map?
Answer:This is introduced in Section 1.3.3 of the book.

On upper pressure level maps, the temperature is always plotted with units of ^oC throughout the world, and as for surface weather maps is plotted in the upper left corner of the station model. The dew point depression, rather than the dew point temperature, is plotted in the lower left corner of the station model. The central circle of the station model is filled if the dew point depression is less than 5^oC, since a small dew point depression is indicative of areas that may be cloudy. Otherwise, the circle is left unfilled. In the top right quadrant, the height of the pressure surface in decameters is reported. The wind speed and direction is reported as for the surface maps.
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Question: How do I interpret the wind direction information in a station data file?
Answer:The wind direction given by meteorologists indicates the direction that the wind is coming from. Wind directions given by numerical values have the following geographic directions: 90^o=East, 180^o=South, 270^o=West, and 360^o=North, with intermediate numerical values corresponding to intermediate directions.
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Working with hodographs and skew-T diagrams

Question: How do I plot a hodograph?
Answer:A blank hodograph diagram is provided here.

The wind direction given by meteorologists indicates the direction that the wind is coming from. Wind directions given by numerical values have the following geographic directions: 90^o=East, 180^o=South, 270^o=West, and 360^o=North, with intermediate numerical values corresponding to intermediate directions.

To make the hodograph, label the wind speed rings on the blank diagram with values appropriate for the data you are using. Then, for each height level, place a dot on the graph at the appropriate wind speed (given by the circles) and wind direction (given by the radial lines). Note that the wind directions plotted on the hodograph are the opposite to that which is normally used: a North wind is plotted at the bottom of the hodograph, and you proceed counter clockwise to East, South, and West. This convention is used so that the final plot on the hodograph indicates the direction the wind is going towards.

When you have placed all the dots, draw a line joining the dots from the surface point to the upper most point.
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Question: How do I plot a skew-T diagram?
Answer: A blank skew-T diagram is provided here.

Skew T-log P diagrams, often referred to simply as skew T diagrams, are commonly used by meteorologists to diagnose the structure of the atmosphere at a particular location and to determine how air parcels will be modified due to ascent or descent. The logarithm of pressure is used as the vertical axis of the plot, and temperature is plotted using an axis that is skewed at a 45^o angle to the horizontal (hence the name of the diagram). Lines of constant temperature (isotherms) are shown as black lines that slope up and towards the right on the diagram. The saturation mixing ratio is also accorded a skewed axis, and lines of constant saturation mixing ratio are shown by blue dashed lines that slope up and towards the right on the diagram. Lines that represent dry adiabatic processes (dry adiabats) are shown by yellow lines sloping up and towards the left, while lines that represent moist adiabatic processes (moist adiabats) are shown as curved black lines on the diagram.

To plot sounding data on a skew T diagram, place a dot on graph at the appropriate temperature for each pressure level in your data. Do the same for the dew point temperature if it is known. When you have placed all of the dots, draw a line joining the dots from the highest pressure (the surface) to the lowest pressure for each temperature point plotted. Repeat this for the dewpoint temperature. The resulting diagram will have two lines plotted that represent the vertical variation of temperature and dewpoint temperature in the atmosphere.

Question: How do I determine mixing ratio and saturation mixing ratio using a skew T diagram?
Answer:The saturation mixing ratio can be determined by locating the saturation mixing ratio line that intersects a point defined by the measured temperature and pressure. The saturation mixing ratio lines are the blue dashed lines that slope up and towards the right on the skew T diagram. If the point defined by the temperature and pressure of interest does not lie on a saturation mixing ratio line you will need to estimate the saturation mixing ratio based on the saturation mixing ratio lines that lie on either side of our point of interest. The saturation mixing ratio lines are labeled at both the bottom and top of the diagram, with the leftmost line having a value of 0.1 g k^-1 and the rightmost labeled line having a value of 36 g kg^-1.

Question: How can I represent a dry adiabatic process on a skew T diagram?
Answer:First start with two points defined by the initial temperature and pressure and the initial dew point temperature and pressure of the air parcel that will be modified by the dry adiabatic process. Changes in the temperature of this air parcel as it undergoes a dry adiabatic process will be given by a line that intersects the initial pressure and temperature of the air parcel and that is parallel to the dry adiabats plotted on the skew T diagram. The dry adiabats are yellow lines that slope up and to the left on the diagram. If this air parcel sinks (i.e. moves towards higher pressure) the temperature will increase. If this air parcel rises (i.e. moves towards lower pressure) the temperature will decrease. Changes in temperature that are parallel to dry adiabats on a skew T diagram represent conservation of potential temperature.

The dew point temperature of the air parcel will also change as the parcel experiences a dry adiabatic process. First, we note that during a dry adiabatic process the total amount of water vapor in the air parcel remains constant, and thus the mixing ratio will also remain constant. The mixing ratio of an air parcel is given by the intersection of the mixing ratio lines with the parcel dew point temperature and pressure. The mixing ratio lines are plotted on the skew T diagram as blue dashed lines that slope up and towards the right. To represent the conservation of mixing ratio draw a line that starts at the value of the initial mixing ratio and is parallel to the mixing ratio lines on the skew T diagram. Each point along this line has a constant mixing ratio and defines the dew point temperature and pressure of the air parcel as it undergoes a dry adiabatic process.

As long as the dew point temperature is less than the temperature the air parcel will remain unsaturated. See “How can I represent a moist adiabatic process on a skew T diagram?” to find out how to determine how the temperature and dew point temperature of a saturated air parcel will change as it is lifted.

Question: How can I represent a moist adiabatic process on a skew T diagram?
Answer:If a saturated air parcel (i.e. an air parcel in which the temperature is equal to the dew point temperature) is lifted the potential temperature and mixing ratio will not be conserved as was the case for an air parcel undergoing a dry adiabatic process (see “How can I represent a dry adiabatic process on a skew T diagram?”). The potential temperature will increase due to latent heat release as water vapor condenses, and the change in air parcel temperature will now be given by the moist adiabats (curved black lines) on the skew T-log P diagram. The mixing ratio will decrease due to the condensation of water vapor, and the dew point temperature will remain equal to the air temperature (that is, the rising air parcel will remain saturated). Thus the dew point temperature of the rising air parcel will also be given by the moist adiabats on the diagram. Graphically this is determined by drawing a line, starting at the point defined by the initial temperature, dewpoint temperature, and pressure, that is parallel to the moist adiabats.

Question: How can I find the lifting condensation level on a skew T diagram?
Answer:First identify the two points defined by the initial temperature, dew point temperature, and pressure on the skew T diagram. If the dew point temperature is less than the temperature the air parcel is unsaturated. Dry adiabatic ascent will cause the temperature of the air parcel to decrease while the dew point temperature will increase. To determine how the air parcel temperature and dew point temperature will change due to dry adiabatic ascent see “How can I represent a dry adiabatic process on a skew T diagram?”. The point at which the dry adiabat (defined by the initial temperature and pressure) and the mixing ratio line (defined by the initial dew point temperature and pressure) intersect is the lifting condensation level, and will be the base of a cloud formed by dry adiabatic lifting of the air parcel.

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