METR 104:
Our Dynamic Weather
(Lecture w/Lab)
Responses to Thought Questions
on Radiosonde Soundings
Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Spring 2012

The questions below, and the responses to them, refer to the two vertical profile plots of temperature, dew-point temperature, and wind speed and direction ("soundings") provided as handouts accompanying this one. The observations on these plots were recorded by radiosondes launched from Oakland International Airport (KOAK) at 00Z and 12Z (respectively) on January 13, 2012:

For reference, the vertical profile of long-term, global average temperature in the atmosphere can also be useful:

  1. There are two line plots on each graph, showing temperature and dew-point temperature. Which one is which, and how can you tell? [Hint: refer to what you've learned about temperature and dew-point temperature on meteograms.]

    Dew point temperature can never exceed temperature (though it can equal temperature). Hence, the right-hand profile must be temperature while the left-hand profile must be dew-point temperature, because temperature is plotted on the horizontal axis of the graph and increases to the right.

  2. What was the temperature (in °C) recorded at the earth's surface on each sounding? What was the surface pressure (in millibars [mb])? How do you think this would this compare to the sea-level pressure at KOAK? (The elevation of KOAK is 3 meters above sea level.)

    The radiosonde begins recording data at the ground from where it is launched. Hence, the surface temperature is the lowermost value on the temperature plot. The surface temperature at 00Z Jan. 13, 2012 was about 19°C or slightly more. (That's about 66° or 67°F). At 12Z, it was only 4°C (about 39°F). The surface pressure on both days (which you can read from the vertical axis at the level corresponding the lowermost value on the temperature and dew point profiles) was around 1015 or 1020 millibars (mb).

  3. An important property of the atmospheric temperature profile is the rate at which the temperature decreases with increasing altitude, expressed as the decrease in temperature per unit of vertical distance. This is called the atmospheric lapse rate (or environmental lapse rate). How can you tell by looking at the temperature plot on a radiosonde sounding where the atmospheric lapse rate is relatively large (that is, temperature decreases rapidly with increasing altitude), where it decreases slowly with altitude, where it doesn't change at all with increasing altitude, and where it increases with increasing altitude?

    Where the temperature profile is oriented vertically (that is, straight up and down), the temperature doesn't vary at all with increasing altitude. In contrast, the flatter the profile appears to be, the faster the temperature varies with increasing altitude.

  4. The troposphere is the lowest layer in the atmosphere. Like the other layers, it is defined in terms of how the temperature varies with increasing altitude within it.

  5. The tropopause is the boundary between the troposphere below and the stratosphere above. At the tropopause, the temperature generally stops decreasing rapidly with increasing altitude and either starts decreasing much more slowly, hardly changes at all with increasing altitude, or actually increases with altitude. At what level (in terms of altitude in meters and in kilometers, and in terms of pressure in millibars) is the tropopause located on each sounding? How can you tell where it is visually, in terms of the shape of the temperature plot? Where is the troposphere on each sounding?

    On the 00Z sounding the tropopause is located at the level where the pressure is about 200 mb, while on the 12Z sounding it is at about 210 mb. Both are just below 12 km (12,000 meters, or about 7.5 miles) above sea level. (There is a pronounced "kink" in the temperature profile there, and it lies well within the normal range of tropopause heights observed on the planet, not far from the global average).

    A plot of the long-term, global average temperature profile shows that there is a "pause" in the rapid decrease of temperature with increasing altitude at around 11 km above sea level (on the average). This is called the tropopause (the top of the troposphere). Although the individual profiles at KOAK don't extend high enough to show it, the "pause" eventually changes to an increase in temperature with increasing altitude, until there is another "pause" (the stratopause) at around 50 km (about 30 miles), where the temperature starts decreasing again. The layer between the tropopause and the stratopause is the stratosphere.

    The stratosphere exists because there is a layer above the tropopause (centered at an altitude of around 25 km, or about 15 miles) where concentrations of O3 (ozone) are higher than anywhere else in the atmosphere. This layer of maximum ozone concentration is called the ozone layer. (Even in the ozone layer, though, only about 1 out of every 10,000 air molecules is an ozone molecule, so there simply isn't much ozone in the atmosphere anywhere, even in the ozone layer. If all of the ozone in the atmosphere were gathered in a pure layer at sea level pressure, it would be only 0.25" deep.) Ozone is a very effective absorber of ultraviolet (UV) radiation, which constitutes about 10% of the energy in solar radiation. When ozone in the ozone layer absorbs most of the UV radiation in solar radiation, it warms up, heating the air and creating the distinctive vertical profile pattern of temperature that we assign the name "stratosphere". (Of course, without the tropopause and the stratosphere, there would be no basis for defining the troposphere, either, and the atmosphere would be a very different place.)

    Ultraviolet radiation destroys living cells, so without the ozone layer life would not exist on land or in the upper parts of the ocean. (As it is, a little bit of UV radiation still reaches the surface, where it can burn our skin if we get too much exposure to it!)

    The story of ozone shows how a gas present in the air in only very tiny amounts can still have a profound impact, by virtue of its ability to absorb certain wavelengths of radiation.

  6. A temperature inversion is a layer in which the temperature increases with increasing altitude. On each sounding, are there any temperature inversions in the troposphere? If so, where? How can you tell, based on the shape of the temperature plot?

    In the troposphere 00Z on the KOAK sounding, there are at least four temperature inversion layers. Each is quite shallow. The are located between 900 and 905 mb, between 820 and 840 mb, between 680 and 700 mb, and between 650 and 670 mb. You can spot them because the profile slopes to the right instead of to the left, showing that the temperature increases instead of decreases with increasing altitude.

