Geography and Climatology Lecture 1. Summary Fair success may be achieved in deducing the location...

Geography and ClimatologyLecture 1

Summary

Fair success may be achieved in deducing the location of a place from a given small set of climatic data.

The above success is achieved by using crude empirical relationships between climate and geographic averages.

Conversely, monthly mean temperatures can be inferred approximately from geographic information.

To what extent that these normative estimates differ from actuality offers clues on the effects of regional features of topography and ocean circulation on climate, as well as on microclimatic effects.

Q: Is climate largely controlled by location, i.e. geography? The Greek philosopher Pythagoras recognized the

sphericity of the Earth and the dominance of latitude in explaining climate variation.

Aristotle expanded on Pythagoras's foundation and introduced five climate zones, ranging from tropical to northern frigid.

It is not coincidental that in the early 20th century German scientist Koeppen also used 5 climate zones in his climate classification (classified with the 5 letters A-E).

Q: Is climate largely controlled by location, i.e. geography? Koeppen's classification was developed at a time when it was widely believed, especially in the German scientific community, that climate (therefore geography) determined flora and fauna, even the physical and behavioral traits of human societies.

Such determinism has its limitations, but it highlights the widespread and longstanding belief that location determines climate.

Work done by Geiger (1960) indicates that even the microclimate is largely controlled by the local 'geographical' conditions, such as orography and coastlines.

Given this control, therefore, one could hypothesize that one can infer the place where given climatic data were obtained. In other words, can we work out the one or more locations where a weather station may be, if we are given its climatic record? This is the key question addressed herein.

Q: Is climate largely controlled by location, i.e. geography?

Main governing factors

1. hemisphere - which leads to warmer conditions, or, in the Tropics (23.5S-23.5 N), to wetter conditions, in either January (south) or July (north)

2. latitude - affects a) the annual mean temperature, b) the annual range, c) the annual total rainfall (which is least around the Tropics and near the poles), d) the prevailing wind direction and strength

3. elevation - affects a) the annual mean temperature, b) precipitation, and c) the daily range of temperature

Main governing factors

4. sea surface temperature upwind - anomalously high SSTs (for a given latitude) means warmth and rain onshore - conversely, upwelling or cold water advection stabilizes the atmosphere and inhibits rainfall

5. upwind topography - an upwind mountain chain implies drier conditions, but also colder winters, because of easy advection of polar airmasses along the mountain chain

6. local topography - e.g. low daily minima in broad valleys, but higher ones in urban canyons; or reduced daily maxima near large bodies of water on account of a lake breeze.

The seasonal march of global sea surface temperature

The above mentioned 6 factors suggest that the following climatic/geographical information should be sufficient in providing the guidelines about climate as a function of geography.

1. Annual mean temperature2. Monthly mean temperatures (possibly reduced to

annual temperature range)3. Monthly-mean daily temperature range (or merely

its annual mean)4. Monthly mean precipitation5. Distance inland, relative to the prevailing surface

wind direction6. Local topographic information

1. Annual mean temperature

1. Annual mean temperature

1. The annual mean temperature is given approximately by the average temperature of the maxima and minima for the warmest and coldest months.

2. This temperature varies primarily with latitude, although warm and cold ocean currents can be also identified.

3. The western ocean margins (i.e. near the east coast of a continent) are warmer around 20-30° (in both N.H. and S.H.), but they are colder around 40-50° N, when compared to the eastern margins (west coasts).

4. Also, subtropical deserts are distinctly hot, on average.

1. The zonal averaged annual mean temperature

Thick curve lines: observations Thin straight lines are the linear regression lines.

1. The zonal averaged annual mean temperature is around 27 C within 20 S - 16 N degrees and then falls by about 0.85 K/degree latitude in the N.H. and 0.63 K/degree latitude in the S.H.

