Warm fronts

A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fronts, and move more slowly than the cold fronts which usually follow because cold air is denser and harder to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale.
Clouds ahead of the warm front are mostly strati form, and rainfall gradually increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the strati form clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.

Development

Air masses are large bodies of air with similar properties of temperature and humidity that form over source regions, and the warm air masses behind warm fronts are not only warmer but higher in humidity than the colder air preceding them. Because of a warm air mass’s higher temperature and thus lesser density, mixing between the two air masses is unlikely. Being lighter, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 130 and lifting is slow but persistent. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, water vapor condenses and forms extensive cloud coverage. The first clouds to form along the sloping surface of the cold air are high cirrus, which thicken to cirrostratus and altostratus. Once the clouds have thickened to 2,500 metres (8,200 ft) from the earth’s surface, rain can begin to fall from the heavy nimbostratus cloud.

References:  http://passporttoknowledge.com/scic/jetstreams/educators/fronts.pdf

HOT WEATHER! -due to season changing

Today, I would explain why our weather recently is SO HOT! Yet, do you realize it is only really really hot during the morning to afternoon, then suddenly at around 5pm it starts to rain? Well, below is an explanation for it. (This relates to Singapore’s seasons.)

Singapore is currently going through something called the inter-monsoon season, which means a season between two monsoons. This season is characterized by hot, intense heat during the afternoons followed by short but heavy thunderstorms later in the day. The atmosphere is also very humid, with weak winds in the afternoon but heavy winds during the thunderstorms.

Contrary to much belief, Singapore does have seasons! The uniform weather in Singapore is affected by two specific season-Northeast Monsoon Season (December-March), where the season north easterly winds blow through Singapore and Southwest Monsoon Season (June-September), south easterly or south westerly wind blow through the country. Between each monsoon season, there is another kind of season called the inter-monsoon season which occurs during May-April and October-November.

The difference between the Monsoon seasons and inter-monsoon seasons is the strength of the wind as well as the effect of the sun. During monsoon seasons, the wind is constant and strong and it blows from a uniform direction. This uniform direction of wind leads to a more predictable and fairly constant weather. However, during the inter-monsoon period, the winds are weaker and they do not come from a fixed direction, leading to more erratic weather, which is occurring now.

Furthermore, during this season, due to the earth’s tilt, the equator is directly below the sun, which explains the hot noon temperature.  This hotter temperature would then lead to the heavy downpour later in the day. As said above, as the surface heats up, the warm air would rise and cool to form water vapor, which would condense to produce clouds that form rain. Since there are higher temperature in the afternoon, it would likely result in heavier rains later in the day as more hot air rise to form clouds. Hence, the weather now is not only hot, but it also has a high humidity, with drastic weather changes.

Taken from: http://www.climatetemp.info/singapore/

Breakdown/ Summary of graph:

Months with high average temperature: March, APRIL, MAY, June

Months with high humidity: Jan, APRIL, MAY, November, December

Well, now we can see why it is so hot and so humid!

Bibliography

http://wildsingaporenews.blogspot.com/2011/04/singapore-hot-and-wet-days-may-last.html

http://wildsingaporenews.blogspot.com/2011/05/singapore-hottest-days-of-year.html

http://monsoonseason.com/

http://weather.about.com/od/monsoons/f/monsoons.htm

Done by: Tay Li Lin (14)

River Delta

River delta is defined as the place where river unload their deposits, which is built up from primarily river-borne sediment. It is usually formed when the amount of sediment delivered at the mouth of a river exceeds the amount removed by waves and tidal currents, decreases the current velocity and also the transport capacity of the river.

There are a lot of types of river delta:

Fig 1: An arcuate delta

• Arcuate (fan-shaped) delta - e.g., Nile River.  Has many active, short distributaries taking sediment to their mouths.  The receiving (ambient) waters are rather shallow and have relatively even wave action arriving perpendicular to the shore with minimal longshore current.  As the sediment exits the many distributary mouths, the waves push it back, so the coastline is rather smooth.

Fig 2: a bird-foot delta

• Bird-foot (shaped like a bird foot) delta - e.g., Mississippi River.  Tend to have one or a very few major distributaries near their mouths.  The receiving basin has currents that carry the sediment away as it exits the distributary mouth.  There is a broad, shallow shelf that deepens abruptly, so the trend is to grow long and thin like a bird's toe.

Fig 3: a cuspate delta

• Cuspate (tooth-shaped) delta - e.g., Tiber River of Italy.  Usually has one distributary emptying into a flat coastline with wave action hitting it head-on.  This tends to push the sediment back on both sides of the mouth, with a "tooth" growing out onto the shelf.

Fig 4: an estuarine delta

• Estuarine delta - e.g., Seine River of France.  This type of delta has a river that empties into a long, narrow estuary that eventually becomes filled with sediment (inside the coastline).

Reference:

http://home.swipnet.se/valter/main%20Ri.htm

http://web.bryant.edu/~dlm1/sc366/deltas/deltas.htm

http://www.americaswetlandresources.com/background_facts/detailedstory/RiverDelta.html

Wrap-up with Term 2 Booklet

Hey guys, Luo Yang here.

My post would a summary of the things we had done this term using mindmap :)

Luo Yang (22)

Made using https://bubbl.us/

Chapter 3: Rivers

Hello everyone,

This post is about the stages of the river and how they are formed.