    On the 12Z sounding, there are three inversions layers in the troposphere. The most pronounced one is between the surface and about 990 mb. This formed when the air next to the earth's surface cooled off at night but the air not far above the surface hardly changed temperature at all. (Compare to the 12Z sounding.)

    Why are temperature inversions significant? It turns out that air has a difficult time moving vertically (that is, rising or sinking) through an inversion layer. When there is an inversion layer at or close to the earth's surface, the air near the surface can be trapped there. If there is no wind, this can be a problem when there are sources of air pollution at the surface, such as the emissions of cars or some industrial facilities. The pollutants will accumulate in the unmoving air trapped near the surface, and their concentrations can increase to the point where they can damage living organisms, such as humans and trees. The Los Angeles Basin and the Central Valley of California are places where this problem is particularly severe, especially in the summer.

    [This is as far as we got on Monday, Feb. 27.]

  7. For clouds to form, air must first become saturated with water vapor, which occurs when the temperature falls to equal the dew-point temperature, or (less common) the dew-point temperature rises to equal the temperature, or (also less common) some of each. If there were any clouds present at the times of these radiosonde soundings, at what level(s) would they most likely have been? Why?

    The most likely levels where clouds might be located is where the temperature and dew point temperature are equal or nearly equal. On these soundings, there are no such levels, though, so we wouldn't expect to see clouds reported. (A quick check of the meteogram that covers these two times confirms that contention.)

    However, the radiosonde sounding at KOAK at 00Z on Feb. 29, 2012, shows several levels where clouds are likely, including around 870 mb and between 660 to 710 mb. In neither case is the temperature exactly equal to the dew point, but on temperature soundings its common for the two not to be quite the same while there is still a cloud present.

    Another radiosonde sounding at KOAK at 12Z on Feb. 29, 2012, just 12 hours later, shows an unambiguous layer of cloud extending from nearly the earth's surface to 650 mb (about 3.6 km, or around 12,000 ft. above sea level). The meteogram for this time shows that the sky was completely overcast at KOAK and it was raining lightly.

  8. Describe how the wind speed varies with increasing altitude on each sounding. At what level is the wind the fastest? How fast is it there?

    At 00Z the wind was in the 5 to 10 knot range near the earth's surface, but it increased to a maximum of 50 knots a little below the tropopause, at around 270 to 300 mb (between 9 and 10 km). At 12Z the wind was relatively light in the lower troposphere (with the exception of the wind right at the surface, which was 20 knots), but it increased above that to a maximum of around 25 knots a little below the tropopause. (This is pretty slow, actually.)

    The wind profile appearing on the radiosonde sounding at 12Z March 5, 2012 is more representative of winter conditions over KOAK. In that case the winds near the surface were very light, but they increased in speed with increasing altitude until reaching a maximum of 65 knots a little below the tropopause.

  9. Describe how the wind direction varies with increasing altitude. Is the wind observed at the surface representative of the wind at higher altitudes?

    At 00Z the winds were roughly from the northeast through most of the troposphere, and then varied with altitude erratically above that. However, at 12Z the winds in the lower troposphere were from the south and east, while in the upper troposphere they shifted to the north-northeast, then in the lower stratosphere they shifted to the northwest.

    As another example, the radiosonde sounding at 12Z March 5, 2012 shows light winds from varying directions near the earth's surface, then becoming first northwest and then consistently west as we go higher in the troposphere.

    Based on these soundings, the winds that we experience at the surface do not seem usually to be very representative of winds at higher altitudes, in either speed or direction.

  10. Dew-point temperature tells us something about the amount of water vapor in the air. Based on these soundings, where does the air tend to have the most water vapor in it, generally speaking?

    The dew point temperature is generally highest very close to the earth's surface, and tends to decrease with increasing altitude to the stratosphere, where it is very dry. (There is quite a bit of variation in dew point temperature in the troposphere on the way up through it, though, with occasional moister and drier layers; that is, the dew point doesn't always decrease steadily with increasing altitude by any means.)

    It shouldn't be very surprising that the air with the highest amounts of water vapor in it tends to be near the earth's surface. After all, how does water vapor get into the atmosphere in the first place? It's usually by evaporation of liquid water, especially from the oceans but also from lakes, rivers and streams, vegetation, and soil, all of which are at the earth's surface. (The reason why water vapor is most abundant near the earth's surface is actually more complicated than this, but this explanation will serve as a first stab at it.)

  11. Between 00Z and 12Z, where in the troposphere did the largest temperature change occur? How large was it?

    This question is easiest to address if we can plot the 00Z and 12Z temperature profiles on the same graph (without the dew point temperatures or winds). Doing this shows clearly that the level where by far the biggest change in the two temperature profiles occurs is at the earth's surface (a change of roughly 16°C, or about 29°F).

    Since 00Z is 4 pm PST, which is not that far off of the time when the temperature tends to be highest during the day, and 12Z is 4 am PST, which is not that far from the time when the temperature is typically at its lowest, the temperature difference between 00Z and 12Z is not that far from the daily temperature range. Based on these two soundings, at least, the daily temperature range is by far the largest at the earth's surface, and is typically much smaller throughout the rest of the troposphere.

    Was this a fluke? Here's another comparison, for 00Z and 12Z on March 5, 2012. The result is very similar! (It won't always be like this, but more often than not it is—the biggest day to night temperature change in the troposphere is usually, though not in every case, right at the earth's surface, and air temperatures at higher altitudes don't change much over the course of a day. An exception: when a warm "tongue" of air along the polar front replaces a cold "tongue", or vice versa. Those temperature changes tend to extend into much more of the troposphere, especially the lower half of it. For example, from 12Z March 1 to 00Z March 2 (that is, from 4 am to 4 pm on March 2, PST), the temperature warmed through a good part of the troposphere as the back edge of a cold tongue edged away from the California coast coast and a warm tongue began to replace it.

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