2. This implies that on average the S.H. is warmer than the N.H. (roughly 17C vs. 11C, respectively).

3. The above statement is true, only if the Antarctic plateau stations are excluded. These stations, 30-40K colder than the curve, suggests on account of their altitude and their isolation within the circumpolar vortex.


the water in S.H. is more salty than in N.H. (exception occurs in the North Atlantic Ocean)

4. A mean temperature of 19C in the N.H. is likely to be found at sea level at about 25N (i.e. 16 + [27 - 19]/0.85).

5. However, (i) a lower latitude is possible if the place is elevated or near a cold ocean current; (ii) a place 1,000 metres high may be 4 K cooler on average than another at sea level at the same latitude.

6. A temperature of 19C may be found below 16N at 2,000 m elevation (i.e. 27 - 2,000x4k/1,000]).

7. Choosing between these alternatives is facilitated by considering the annual range and monthly rainfalls.


2. Annual temperature range

1. [Definition] Difference between the hottest and coldest months, taking monthly mean temperatures in each case.

2. Over land, it is given approximately by the difference between the average of the January minimum and July maximum temperatures. This difference is called the apparent annual range. [Q: How about over the ocean?]

3. The error in assuming that the January and July temperatures are the coldest and warmest (or vice versa in S.H.) is small at latitudes above about 40 degrees

4. Data from Africa, South America and Europe show that the true range exceeds the apparent range on average by about 1 K at latitudes 0 - 10, 1.5 K over 11 - 20 and 0.5 K over 21- 40 degrees.

Annual Range of Surface Temperature

陸性率 (Continentality) = (1.7 A) / sin(Φ + 10) – 14

(V. Conrad formula)

海性率 (Oceanicity) = (Toct - Tapr) / A

5. The 11-20 degree belt generally experiences a wet season in summer, cloudiness and rain thus reduce the Tmax. The hottest month usually falls 1-2 months before the summer solstice (i.e. April or May in the N.H.), when the noon sun is near the zenith and the skies are often clear.

6. Many places near a west coast in the 21- 40 degree belt are affected by low sea surface temperatures due to upwelling of deep-ocean water, and this upwelling is often most intense in summer. Therefore, the warmest month here may be a few months after the summer solstice, e.g. September in the northern hemisphere.

7. A small annual range indicates (i) either a low latitude and/or (ii) proximity to the sea, especially on the coast facing the prevailing wind.


Example: Variation of the annual temperature range over South America

Questions:1. Why? 2. Any consequences?

0

0

4

4

8

12


Empirical relation between latitude (A, x-axis) and annual temp. range (R, y-axis): TR = 0.4 A.(This is Only a first order approximation)


Refinement: the effect of distance from sea (and/or the prevaling wind direction) needs to be considered.Tr = 0.13 A d0.2

d: distance downwind from the sea

How far is the beach?d = [Tr / 0.13 A]5 km


3. precipitation

Rain on earth can be triggered by any of 4 uplift mechanisms (or a combination):1. orographic: forced ascent of the low-level flow over

mountains;2. convective: spontaneous ascent due to local static

instability (This is due to surface heating and moisture convergence, and is found mainly at latitudes of 30 degrees latitude or less, but occurs up to 60 degrees in summer over the continents.);

3. Convergence: air is no where to go in low latitudes4. frontal: uplift over frontal surfaces and in lows is due

to baroclinic instability, i.e. to the interplay of warm and cold airmasses.

3. precipitation1. A wet summer and dry winter suggests a low-latitude

zenithal-rain or monsoonal climate. 2. The former, zenithal rain, is due to latitudinal shifts of the

I.T.C.Z. The latter, monsoonal regime differs only from the more general zenithal-rain climate by the requirement that the surface winds change direction at least 120° between seasons, according to Ramage (1970).

3. Monsoons generally occur between 10-30 degrees of the equator, but over a few degrees further north in late summer over India, and, especially, southern China.

4. Frontal precipitation falls at latitudes between about 35-70 degrees, but lower in winter. Between 30-40 degrees, mainly at west coasts, the rainfall in winter is much greater than in summer. This is because fronts stay poleward in summer.