                                  Fig 1.   Overall of three main stages of river

1. Upper course

                                 Fig 2. The V-shaped Valley of the upper couse

    Most of the rivers begin from the top of the hills and mountains. Since the clouds move upwards air will cool down and form the rain, there will be a lot of rain. When rain falls on the land, some of the water will seep into the soil, and some of it stays on the surface. The water on the surface will flow down and form tiny streams. These streams will gradually join with other streams to form the beginning of the river. The beginning may be a spring, or a marsh or even a glacier. When the river flows down the steep and rocky sides of the hill or mountain, it will get faster and wilder as well as pick up and carries along with the large boulders and rocks. This wearing away of the land is called erosion.

2. Middle course

                                    Fig 3.  The meanders of the middle course
                                      

    As the river continues on its journey the ground starts to have a gentler slope. The river carries on eroding the land, making the river wider and deeper. Bends and turns called meanders then start to form, as the water begins to wears away the sides. The volume of water also increases, as smaller rivers called tributaries also flow in.

3. Lower course

                                               Fig 4.  The lower course and end of the river

    The river is very wide and deep by the time it reaches its final stage which is lower course. The surrounding land will have also flattened out, making the water slow down, and stones that have been carried along will drop down to the riverbed. It is also at this stage that the river meanders into large horse-shoe shapes, and this is where flooding is most likely to cause problems. When the river comes to an end, it will either flow into a loch or into the sea. This parts is known as the mouth of the river.

Done by Yu Jiacheng (25)

Human influences on Hydrograph

Urbanization

Urbanization replaces permeable land surface with impermeable one, which is covered by streets, parking lots, buildings etc. Water will run off the surface rather than being absorbed by soil. Base flow will decrease while overland flow will increase. With more overland flow, river will respond more quickly to rainfall, resulting in a shorter lag time. The construction of sewers during urbanization effectively increases the drainage density, resulting in a higher peak discharge.

Deforestation

Trees lengthen the lag time by intercepting the water. They also increase water infiltration by taking up water by their roots. Deforestation will reduce the interception and water-taking up; there will be more overland flow. Rivers in areas that have been cleared by deforestation will also respond quicker to rainfall, resulting in a higher peak discharge and shorter lag time.

Building of dams

The presence of a dam, will allow flow to be controlled, reducing the risk of flooding and allowing rivers to gradually respond to heavy rainfall in a controlled way; the river has a longer lag time.

Done by Nguyen Trung Hieu (23)

REFERENCE:

Channel Geometry and Flow Characteristics. The Physical Environment. Retrieved on 26 April 2011.URL: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/channel_geometry_and_flow.html

Factors affecting Hydrographs / Lag Time. Retrieved on 26 April. URL: http://cgz.e2bn.net/e2bn/leas/c99/schools/cgz/accounts/staff/rchambers/GeoBytes/GCSE%20Revision/Unit%201/Rivers/rivers_factors_affecting_hydrographs.htm

Hydrographs

Hi guys, today I'll be blogging about hydrographs.

Recap: a hydrograph is a graph that shows the discharge of a point in the river over a period of time, usually to show the change in discharge caused by rainfall. Discharge is just the amount of water that flows through that certain point and is measured in cumecs - cubic meters per second. The significance about hydrographs is that we can find out about the patterns in a certain drainage basin, how much water passes through a point and how long it takes for the water to get there, and from there we can implement protective measures to ward against floods and things like that.

Since all that water has to come from somewhere, let's look back to the hydrological cycle.
Overland flow and through flow all contribute to the amount of water in the drainage basin. The amount of overland flow/surface runoff is dependent on rainwater intensity and the infiltration capacity of the soil. If it's been raining heavily for a while and the soil is already saturated, then it stands to reason that there would be more overland flow. Throughflow is the water that flows horizontally in the soil (it's different from groundwater flow, that is in, well, groundwater but this is in the soil).The factors that would affect the amount of water would be:

1. Urbanisation
In this case, we're assuming urbanisation doesn't include drains. (But if there were, it would lead to a longer lag time, because the water takes a longer time to get to the river, or if the drains lead to reservoirs or water storage areas, then a lower peak). Urbanisation would basically entail a concrete jungle, and concrete has really poor permeability. As a result, there would be a lot more water flowing on the surface. 
2. Reforestation
Reforestation would mean more trees, and more interception (and also more evapotranspiration). So there would be less water reaching the surface, and less overland flow. And of course, it would offset some of the carbon in the atmosphere, and become a carbon sink.
3. Deforestation
The lack of trees would conversely mean less interception, and more water would reach the surface.
4. Permeability of soil
Very permeable soil means that it is more likely to absorb water and not so permeable soil means that more water would "pass" over it rather than being absorbed. This is assuming that infiltration capacity is constant, because the soil could be very permeable but cannot absorb a lot of water.  

(If you can think of some more factors, leave it in the comments :>)

Since we've already went through in class the factors that affect the lag time and shape of hydrographs, I won't talk about them here.

But I think the thing to note here is that, a lot of human activity is affecting not only the hydrological cycle, but also the discharge in the rivers. In reality, a lot of the floods and water-related disasters could be due us, our need to build buildings, cut down trees and so on. So we should balance our activity with our environmental conscience - because the environment can only take so much and bring the conditions back to equilibrium.