3. precipitation

5. Rainfall is more uniformly distributed along east coasts and in the interior, not because fronts penetrate into these regions in summer, but because of convection.

6. The global average rainfall is 860 mm/yr, averaged over oceans and land together. Precipitation is heaviest near the I.T.C.Z. and in areas where monsoon winds come from over a warm ocean.

7. Arid conditions are generally found in the subtropics (mainly between 18-33° ) (except where the Trade winds blow onshore or where the monsoon penetrates further poleward) and near the poles.

8. Precipitation increases with elevation on the windward side of mountains up to about 2,000 meters, and then decreases with height (why?).

3. precipitation9. Rainfall is less on the leeside, as in the case of the

Patagonian desert in Argentina, to the east of the southern Andes, where the wind is from the west. It is also less in the Gobi desert in Mongolia, not only because monsoonal moisture is trapped by the Tibetan plateau, but also because frontal systems have lost most of their moisture on their long travel from the Atlantic.

10. Dry conditions thus imply (i) a subtropical or polar latitude, (ii) an extreme elevation, (iii) a rain shadow, situation near an anomalously-cold ocean (e.g. the Atacama desert in northern Chile) and/or (iv) great distance downwind of the ocean.

11. Dry conditions are indicated not only by low rainfall figures but also by a large daily range of temperatures.

4. daily temperature range1. [definition] the difference between the maximum and

minimum temperatures in 24 hours. It has no meaning at latitudes above 66.5 degrees, where the Sun does not appear for 24 hours during the winter solstice.

2. If the daily range is large, the atmosphere is dry and there is little cloud, probably indicated by low rainfall figures.

3. Thus a large range may indicate (i) a position well inland, (ii) considerable elevation above cloud level, and/or (iii) a latitude near the Tropics, where anticyclones are common.

4. Within anticyclones the air is dry and the sky often cloud-free; also, there is little wind, allowing the build-up of a strong nocturnal radiation inversion.

4. daily temperature range

5. Near the ITCZ and in areas frequented by frontal systems, not only does cloudiness limit daytime heating and nighttime cooling, but also, the air is too humid to cool much at night (near the ITCZ) or too windy (at mid-latitudes).

6. There appears to be no systematic variation of the annual mean daily range with latitude, because of the annual movement of the ITCZ, and the key dependence of a station's location on a continent.


Daily ranges at various elevations. The horizontal lines show the median values for each band of elevations


7. The effect of the station elevation (h, in km) can be isolated: there is a slight tendency for the daily range to be given by Rd = 8 + 2 h

8. This equation means that a range of 12 K, say, suggests an elevation of 2 km, i.e. [12 - 8] / 2 . However, it would be better to use Figure directly, as a guide to Rd; in that case, the deduced elevation for a daily range of 12 K would be about 1 km.

9. The broad scatter in Figure is not surprising: a place on the windward side of mountains has a much smaller daily range than a place at the same elevation on the lee side.

Remarks: A rough connection

between rainfall P (mm/month) and daily range is shown in the right figure, yielding the very approximate equation -

Rd = 13.5 - 2 log P

Estimates of Rd from rain may be compared with that from the relationship with elevation.

Unfortunately, formula is of little use for estimating P, not only because of the great scatter seen in Figure, but also because of the formulas nature; a small error in Rd leads to a huge difference to the value of P.

5. Surface wind

The prevailing wind direction is important in determining the climate because it (i) affects whether the incoming wind is cooled by an

west-coast ocean current, (ii) governs whether or not a place is dry because it is

far from the upwind ocean (as in the Gobi desert, for instance),

(iii) determines whether a place is dry because it lies in the lee of a high mountain range or wet because the wind has lately come from a warm ocean.

5. Surface wind

The direction of the wind depends primarily on the latitude, which is assumed already assessed from consideration of the annual temperature mean and range, and precipitation.

As you have noticed, winds tend to be light and variable from the equator to about 10 degrees latitude, generally easterly and steady over 10 - 25 degrees, variable between 25 - 35 degrees, mainly westerly but variable over 35 - 60 degrees, variable between 60 and 70 degrees, and easterly nearer the pole.

How can one proceed to estimate the location of the place with a given climate by calculating the annual mean temperature, the annual range and daily range, and by examining the rainfall figures?

A place with an annual average temperature T of 12° C, a January/July difference of 3.5 K and rainfall of 163 mm in January and 5 mm in July. The daily range is 13 K in January and 22 K in July, i.e. around 17 K throughout the year.

1. The higher temperatures in January show that the place is in the S.H.

2. The value of T implies a sea-level place around 36 degrees latitude [i.e. (27-17)/0.63 + 20]. Or it might be at a lower latitude and either greater elevation or located near a west coast. Of these options, a coastal location seems unlikely because the daily ranges are high, i.e. the air is dry, especially in winter. So the place may be elevated, at a latitude less than 36 degrees.

Example 1


3. A lower latitude is also indicated by the difference between the wet summer and dry winter, which suggest zenithal rain, and a location between about 8-16° S. In addition, the apparent annual range of about 3.5 K yields a rough estimate of about 9° S, i.e. 2.5 x 3.5. There the true annual range is roughly one degree more than the apparent range, i.e. 4.5 K in the present case, and so the estimate of latitude becomes 11 degrees, i.e. 2.5 x 4.5.

Example 1


4. The sea-level temperature at that latitude would be about 27° C. Consequently, a mean temperature of 12° C would be found at 3.7 km, i.e. (27 - 12)/4 . Such an elevation accords with Figure, which suggests above 3 km for a daily range of 17 K.

5. The formula for the distance inland (based on the apparent range) yields a figure of about 100 km if the latitude is 11 degrees, i.e. [3.5/{0.13 x 11}]5 .

6. Thus we estimate a place around 11° S, 3.7 km high and roughly 100 km from the coast. It is actually Cuzco in Peru, at 13° S, 3.2 km high, and about 350 km inland.

Example 1 (cont.)

Example 2 (an opposite estimate)

Determination of approximate temperatures given locale is also possible. For instance, take Chicago, which is at 42° N, 250 m elevation and about 3000 km downwind of the Pacific ocean (but only 1400 km from the Gulf of Mexico). 1. The sea-level temperature is estimated to be 5° C, i.e. (27 - 0.85 x

[42 - 16]) . 2. However, the height means that the value has to be reduced to

4° C, i.e. 8 - 250 x 4/1000. The formula for the annual range shows it to be 23-27 K, i.e. 0.13 x 42 x d0.2, where d is either 1400 (less common) or 3000 km (more common).

3. Thus the January and July monthly mean temperatures would be about (4 - 26/2) and (4 + 26/2), respectively, i.e. -9° C and 17° C. The observations are -4° C and 23° C, both about 5 K higher than the estimates. This is warmer than expected (why?)

Example 2 (an opposite estimate)

Determination of approximate temperatures given locale is also possible. For instance, take Chicago, which is at 42° N, 250 m elevation and about 3000 km downwind of the Pacific ocean (but only 1400 km from the Gulf of Mexico). 4. It is harder to derive the daily range because of the large

scatter in Figure. For what it is worth, the estimate from Figure is 13 K, which is more than the observed values of 8 K in both January and July.

5. We hypothesize that the daily ranges are reduced by strong winds in winter and the high humidity in summer.

Conclusions

1. Above examples illustrate how far empirical relationships between elements of climate and geography can be used to estimate either location or climate from the other.

2. It is interesting to see how much can be deduced, even when ignoring local circumstances.

3. Ideally, the procedures for estimating either geography or climate norms from the other would be simplified into the format of a decision tree.

Geography and Climatology Lecture 1. Summary Fair success may be achieved in deducing the location...

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